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Abstract
A diode-pumped, passively mode-locked laser emitting at 915 nm with a praseodymium-doped yttrium lithium fluoride (Pr:YLF) crystal was demonstrated for the first time, to the best of our knowledge. Utilizing two polarization-combined blue pumping laser diodes (LDs) and a semiconductor saturable absorber mirror (SESAM), stable continuous-wave (CW) mode-locking operations were achieved with a maximum average output power of 408 mW and a slope efficiency of 10.8%. Laser pulse durations of 15 ps were obtained with a spectral full width at half maximum (FWHM) of 0.15 nm and a repetition rate of 1.53 GHz.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Ultrafast mode-locked lasers in the near-infrared band have facilitated many important cross-disciplinary research applications in the biological [1], bio-medicine [2], microscopy [3,4], optical coherence tomography [5], multi-modal imaging [6], and 3D vision areas [7,8]. For ultrafast laser sources covering the 650 to 1100 nm band, the traditional titanium-doped sapphire (Ti:Al2O3) mode-locked lasers have achieved tremendous success [9,10]. These systems typically use relatively bulky diode-pumped solid-state lasers (DPSSLs) or optically-pumped semiconductor lasers (OPSLs) as the pump sources, while there have been many recent efforts to utilize more compact and less expensive LD pumps instead [11–13].
Praseodymium-doped crystals are one of the most significant and alternative laser crystals developed in recent years [14–16]; praseodymium-doped yttrium lithium fluoride (Pr:YLF), in particular, has been proven as an excellent candidate for laser emissions not only in the visible region [17], but in the specific near-infrared wavelength around 900 nm (3P0→1G4 transition). Furthermore, Pr:YLF also possesses excellent photo-chemical stability and thermo-mechanical properties. The first CW near-infrared Pr:YLF laser emitting at 907 nm was demonstrated with an argon laser pump in 1994 [18]. CW Pr:YLF lasers at 915 nm pumped with 444-nm InGaN LDs were recently reported [19] and their power scaling was later achieved with an OPSL pump at 479 nm [20]. Further development included the realization of widely tunable (> 60 nm) CW laser operations ranging from 863 to 924 nm that peaked at 915 nm with a maximum output power of ~50 mW [21].
The above-mentioned 915-nm wavelength is crucial for certain laser spectroscopy and biophotonic applications. E.g., 915-nm laser has been used as the secondary excitation for dual-wavelength excited photoluminescence to study deep-level hole traps in Ga(In)NP alloys [22]; while a wide variety of proteins have two-photon absorption bands within the 910-920 nm wavelength regime [23,24]. The Pr:YLF crystals’ achievement in broad wavelength tunability around 900 nm indicates strong possibility for ultrafast laser generation in this spectral region, particularly at the wavelength of 915 nm, which has yet to be reported to date. Furthermore, the feasibility for Pr:YLF crystals to work in the mode-locking status has also been confirmed in the visible [25,26].
Here we report, for the first time to our best knowledge, the demonstration of a diode-pumped, passively mode-locked Pr:YLF laser emitting at 915 nm with a SESAM. To overcome the extra losses introduced by the mode-locking elements, i.e., the saturable absorbers (SAs), we utilized an efficient pumping scheme with two multimode InGaN blue LDs with a combined pump power of 7 W. Stable CW mode-locking operation was achieved with a maximum average output power of 408 mW and a slop efficiency of 10.8%. Pulse durations of 15 ps were obtained with a FWHM of 0.15 nm and a fundamental repetition rate of 1.53 GHz.
2. Experimental setup
Figure 1 illustrates the scheme of the experimental setup. The laser gain medium was an a-cut Pr:YLF crystal (Unioriental Inc.) with Pr3+ dopant concentration of 0.5 at. % in melt. The crystal was 8 mm in length and 3 × 3 mm in cross-section. Both the front and back surfaces of the crystal were anti-reflection coated in the visible and ~900 nm band to minimize the insertion losses. The crystal was wrapped with indium foils and mounted on a water-cooled copper holder for heat dissipation.
Fig. 1 Schematic of the passively mode-locked Pr:YLF laser with a SESAM and pumped by InGaN blue LDs.
