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Abstract
The spectral properties of 5 at.% doped Yb:LaMgB5O10 (Yb:LMB) crystal were presented and discussed in detail. The large absorption and emission cross sections, broad emission spectral lines, and considerably good thermal conductivity indicate the considerable potential of Yb:LMB for efficient and powerful ultrashort laser application. In the experiment, the semiconductor saturable absorber mirror (SESAM) mode-locked Yb:LMB laser generated pulses with the pulse duration of 182 fs and the central wavelength of 1025.2 nm. The maximum average power was 2.49 W with the slope efficiency of 48% and the optical to optical efficiency as high as 32%. Furthermore, pulses of 148 fs with average output power of 1.52 W and 108 fs with 0.61 W were obtained based on Yb:LMB crystal. To the best of our knowledge, this is the first demonstration of mode-locked pulse generation from the Yb:LMB laser.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The interest in Yb3+-based mode-locked laser technology is motivated by the fact that Yb-doped materials has advantages of low quantum defect, broad emission bandwidths, and compatibility with diode pumping. Of particular interest is the laser oscillator that could deliver high average power with high efficiency and short pulses. Reaching high power and short duration requires wide-bandwidth gain materials with reasonably thermal-mechanical features, together with the fast-developing high-power laser diode technology for pumping near 980 nm. Most important progress in this area was triggered by the pioneering experiments on laser operation of Yb:CALGO and Yb:KGW. With Yb:CALGO crystal, 94-fs pulses with high average output power up to 12.5 W were achieved [1]. Average output powers as high as 10 W with 290-433 fs pulses [2], 1.5 W with 85 fs pulses [3], and 162 fs pulses with peak power as high as 0.97 MW and average power of 8.8 W [4] were demonstrated for bulk lasers based on Yb:KGW crystals. Besides the effective laser design, these two crystals exhibit an attractive combination of spectroscopies and thermal properties ideally suited for the realization of high energy laser. Therefore, the quest for superior performance in terms of higher power and shorter pulses has pushed the material research to find new extremely promising crystals.
Very recently the LaMgB5O10 (LMB) crystal with the P21/c space group is used as the laser host material owing to its considerable large thermal conductivity (5.0 W K−1 m−1 at 300 K) and nice optical properties [5–8]. LaMgB5O10 compounds have a small La/O atomic ratio and the La ions are isolated from each other, which will lead to relatively low concentration quenching and high luminescence efficiency after La atoms were substituted by the active ions. Unfortunately, however, because of the strong volatility of MgO and B2O3 in the growth process, it is very difficult to obtain high-quality and large size LaMgB5O10 crystal by the conventional Czochralski method. It must be due to this reason, up to now, little progress has been made in developing lasers based on such sort of crystals. The only work reported of Yb:LMB (2 at.% Yb3+-ion doped) laser in the past long period was on the continuous-wave operation, in which 2.76 W of output power was generated with a slope-efficiency of 64.5% [9]. As for the ultrafast laser performance of Yb:LMB crystal, there is no related report.
In this paper, the spectroscopic properties of 5 at.% doped Yb:LMB crystal were systematically studied. Moreover, with a semiconductor saturable absorber mirror (SESAM) for passive mode-locking and two Gires-Tournois interferometer (GTI) mirrors for dispersion compensation, the highly efficient femtosecond operation of Yb:LMB crystal was achieved. 182 fs pulses at the central wavelength of 1025.2 nm were obtained with a maximum average output power of 2.49 W, corresponding to a maximum optical to optical efficiency of 32%. In addition, pulses of 148 fs with average output power of 1.52 W and 108 fs with 0.61 W were obtained based on Yb:LMB crystal. The results reveal that Yb:LMB crystal is a promising gain medium suitable for realizing high power ultrafast lasers.
2. Spectroscopic properties
In this work, Yb:LMB crystal was grown by the top-seeded solution growth (TSSG) method from Li2O-B2O3-LiF flux. The measured concentration of Yb3+ ions in the Yb:LMB crystal was 2.7 × 1020 atoms per cm3 (5 at.%-doped). A cuboid sample with dimensions of 6.1 × 5.2 × 4.6 mm3 was cut and polished along the principle optical axes (X, Y and Z) and used for the measurements of the spectral features. Room temperature (RT) polarized absorption spectra were conducted on a Perkin-Elmer UV-vis-NIR spectrometer (Lambda-950). An Edinburgh analysis instrument spectrophotometer (FLS980) at 77 K with a pulse xenon lamp was used to obtain the fluorescence spectra.
