Open Access
Add to BibSonomy
Abstract
A Tm-doped mixed sesquioxide ceramic laser is mode-locked near 2 µm using InGaAsSb quantum-well semiconductor saturable absorber and chirped mirrors for dispersion compensation. Maximum average output power of 175 mW is achieved for a pulse duration of 230 fs at a repetition rate of 78.9 MHz with a 3% output coupler. Applying a 0.2% output coupler pulses as short as 63 fs are generated at 2.057 µm.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Thulium (Tm3+) based solid-state lasers generating ultrashort pulses in the eye-safe spectral region around 2 µm are currently actively investigated for potential applications in environmental monitoring [1] and material processing [2]. They have been already widely applied for frequency down-conversion into the mid-IR, as pump sources for synchronously pumped optical parametric oscillators [3]. As seeding sources for high power ultrafast amplifiers they are expected to play an important role also in high-harmonic and soft X-ray generation [4]. Bulk materials exhibit certain advantages over fibers [5] and few families of Tm3+-doped laser gain media have been successfully employed so far in passively mode-locked bulk solid-state lasers emitting in the 2-µm spectral range. The monoclinic tungstates Tm:KLu(WO4)2 (KLuW) and Tm:MgWO4 (MgW), exhibiting large emission cross-sections and bandwidths, have been one of the most successful ones generating pulses as short as 141 fs [6] and 86 fs [5]. However, in the quasi-3-level Tm-laser system what counts are the gain cross-sections and bandwidths. Another family known for their superior thermo-mechanical and thermo-optical properties are the cubic (C-type, bixbyite structure) rare-earth sesquioxides A2O3 (where A = Lu, Y, Sc). Employing a Tm:Sc2O3 crystal, 218 fs pulses were generated with an ion-implanted InGaAsSb-quantum-well based semiconductor saturable absorber mirror (SESAM) [7] and 166 fs by Kerr-lens mode-locking [8]. With Single-Walled Carbon Nanotube Saturable Absorbers (SWCNT-SAs) pulses as short as 175 fs were generated employing a Tm:Lu2O3 crystal [9]. Pulse durations as short as 105 fs were mentioned in a short report [10] employing the mixed LuScO3 crystal with the same SESAM as in [7], however, the optical spectrum exhibited a continuous-wave (CW) component.
Due to the high melting point of the cubic A2O3 crystals (e.g. 2450°C for Lu2O3), their growth typically requires the use of expensive rhenium (Rh) crucibles and Rh could be the source of the observed crystal coloration [11]. The growth of large-volume highly Tm3+-doped single crystals with high optical quality is complicated and the fabrication of transparent sesquioxide ceramics is a promising alternative. There are few reports on CW lasing of transparent Tm:Lu2O3 ceramics in different schemes including thin-disk lasers [12–14]. Femtosecond lasers based on Tm:Lu2O3 ceramics have also been investigated: using single-layer graphene for mode-locking resulted in pulse duration of ~410 fs [15] while SESAM mode-locking produced pulse durations as short as 180 fs [16].
Recently, we reported on fabrication, spectroscopy and CW laser operation of novel Tm3+:(Lu2/3Sc1/3)2O3 (shortly Tm:LuScO) mixed ceramics [17]. As a consequence of the compositional disorder (inhomogeneous spectral line broadening), mixed Tm:LuScO ceramics exhibit very broad absorption and emission bands [17] similar to the mixed single crystals [18]. Moreover, pumped by a laser diode at 802 nm, a Watt level Tm:LuScO ceramic micro-laser at 2.1 µm was demonstrated with a slope efficiency of 24% [17]. These promising spectroscopic features and laser performance motivated us to investigate passive mode-locking of such Tm:LuScO ceramics. Here we report on the realization of this regime employing a SESAM and chirped mirrors (CMs) for dispersion management, setting a pulse duration record for a mode-locked bulk solid-state laser operating in the 2-µm spectral range.