Due to advancing in the high-power GaN/InGaN-based blue LDs, which are emitting in the Pr:YLF absorption band of 441 and 444 nm, power scaling of the 915-nm lasers can be achieved with a more cost-effective LD pumping scheme comparing with the more bulky OPSLs [20]. In the experimental setup, two multimode InGaN blue LDs (Nichia Inc.) were utilized as the pump sources. Each LD was encapsulated with a thermoelectric Peltier-cooler for efficient thermal management. The maximum output power of each LD was 3.5 W and the combined power was 7 W. The divergence angle of the LD emitted laser beam was about 14 × 45° (fast × slow axes). The LDs were collimated by aspherical lenses (MDTP OPTICS Inc.) of 4.3-mm focal length at 450 nm, and the beams were expanded along the slow axis with cylindrical lens pairs by a factor of 3.3. The lenses were also anti-reflection coated in the visible range. The pair of pump beams were combined by a polarization beam combiner and focused into the laser crystal with a plano-convex focal lens (f = 75mm). The spot size of the focused pump beam in the gain medium was measured to be 38 × 45 μm, and the corresponding Rayleigh lengths were 10 × 14 mm in the two directions, respectively. The Pr:YLF crystal has its absorption peaks at 441 and 444 nm for linear polarizations parallel to the a-axis and c-axis of the crystal, respectively. The maximal pump absorption was achieved by temperature control of the blue LDs.
The designated v-type laser cavity was composed of three mirrors. The folding input mirror (IM) was a concave mirror with a 100-mm radius of curvature. It was anti-reflection coated for the pump wavelengths (transmission > 96% at 441 and 444 nm) and highly reflective at the lasing wavelength (reflectivity > 99.9% at 915 nm). The IM also had high transmissions (> 60%) in the green, orange, red, and deep red spectral regions to further suppress potentially competitive lasing emissions in the visible. The output coupler (OC) was a coated plane mirror whose transmission was 2.0% at 915 nm. The third mirror in the cavity was a commercial SESAM (BATOP GmbH Inc.).
The blue pumping LDs are multimode in nature at high-power levels when the beam qualities generally suffer serious degradation and are far from diffraction-limited. On the other hand, for mode-locking operations, particularly in the Kerr lens mode (KLM), it has been found that the mode matching process is very sensitive to the modal quality of pump beams [27]. Consequently, SAs become an excellent candidate to initiate and stabilize the mode-locking [28]. Thus, in our experimental setup, the SESAM was a critical mode-locking element, which had a carrier relaxation time of ~1 ps, a saturation fluence of 70 µJ/cm2, a non-saturable loss of 1.4%, and a modulation depth of ~2.6% in the high reflection region from 910 to 990 nm. The SESAM was soldered on a copper heat sink and the chip had an active area of 4.0 × 4.0 mm and thickness of 450 μm. The damage threshold of the SESAM was 3 mJ/cm2.
3. Experimental results and discussions
CW laser operations at 915 nm were initially investigated with a highly-reflective plane mirror without the SESAM. The output power was measured by a thermopile power meter (Coherent Inc.). The CW laser performance was shown as the black dots in Fig. 2. The maximum output power of 720 mW was achieved under an absorbed pump power of 4.3 W, resulting in a slope efficiency of 18.4%. The ratio of absorbed pump power with respect to the incident power was measured to be 61.4%. The end mirror was later replaced by the SESAM, and the laser operation switched from CW to Q-switched mode-locking, and, eventually, to stable CW mode-locking with gradually increasing pump powers. To note, the SESAM was prone to damage by the Q-switched mode-locking pulses of high energies, which would cause a substantial decrease in output powers. By adjusting the pump power over the Q-switched regime fast enough, we successfully maintained the SESAM in good condition during our experiments. Once the CW mode-locking was established, we found that it would sustain for at least several hours without noticeable events. The average output power in the CW mode-locking laser operation versus the absorbed pump power was also plotted in Fig. 2, as the red dots. The threshold of CW mode-locking was ~1.9 W of the absorbed pump power. The maximum average output power reached 408 mW with no obvious power saturation and was only limited by the available pump power. Comparing with the mere CW laser, the reduced slope efficiency in CW mode-locking could be attributed to the relatively large non-saturable absorption of the utilized SESAM.