The polarized absorption cross-sections of the Yb:LMB crystal in a wavelength range from 880 to 1000 nm are shown in Fig. 1. The absorption spectrum is dominated by a main absorption band with the peak located at 977 nm, at which the absorption cross section amounts to 3.7 × 10−20 cm2 for E//Z, 3 × 10−20 cm2 for E//Y and 2.5 × 10−20 cm2 for E//X, respectively. The FWHM of the absorption band around 977 nm is about 3.5 nm, which is comparable with Yb:YAG, Yb:KGW and Yb:YCOB, hence the Yb:LMB crystal will be suitable for diode laser pumping
Considering the strong electron-phonon coupling in Yb3+-doped materials, which disturbs the Yb3+ energy transition and generates some additional emission peaks [10, 11], it is difficult to interpret the information from the emission spectra at room temperature. Measuring the low-temperature fluorescence spectrum can avoid these problems and provide the accurate energy level scheme. Figure 2 shows the polarized emission spectra at 77 K and the energy level diagram of Yb3+ ion in the Yb:LMB has been identified. The stark level positions of Yb3+ in LMB crystals are as follows: 2F7/2 (0, 348, 486, 756 cm−1) and 2F5/2 (10235, 10629, 10905 cm−1).
The RT stimulated emission cross-section of Yb:LMB is calculated by the reciprocity method [12]:
where σabs(λ) is the absorption cross-section at wavelength λ, h is the Plank constant, k is the Boltzman constant, c is the velocity of light, and λZL is the zero phonon line wavelength (977 nm). Zl and Zu are the lower and upper manifold partition functions, respectively, and is the degeneracy of the energy level , a = l or u. The partition functions Zl and Zu are calculated to be 1.298 and 1.184, respectively, based on the energy-level diagram in Fig. 2 (inset). The calculated emission cross-section is shown in Fig. 3. The strongest emission occurs at 977 nm, while because of the overlapping with the absorption peak at 977 nm, this emission peak seems to be less important for laser action. Of more practical importance is the emission that occurs at wavelengths in its trailing edge (approximately 1000-1080 nm). Over the wavelength range of 1000-1080 nm where practical laser oscillation is expected, the emission spectrum comprises three well-resolved emission peaks (1011 nm, 1025 nm and 1055 nm), with the strongest occurring at 1025 nm. The maximum emission cross section at 1025 nm is 2.88 × 10−20 cm2 for E//Z, 1.57 × 10−20 cm2 for E//Y, 1.36 × 10−20 cm2 for E//X, respectively.For a quasi-three-level system such as Yb:LMB, the laser action in free-running mode is determined by the effective gain cross section, σg(λ), rather than simply by σem(λ). Figure 4 shows the σg(λ) of Yb:LMB calculated from the following equation:
Here β denotes the fraction of Yb3+ ions excited to the upper manifold. A wide emission wavelength range can be expected, indicating that Yb:LMB should be a favorable candidate for achieving ultrafast lasers. For comparison, the main spectroscopic parameters for Yb:LMB and several typical Yb-crystals are listed in Table 1. In this table, the undefined symbols are peak wavelength λpeak; absorption bandwidth Δλabs; emission bandwidth Δλem; fluorescence lifetime τf; and thermal conductivity к.
Table 1. Comparison of Spectroscopic Properties between Yb:LMB and several typical Yb-crystals.
3. Mode-locking experimental setup
Considering the high thermal conductivity and the broad emission spectrum of Yb:LMB, this work is aimed to reveal the potential of Yb:LMB for achieving high power mode-locking laser. The 2-mm-thick antireflection coated Yb (5 at.%):LMB crystal used in the experiments was cut along the X axis. As can be seen from Fig. 1, Fig. 3, and Fig. 4, the X-cut crystal provides large absorption and emission cross sections and broader gain bandwidth supporting low lasing threshold, high efficiency and a reduced Q-switched mode-locking instability.
The The laser setup is presented in Fig. 5. The fiber-coupled multimode pump diode centered at 977 nm with a 4-nm spectrum (core diameter 200 µm, NA = 0.22) delivers up to 30 W output power. With a 1:1 optical collimation system, the pump beam was focused into Yb:LMB crystal with a radius of 100 µm. The gain medium was water cooled to be 8 degrees centigrade and placed with a small incident angel with respect to the cavity axis to suppress the etalon effect. The beam waist in the gain medium was calculated to be 105 µm × 100 µm with the propagation ABCD matrix theory, which matched the pump laser mode well. An InGaAs SESAM with the following parameters provided by BATOP GmbH: a non-saturable loss of 0.8%, a saturation fluence of 80 µJ/cm2, a modulation depth of 1.5%, and a relaxation time of 1 ps was used for starting and stabilizing the mode-locking laser. The laser beam was focused by a 500-mm radius-of-curvature mirror onto SESAM with a radius of 200-220 µm. To reduce the intracavity losses and keep our oscillator compact, we compensated the intracavity dispersions with two plane Gires-Tournois interferometer (GTI) negative-dispersion mirrors. An amount GDD of −3600 fs2 per round trip was introduced, which compensated the GDD inside the cavity and balanced the self-phase modulation (SPM) introduced by the gain medium.