2. Tm:LuScO mixed ceramics
High quality Tm:(Lu,Sc)2O3 ceramics were fabricated by the Hot Isostatic Pressing (HIP) sintering method using powders of Sc2O3, Lu2O3, and Tm2O3 (purity: 99.99%, Alfa Aesar). The raw materials, with a stoichiometric amount of 100 at. % Lu + Sc (taken in a proportion of Lu:Sc = 2:1, the only ratio for which good optical quality could be obtained) and 5 at. % Tm over it, were mixed uniformly by ball milling for 24 h, dried for 6 h at 70°C, sieved, dry-pressed at 10 MPa, and cold isostatically pressed at 200 MPa. The green bodies of Tm:(Lu,Sc)2O3 ceramics were first pre-sintered at 1750°C for 10 h under vacuum (pressure, P < 10−3 Pa) to densify the preforms. For further densification, the pre-sintered ceramic samples were post-sintered by HIP at 1800°C for 2 h in an Ar atmosphere (P = 195 MPa) to eliminate the closed pores around the grain boundaries. Finally, the ceramics were annealed at 1500°C for 10 h in an O2 atmosphere to eliminate the oxygen vacancies and remove internal stresses. The composition of the ceramics can be represented as 4.76 at. % Tm:(Lu2/3Sc1/3)2O3 or shortly Tm:LuScO. Samples with a diameter of 15 mm and a thickness of 5 mm were obtained, see Fig. 1(a). For the laser experiments, a 2.95 mm thick active element was cut and its input and output surfaces (aperture ~3 × 3 mm2) were polished to laser-grade quality.
Fig. 1 (a) The fabricated 4.76 at.% Tm:LuScO mixed ceramic disks (laser-grade-polished) and (b) calculated gain cross section σgain for different inversion levels β of the Tm:LuScO mixed ceramic in the 2-µm spectral range.
In order to estimate the potential spectral emission of the Tm:LuScO ceramics, the gain cross section σgain = βσe − (1−β)σa where σe and σa denote the emission and absorption cross sections, respectively, was calculated for several values of the population inversion parameter β, see Fig. 1(b) [17]. β is the ratio of the excited Tm3+-ions in the 3F4 manifold to the total Tm3+-ion density. All the Tm:LuScO ceramics cross sections are very similar to those of their single crystal counterparts [18]. Similar to the mixed crystals, the mixed ceramics exhibit smoother wavelength dependence compared with the single ceramics which is potentially advantageous for enhancing the mode-locked pulse bandwidth. For low inversion levels emission of the free running laser is expected above 2 µm. This is beneficial for femtosecond pulse generation because the strong water absorption below 1.95 µm is avoided.
3. Experimental setup
The experimental setup of the Tm:LuScO ceramic laser shown in Fig. 2 was based on an X-shaped cavity. A narrow-linewidth Ti:sapphire laser tuned to 0.795 µm was used for pumping in the absorption peak corresponding to the 3H6 → 3H4 transition. The single pass absorption amounted to ~74%. Note that this absorption band at 0.74-0.83 μm is rather broad (FWHM = 25 nm) and thus suitable for AlGaAs laser diode pumping [17]. The pump beam was focused with a 70 mm lens. The Tm:LuScO ceramic sample was placed at Brewster angle between two dichroic folding mirrors M1 and M2 with a separation of ~100 mm. It was wrapped in In-foil for good contact (4 lateral sides) with the water cooled (14°C) Cu-holder.
Fig. 2 Scheme of the Tm:LuScO mixed ceramic laser (L: lens, M: dichroic mirror, CM: chirped mirror, OC: output coupler, r: radius of curvature, f: focal length).
For CW tunable operation, a 3.2 mm thick quartz plate was inserted as a Lyot filter close to the wedged output coupler OC1 (transmission T = 1.5%). A plain mirror M3 was used as an end mirror in this case leading to a cavity length of ~150 cm. In the mode-locking experiments, 5 chirped mirrors (CM1-5) with a group-delay dispersion of GDD = −125 fs2 per bounce were used for intracavity dispersion management. This leads to a total GDD of −1250 fs2 per round-trip. An InGaAsSb quantum-well based SESAM with 2 quantum-wells and 50 nm thick cap layer design [19] served as an end mirror in this case leading to a cavity length of 190 cm. Output couplers OC2 with T = 3.0%, 1.5%, 0.5% and 0.2% were tested.