In characterizing the CW mode-locked Pr:YLF laser, the emitted spectrum was first measured by an optical spectrum analyzer (Yokogawa, AQ6370D) with a resolution of 0.02 nm. The laser spectrum was centered at 915 nm with a full width half maximum bandwidth (FWHM) of 0.15 nm at the maximum output power of 720 mW, as shown in Fig. 3. The recorded spectral shape and FWHM were typical for mode-locking operations with laser crystals [14,29]. The mode-locked pulse trains were detected by a time-domain-optimized InGaAs photodetector (New focus, model 1024) with a typical impulse response of 11 ps. The data were recorded with a digital oscillator (Tektronix, DPO72004C) with a 20-GHz bandwidth, a 100-GS/s maximum sample rate, and a 250-MS maximum record length. Figure 4 shows the typical mode-locked pulse trains in nanosecond and microsecond (inset) time scales.
Fig. 3 Optical spectrum of the mode-locked 915 nm Pr:YLF laser emission, measured at maximum output laser power.
To further investigate the emitted laser pulses, the radio frequency (RF) spectra of laser emissions, as shown in Fig. 5, were registered with a frequency-domain-optimized photodetector (New focus, model 1014) with a bandwidth of 45 GHz and a rising time of 9 ps. The RF spectra were recorded with a spectrum analyzer (Keysight-Agilent N9322C) up to 7 GHz to verify stable mode-locking. The resolution was 2 MHz with the 7-GHz span in Fig. 5(a), and 20 Hz with the 5-kHz span in the more detail-revealing Fig. 5(b) centered at the fundamental repetition rate. The fundamental frequency of 1.53 GHz, corresponding to a cavity length of 9.8 cm, possessed a signal-noise ratio of 53 dB. In Fig. 5(a), several harmonic signals of the fundamental rate of 1.5 GHz were also observed possessing nearly the same amplitude, which further confirmed the stable mode-locking laser operations.
Fig. 5 RF spectrum of the mode-locked Pr:YLF laser emission at 915 nm. (a) 7 GHz span, 2 MHz resolution bandwidth (RBW); (b) 5 kHz span, 20 Hz RBW.
Finally, the autocorrelation trace of the mode-locked laser pulses at the maximum output power is illustrated in Fig. 6. The autocorrelation trace was recorded with a commercial autocorrelator (APE, Pulse Check 150) and fitted well assuming a sech2-pulse shape. The obtained pulse duration was 15 ps. It was observed that the duration of mode-locked pulses was slightly shortened from ~17 ps at the threshold of stable CW mode-locking to 15 ps at the maximum output. Considering FWHM of the emitted spectrum was 0.15 nm, the time–bandwidth product was calculated to be 0.806, which was 2.6 times of the Fourier transform limit of the sech2-shaped pulses.
Fig. 6 Autocorrelation trace of the mode-locked 915 nm Pr:YLF laser, measured at maximum output laser power.
Diode-pumped Ti:Al2O3 KLM lasers currently have achieved mode-locked pulses with 219-fs duration and 164-mW output power at the longest available wavelength of 875 nm [13]. On the other hand, the diode-pumped Pr:YLF lasers presented here can readily provide mode-locked pulses around 900 nm band, particularly, the specific 915-nm emission with few other viable solutions to date. From our experimentation, the output power and repetition rate of Pr:YLF lasers are on par with the Ti:Al2O3 counterparts, while the current pulse duration is much longer. However, the gain bandwidth of Pr:YLF crystals around the 915-nm emission line was reported to be ~4.5 nm [20], which corresponded to a potentially minimal pulse duration of 195 fs for the bandwidth-limited sech2-shaped pulses. Therefore, we believe the Pr:YLF 915-nm mode-locked laser pulses can be substantially shortened by alternative laser configurations and/or optimal dispersion management.
4. Conclusions
In conclusion, we experimentally demonstrated a passively mode-locked Pr:YLF laser at 915 nm that was pumped by blue LDs with a SESAM. The polarization-combined LDs provided ~7 W pump power and the SESAM initiated stable CW mode-locking laser operations. The achieved maximum average output power was 408 mW with a slop efficiency of 10.8%. The pulse duration was 15 ps with a FWHM of 0.15 nm and a repetition rate of 1.53 GHz. Further efforts to optimize the dispersion management and utilize SAs of faster relaxation time towards femtosecond pulse generation are currently under investigation.
Funding
National Natural Science Foundation of China (No. 61627802), the Fundamental Research Funds for the Central Universities (No. 30916011103), and the High-Level Educational Innovation Team Introduction Plan of Jiangsu Province, China.