Fig. 5 Experimental setup of the mode-locked laser. M1, flat input mirror: dichroic mirror coated for high transmission (HT) at the pump wavelength and high reflection (HR) in 1010-1100 nm; M2 and M3, HR fold mirrors. GTI1, GTI2: HR Gires-Tournois interferometer mirrors with dispersion of −1250 and −550 fs2 per reflection, respectively; M4: output coupler.
4. Experimental results and discussions
The mode-locking performance of the laser was investigated with three different output couplers (OCs) of 2%, 5% and 8% output coupling. Yb:LMB laser with all the three OCs could operate in the ML mode. Figure 6 shows the output power as a function of absorbed pump power, which was measured with an output coupler of T = 2%, 5%, and 8%, respectively. The absorbed pump power (Pabs) was determined from the pump power incident into the Yb:LMB crystal, by Pabs = γPin, here γ represents the small signal or unsaturated absorption. For the 2-mm-thick 5 at.% doped X-cut Yb:LMB, γ was measured as 0.75. The maximum output power was obtained with an 8% OC and reached up to 2.49 W, corresponding to a slope efficiency (η) of 48%. The continuous-wave mode-locking (CWML) regime can be sustained while the absorbed pump power from 5.38 to 7.83 W, giving a maximum optical to optical efficiency of 32% [22% (with respect to the incident pump power)]. We measured the stability at the maximum output power for an hour, and the recorded output power showed an instability of about ± 2%. In the case of lower output coupling (Toc = 2% and Toc = 5%), a maximum output power of 0.61 W and 1.52 W were obtained at a pump power level of 4.95 W and 6.59 W, resulting in an optical to optical efficiency of 12.3% and 23%, respectively. For all OCs, approximately the same value of the intracavity pulse energy of 0.3-0.6 µJ was estimated for the stable mode-locked operation. For higher intracavity pulse energy, the laser demonstrated CW or multi-pulse instabilities.
Figure 7(a) shows the typical CW mode-locked pulse trains with time span of 20 ns and 1 ms, demonstrating the good amplitude stability at the pulse repetition rate (RR) of 52 MHz. The typical radio frequency (RF) spectrum is shown in Fig. 7(b), revealing a nice signal-to-noise ratio. The peak at the fundamental beat note near 52 MHz with an extinction ratio of > 60 dB from the noise level can be seen. The absence of any spurious modulation prove clean CWML operation of the Yb:LMB laser.
Fig. 7 (a) Typical mode-locked pulses trains recorded in 20 ns and 1 ms per division (div) time scale. (b) The recorded frequency spectrum of the mode-locked laser (resolution bandwidth (RBW): 30 Hz). Inset: 1 GHz wide-span spectrum (RBW: 11 kHz).
With the case of Toc = 8%, an autocorrelation measurement gave a pulse duration of 182 fs at an absorbed pump power of 6.59 W, assuming a sech2 pulse shape [see Fig. 8(a)]. The spectrum of the mode-locked pulses was centered at 1025.2 nm with 6.2-nm bandwidth [see red curve in Fig. 8(b)]. The maximum output pulse energy of 47.9 nJ resulted in a corresponding peak power of about 0.263 MW. In contrast, the minimum pulse duration of 148 fs with 7.9 nm spectral bandwidth at FWHM was demonstrated for 5% OC. With decreasing OC transmittance, the central wavelength shifted toward longer wavelength to 1033.6 nm. The maximum pulse energy reached to 29.2 nJ with corresponding peak power of 0.196 MW. The pulse spectrum and autocorrelation trace are presented in Fig. 9. Using output couplers with the transmittance of 2%, as is shown in Fig. 10(a), pulses as short as 108 fs were obtained, and the central wavelength was further shifted to 1055.2 nm with a spectral bandwidth of 11.6 nm [see Fig. 10(b)]. For all OCs, the output pulses have a time bandwidth product (TBP) of 0.322-0.337. With higher output coupling, the TBP got slightly closer to the transform limit for a sech2 pulse (0.315)
The above results indicate that further power scaling and pulse shortening with high efficiency can be obtained in the SESAM mode-locked Yb:LMB bulk lasers by carefully designing the laser and taking into account the following challenges. First, SESAMs with larger modulation depth and an output coupler with higher transmittance should be employed. With 15% OC and 1.5% modulation depth SESAM, although it provided higher efficiency, the laser operated only in the CW mode. The other limitation in the current experiment is that the multi-peak structure of the emission spectrum makes it difficult to obtain shorter pulses. The laser output needs to be shifted toward longer wavelength to obtain sub-100 fs pulses. It can be reached by increasing the Yb doping level or crystal thickness and thus increasing the reabsorption losses in the gain crystal. Moreover, thanks to the excellent saturable absorption properties, many two-dimensional (2D) materials have also emerged as promising saturable absorbers (SAs) in bulk lasers, such as graphene, topological insulator (TI), and black phosphorus (BP) [20–22]. Pulses as short as 30 fs were obtained from a mode-locked Yb:CALYO laser based on a graphene SA [23]. Besides, average output power of 0.82 W with 272 fs pulses were achieved from a BP mode-locked Yb,Lu:CALGO oscillator [24]. Therefore, 2D materials mode-locked Yb:LMB laser may be also a potential approach for generating femtosecond pulses.