4. Experimental results and discussion
Smooth and continuous tuning from 1.978 to 2.108 µm was achieved with the 1.5% OC in the CW regime as shown in Fig. 3 for an absorbed power of Pabs = 1.09 W. The total CW tuning range of 130 nm is limited on the long-wave side by the mirror transmissions (3% for the OC).
Mode-locking with the SESAM was self-starting and stable for hours without disruption during the daily operation with all the OCs. The performance applying different OCs was compared at a fixed incident power of 1.67 W (measured in front of L) corresponding to Pabs ~1.2 W. The non-collinear autocorrelation traces and the optical spectra of the mode-locked laser are shown in Fig. 4 and all relevant parameters are summarized in Table 1. The highest average output power of 175 mW was achieved with the 3.0% OC for a pulse duration of τ = 230 fs (FWHM for a sech2-pulse shape), Fig. 4(a). The OCs with decreasing transmission enabled substantial pulse shortening albeit at the expense of the average output power.
Fig. 4 Autocorrelation traces (a, c, e and g) measured by type-I second-harmonic generation in a 3-mm thick β-BaB2O4 crystal and optical spectra (b, d, f and h) of the mode-locked Tm:LuScO mixed ceramic laser measured with a 0.5 nm resolution rotating grating spectrometer for different OCs. Blue lines indicate calculated round trip GDD of the chirped mirrors and the 0.2% OC reflectivity, in (b) and (h), respectively.
Table 1. Mode-locking results with the Tm:LuScO ceramics laser with different OCs (Pout, average output power; τ, pulse duration; Δλ, spectra bandwidth as FWHM; TBP, time bandwidth product).
The shortest pulses with a FWHM of τ = 74 fs and ultimate stability were achieved using the 0.2% OC, at an average output power of 34 mW. No post- or pre-pulses were observed which was confirmed by a measurement in a longer (50 ps) time window. The spectrum was 71.6 nm broad and centered at ~2.057 µm as shown in Fig. 4(h). The secondary peak at longer wavelengths is considered to be leakage through the OC whose transmission increases to about 10% at 2.184 µm. The resulting time-bandwidth product (TBP) was 0.375, i.e. higher compared with the other OCs which yielded nearly transform-limited pulses. Thus a 3 mm thick ZnS plate (GDD = 462 fs2) was placed after OC2 as an extra-cavity compression element. The pulse could be compressed in this way to 63 fs which corresponds to ~9.2 optical cycles. It is also important to point out that the SESAM applied in the experiment has a rather long relaxation time of ~20 ps [19]. Only picosecond or sub-picosecond pulses down to ~650 fs could be generated previously with such a SESAM [20,21]. This indicates that the Kerr effect plays an important role for the sub-100 fs operation in the present work. However, pure Kerr-lens mode-locking was not achieved with an end mirror instead of the SESAM.
To further characterize the stability of the mode-locked Tm:LuScO ceramic laser, the radio frequency (RF) spectra were measured. With different OCs the performance was similar and the results with 0.2% OC are shown in Fig. 5. The wide-span RF measurement with resolution bandwidth of 30 kHz in Fig. 5(a) shows almost constant harmonics intensity within the 1 GHz span range. The narrow-band fundamental beat note was at 78.9 MHz with a high extinction ratio of 78 dB above noise level as shown in Fig. 5(b), measured with a resolution bandwidth of 100 Hz. Both RF spectra indicate clean CW mode-locking without Q-switching instabilities or any multi-pulse behavior [22].
Fig. 5 Radio frequency spectra of the SESAM mode-locked Tm:LuScO ceramic laser with the 0.2% OC: (a) 1-GHz-wide span, (b) fundamental beat note (RBW: resolution bandwidth).
5. Conclusion
In conclusion, sub-10 optical-cycle pulses were generated for the first time with a mode-locked 2 µm bulk solid-state laser. A Tm:LuScO (4.76 at.% Tm:(Lu2/3Sc1/3)2O3) mixed ceramic laser passively mode-locked by a SESAM produced nearly transform-limited pulses as short as 63 fs at ~2.057 µm after careful optimization of the intracavity dispersion using chirped mirrors and minor extracavity compression in a lossless bulk material. The substantial pulse shortening achieved in comparison to previous work with sesquioxide crystalline or ceramic hosts is attributed to the flat and smooth gain spectra of Tm:LuScO mixed ceramics.