References and links
1. L. Shi, L. A. Sordillo, A. Rodríguez-Contreras, and R. Alfano, “Transmission in near-infrared optical windows for deep brain imaging,” J. Biophotonics 9(1-2), 38–43 (2016). [CrossRef] [PubMed]
2. W. Sibbett, A. A. Lagatsky, and C. T. A. Brown, “The development and application of femtosecond laser systems,” Opt. Express 20(7), 6989–7001 (2012). [CrossRef] [PubMed]
3. E. E. Hoover and J. A. Squier, “Advances in multiphoton microscopy technology,” Nat. Photonics 7(2), 93–101 (2013). [CrossRef] [PubMed]
4. J. Klein and J. D. Kafka, “The Ti:sapphire laser: The flexible research tool,” Nat. Photonics 4(5), 289 (2010). [CrossRef]
5. W. Drexler, U. Morgner, R. K. Ghanta, F. X. Kärtner, J. S. Schuman, and J. G. Fujimoto, “Ultrahigh-resolution ophthalmic optical coherence tomography,” Nat. Med. 7(4), 502–507 (2001). [CrossRef] [PubMed]
6. J. Shi, X. Sun, S. Zheng, J. Li, X. Fu, and H. Zhang, “A new near-infrared persistent luminescence nanoparticle as a multifunctional nanoplatform for multimodal imaging and cancer therapy,” Biomaterials 152, 15–23 (2018). [CrossRef] [PubMed]
7. H. Kneipp, J. Kolenda, P. Rairoux, B. Stein, D. Weidauer, J.-P. Wolf, and L. H. Woeste, “Ti:sapphire-laserbased lidar systems,” Proc. SPIE 1714, 270–278 (1992). [CrossRef]
8. A. Velten, T. Willwacher, O. Gupta, A. Veeraraghavan, M. G. Bawendi, and R. Raskar, “Recovering three-dimensional shape around a corner using ultrafast time-of-flight imaging,” Nat. Commun. 3(1), 745 (2012). [CrossRef] [PubMed]
9. R. Ell, U. Morgner, F. X. Kãârtner, J. G. Fujimoto, E. P. Ippen, V. Scheuer, G. Angelow, T. Tschudi, M. J. Lederer, A. Boiko, and B. Luther-Davies, “Generation of 5-fs pulses and octave-spanning spectra directly from a Ti:sapphire laser,” Opt. Lett. 26(6), 373–375 (2001). [CrossRef] [PubMed]
10. A. Bartels, D. Heinecke, and S. A. Diddams, “Passively mode-locked 10 GHz femtosecond Ti:sapphire laser,” Opt. Lett. 33(16), 1905–1907 (2008). [CrossRef] [PubMed]
11. K. Gürel, V. J. Wittwer, M. Hoffmann, C. J. Saraceno, S. Hakobyan, B. Resan, A. Rohrbacher, K. Weingarten, S. Schilt, and T. Südmeyer, “Green-diode-pumped femtosecond Ti:Sapphire laser with up to 450 mW average power,” Opt. Express 23(23), 30043–30048 (2015). [CrossRef] [PubMed]
12. S. Backus, M. Kirchner, C. Durfee, M. Murnane, and H. Kapteyn, “Direct diode-pumped Kerr Lens 13 fs Ti:sapphire ultrafast oscillator using a single blue laser diode,” Opt. Express 25(11), 12469–12477 (2017). [CrossRef] [PubMed]
13. J. C. E. Coyle, A. J. Kemp, J. M. Hopkins, and A. A. Lagatsky, “Ultrafast diode-pumped Ti:sapphire laser with broad tunability,” Opt. Express 26(6), 6826–6832 (2018). [CrossRef] [PubMed]
14. Y. Zhang, H. Yu, R. Zhang, G. Zhao, H. Zhang, Y. Chen, L. Mei, M. Tonelli, and J. Wang, “Broadband atomic-layer MoS2 optical modulators for ultrafast pulse generations in the visible range,” Opt. Lett. 42(3), 547–550 (2017). [CrossRef] [PubMed]
15. H. Yu, D. P. Jiang, F. Tang, L. B. Su, S. Y. Luo, X. G. Yan, B. Xu, Z. P. Cai, J. Y. Wang, Q. W. Ju, and J. Xu, “Enhanced photoluminescence and initial red laser operation in Pr:CaF2 crystal via co-doping Gd3+ ions,” Mater. Lett. 206(1), 140–142 (2017). [CrossRef]