Further possibilities for pulse shortening are seen in the optimization of the Lu2O3 and Sc2O3 mixing ratio and/or co-doping with Holmium for further spectral gain broadening. In such a way, we believe that there is still great potential for such mixed sesquioxide ceramic systems to generate sub-50 fs or even few-cycle pulses around 2 µm.
Acknowledgments
W. J. acknowledges financial support from the Key Laboratory of Science and Technology on High Energy Laser, CAEP.
References and links
1. U. N. Singh, B. M. Walsh, J. Yu, M. Petros, M. J. Kavaya, T. F. Refaat, and N. P. Barnes, “Twenty years of Tm:Ho:YLF and LuLiF laser development for global wind and carbon dioxide active remote sensing,” Opt. Mater. Express 5(4), 827–837 (2015). [CrossRef]
2. K. Scholle, S. Lamrini, P. Koopmann, and P. Fuhrberg, “2 µm laser sources and their possible applications,” in Frontiers in Guided Wave Optics and Optoelectronics, B. Pal, ed. (InTech, Rijeka, 2010), pp. 471–500.
3. V. Petrov, “Frequency down-conversion of solid-state laser sources to the mid-infrared spectral range using non-oxide nonlinear crystals,” Prog. Quantum Electron. 42, 1–106 (2015). [CrossRef]
4. B. Shan and Z. H. Chang, “Dramatic extension of the high-order harmonic cutoff by using a long-wavelength driving field,” Phys. Rev. A 65, 011804 (2002).
5. Y. Wang, W. Chen, M. Mero, L. Zhang, H. Lin, Z. Lin, G. Zhang, F. Rotermund, Y. J. Cho, P. Loiko, X. Mateos, U. Griebner, and V. Petrov, “Sub-100 fs Tm:MgWO4 laser at 2017 nm mode locked by a graphene saturable absorber,” Opt. Lett. 42(16), 3076–3079 (2017). [CrossRef] [PubMed]
6. A. Schmidt, S. Y. Choi, D.-I. Yeom, F. Rotermund, X. Mateos, M. Segura, F. Díaz, V. Petrov, and U. Griebner, “Femtosecond pulses near 2 μm from a Tm:KLuW laser mode-locked by a single-walled carbon nanotube saturable absorber,” Appl. Phys. Express 5(9), 092704 (2012). [CrossRef]
7. A. A. Lagatsky, P. Koopmann, P. Fuhrberg, G. Huber, C. T. A. Brown, and W. Sibbett, “Passively mode locked femtosecond Tm:Sc2O3 laser at 2.1 μm,” Opt. Lett. 37(3), 437–439 (2012). [CrossRef] [PubMed]
8. M. Tokurakawa, E. Fujita, and C. Kränkel, “Kerr-lens mode-locked Tm3+:Sc2O3 single-crystal laser in-band pumped by an Er:Yb fiber MOPA at 1611 nm,” Opt. Lett. 42(16), 3185–3188 (2017). [CrossRef] [PubMed]
9. A. Schmidt, P. Koopmann, G. Huber, P. Fuhrberg, S. Y. Choi, D.-I. Yeom, F. Rotermund, V. Petrov, and U. Griebner, “175 fs Tm:Lu2O3 laser at 2.07 µm mode-locked using single-walled carbon nanotubes,” Opt. Express 20(5), 5313–5318 (2012). [CrossRef] [PubMed]
10. A. A. Lagatsky, P. Koopmann, O. L. Antipov, C. T. A. Brown, G. Huber, and W. Sibbett, “Femtosecond pulse generation with Tm-doped sesquioxides,” Conference on Lasers & Electro-Optics Europe CLEO EUROPE, Munich, 2013, OSA, paper CA_6_3.