16. B. Xu, P. Camy, J.-L. Doualan, Z. Cai, and R. Moncorgé, “Visible laser operation of Pr3+-doped fluoride crystals pumped by a 469 nm blue laser,” Opt. Express 19(2), 1191–1197 (2011). [CrossRef] [PubMed]
17. P. W. Metz, F. Reichert, F. Moglia, S. Müller, D. T. Marzahl, C. Kränkel, and G. Huber, “High-power red, orange, and green Pr3+:LiYF4 lasers,” Opt. Lett. 39(11), 3193–3196 (2014). [CrossRef] [PubMed]
18. T. Sandrock, T. Danger, E. Heumann, G. Huber, and B. H. T. Chai, “Efficient Continuous Wave-laser emission of Pr3+-doped fluorides at room temperature,” Appl. Phys. B 58(2), 149–151 (1994). [CrossRef]
19. B. Qu, B. Xu, Y. J. Cheng, S. Y. Luo, H. Y. Xu, Y. K. Bu, P. Camy, J. L. Doualan, R. Moncorge, and Z. P. Cai, “InGaN-LD-pumped Pr3+:LiYF4 continuous-wave laser at 915 nm,” IEEE Photonics J. 6(6), 1502906 (2014). [CrossRef]
20. Z. P. Cai, B. Qu, Y. J. Cheng, S. Y. Luo, B. Xu, H. Y. Xu, Z. Q. Luo, P. Camy, J. L. Doualan, and R. Moncorgé, “Emission properties and CW laser operation of Pr:YLF in the 910 nm spectral range,” Opt. Express 22(26), 31722–31728 (2014). [CrossRef] [PubMed]
21. B. Qu, R. Moncorgé, Z. Cai, J. L. Doualan, B. Xu, H. Xu, A. Braud, and P. Camy, “Broadband-tunable CW laser operation of Pr3+:LiYF4 around 900 nm,” Opt. Lett. 40(13), 3053–3056 (2015). [CrossRef] [PubMed]
22. D. Dagnelund, Y. Q. Huang, C. W. Tu, H. Yonezu, I. A. Buyanova, and W. M. Chen, “Dual-wavelength excited photoluminescence spectroscopy of deep-level hole traps in Ga(In)NP,” J. Appl. Phys. 117(1), 015701 (2015). [CrossRef]
23. M. Drobizhev, N. S. Makarov, T. Hughes, and A. Rebane, “Resonance Enhancement of Two-Photon Absorption in Fluorescent Proteins,” J. Phys. Chem. B 111(50), 14051–14054 (2007). [CrossRef] [PubMed]
24. M. Drobizhev, N. S. Makarov, S. E. Tillo, T. E. Hughes, and A. Rebane, “Two-photon absorption properties of fluorescent proteins,” Nat. Methods 8(5), 393–399 (2011). [CrossRef] [PubMed]
25. M. Gaponenko, P. W. Metz, A. Härkönen, A. Heuer, T. Leinonen, M. Guina, T. Südmeyer, G. Huber, and C. Kränkel, “SESAM mode-locked red praseodymium laser,” Opt. Lett. 39(24), 6939–6941 (2014). [CrossRef] [PubMed]
26. K. Iijima, R. Kariyama, H. Tanaka, and F. Kannari, “Pr3+:YLF mode-locked laser at 640 nm directly pumped by InGaN-diode lasers,” Appl. Opt. 55(28), 7782–7787 (2016). [CrossRef] [PubMed]
27. M. T. Asaki, C. P. Huang, D. Garvey, J. Zhou, H. C. Kapteyn, and M. M. Murnane, “Generation of 11-fs pulses from a self-mode-locked Ti:sapphire laser,” Opt. Lett. 18(12), 977–979 (1993). [CrossRef] [PubMed]
28. S. Sawai, A. Hosaka, H. Kawauchi, K. Hirosawa, and F. Kannari, “Demonstration of a Ti:sapphire mode-locked laser pumped directly with a green diode laser,” Appl. Phys. Express 7(2), 022702 (2014). [CrossRef]
29. Y. Zhang, H. Yu, H. Zhang, A. Di Lieto, M. Tonelli, and J. Wang, “Laser-diode pumped self-mode-locked praseodymium visible lasers with multi-gigahertz repetition rate,” Opt. Lett. 41(12), 2692–2695 (2016). [CrossRef] [PubMed]