11. R. Peters, C. Krankel, K. Petermann, and G. Huber, “Crystal growth by the heat exchanger method, spectroscopic characterization and laser operation of high-purity Yb:Lu2O3,” J. Cryst. Growth 310(7-9), 1934–1938 (2008). [CrossRef]
12. O. L. Antipov, A. A. Novikov, N. G. Zakharov, and A. P. Zinoviev, “Optical properties and efficient laser oscillation at 2066 nm of novel Tm:Lu2O3 ceramics,” Opt. Mater. Express 2(2), 183–189 (2012). [CrossRef]
13. O. L. Antipov, A. A. Novikov, N. G. Zakharov, A. P. Zinoviev, H. Yagi, N. V. Sakharov, M. V. Kruglova, M. O. Marychev, O. N. Gorshkov, and A. A. Lagatskii, “Efficient 2.1-μm lasers based on Tm3+:Lu2O3 ceramics pumped by 800-nm laser diodes,” Phys. Status Solidi C 10(6), 969–973 (2013). [CrossRef]
14. E. J. Saarinen, E. Vasileva, O. Antipov, J.-P. Penttinen, M. Tavast, T. Leinonen, and O. G. Okhotnikov, “2-µm Tm:Lu2O3 ceramic disk laser intracavity-pumped by a semiconductor disk laser,” Opt. Express 21(20), 23844–23850 (2013). [CrossRef] [PubMed]
15. A. A. Lagatsky, Z. Sun, T. S. Kulmala, R. S. Sundaram, S. Milana, F. Torrisi, O. L. Antipov, Y. Lee, J. H. Ahn, C. T. A. Brown, W. Sibbett, and A. C. Ferrari, “2 μm solid-state laser mode-locked by single-layer graphene,” Appl. Phys. Lett. 102(1), 013113 (2013). [CrossRef]
16. A. A. Lagatsky, O. L. Antipov, and W. Sibbett, “Broadly tunable femtosecond Tm:Lu2O3 ceramic laser operating around 2070 nm,” Opt. Express 20(17), 19349–19354 (2012). [CrossRef] [PubMed]
17. W. Jing, P. Loiko, J. M. Serres, Y. Wang, E. Vilejshikova, M. Aguiló, F. Díaz, U. Griebner, H. Huang, V. Petrov, and X. Mateos, “Synthesis, spectroscopy, and efficient laser operation of ‘mixed’ sesquioxide Tm:(Lu,Sc)2O3 transparent ceramics,” Opt. Mater. Express 7(1), 4192–4202 (2017). [CrossRef]
18. C. Kränkel, “Rare-earth-doped sesquioxides for diode-pumped high-power lasers in the 1-, 2-, and 3-µm spectral range,” IEEE J. Sel. Top. Quantum Electron. 21(1), 1602013 (2015).
19. J. Paajaste, S. Suomalainen, A. Härkönen, U. Griebner, G. Steinmeyer, and M. Guina, “Absorption recovery dynamics in 2 µm GaSb-based SESAMs,” J. Phys. D Appl. Phys. 47(6), 065102 (2014). [CrossRef]
20. A. Gluth, Y. Wang, V. Petrov, J. Paajaste, S. Suomalainen, A. Härkönen, M. Guina, G. Steinmeyer, X. Mateos, S. Veronesi, M. Tonelli, J. Li, Y. Pan, J. Guo, and U. Griebner, “GaSb-based SESAM mode-locked Tm:YAG ceramic laser at 2 µm,” Opt. Express 23(2), 1361–1369 (2015). [CrossRef] [PubMed]
21. Y. Wang, G. Xie, X. Xu, J. Di, Z. Qin, S. Suomalainen, M. Guina, A. Härkönen, A. Agnesi, U. Griebner, X. Mateos, P. Loiko, and V. Petrov, “SESAM mode-locked Tm:CALGO laser at 2 μm,” Opt. Mater. Express 6(1), 131–136 (2016). [CrossRef]
22. C. Hönninger, R. Paschotta, F. Morier-Genoud, M. Moser, and U. Keller, “Q-switching stability limits of continuous-wave passive mode locking,” J. Opt. Soc. Am. B 16(1), 46–56 (1999). [CrossRef]