5. Conclusions
We have reported, to the best of our knowledge, the first mode-locked laser operation of Yb-doped LMB crystal. For E//Z polarization, the most suitable orientation for the generation of the ultrafast pulses, using a commercial multimode fiber-coupled diode, we have built a robust and powerful SESAM mode-locked oscillator at 52 MHz. In this configuration, we have investigated different OCs to optimize the output performance. With an output coupler value of 8%, the oscillator produced pulses with a duration of 182 fs and 2.49 W of output power, resulting in peak power of 0.263 MW and a maximum optical to optical efficiency of 32%. With 2% and 5% OCs, we were able to demonstrate 0.61 W and 1.52 W average output power with stable and self-starting 108 fs and 148 fs mode-locked pulse trains. The results verify the potential of Yb:LMB crystals for realizing high power and high efficiency ultrafast lasers. Considering the promising ultrafast laser performance demonstrated here, and the desirable spectroscopy properties of Yb-doped LMB crystal, one can predict that Yb:LMB could become a new Yb crystal of great potential in ultrafast lasers.
Funding
National Natural Science Foundation of China (61705231); Natural Science Foundation of Fujian Province (2016J01328); Science and Technology Project for the Universities of Shandong Province (No. J17KA180); Science and Technology Project of Qingdao (No.16-5-1-9-jch); Doctoral Foundation of QUST (210/010022849).
References and links
1. A. Greborio, A. Guandalini, and J. Aus der Au, “Sub-100 fs pulses with 12.5-W from Yb:CALGO based oscillators,” Proc. SPIE 8235, 823511 (2012).
2. G. R. Holtom, “Mode-locked Yb:KGW laser longitudinally pumped by polarization-coupled diode bars,” Opt. Lett. 31(18), 2719–2721 (2006). [PubMed]
3. H. Zhao and A. Major, “Megawatt peak power level sub-100 fs Yb:KGW oscillators,” Opt. Express 22(25), 30425–30431 (2014). [PubMed]
4. V. E. Kisel, A. S. Rudenkov, A. A. Pavlyuk, A. A. Kovalyov, V. V. Preobrazhenskii, M. A. Putyato, N. N. Rubtsova, B. R. Semyagin, and N. V. Kuleshov, “High-power, efficient, semiconductor saturable absorber mode-locked Yb:KGW bulk laser,” Opt. Lett. 40(12), 2707–2710 (2015). [PubMed]
5. Y. S. Huang, H. B. Chen, S. J. Sun, F. F. Yuan, L. Z. Zhang, Z. B. Lin, G. Zhang, and G. F. Wang, “Growth, thermal, spectral and laser properties of Nd3+:LaMgB5O10 crystal-A new promising laser material,” J. Alloys Compd. 646, 1083–1088 (2015).
6. H. Chen, Y. Huang, B. Li, W. Liao, G. Zhang, and Z. Lin, “Efficient orthogonally polarized dual-wavelength Nd:LaMgB5O10 laser,” Opt. Lett. 40(20), 4659–4662 (2015). [PubMed]
7. Y. S. Huang, S. J. Sun, F. F. Yuan, L. Z. Zhang, and Z. B. Lin, “Spectroscopic properties and continuous-wave laser operation of Er3+:Yb3+:LaMgB5O10 crystal,” J. Alloys Compd. 695, 215–220 (2017).
8. Y. Chen, Q. Hou, Y. Huang, Y. Lin, J. Huang, X. Gong, Z. Luo, Z. Lin, and Y. Huang, “Efficient continuous-wave diode-pumped Er3+:Yb3+:LaMgB5O10 laser with sapphire cooling at 1.57 μm,” Opt. Express 25(16), 19320–19325 (2017). [PubMed]
9. Y. S. Huang, W. W. Zhou, S. J. Sun, F. F. Yuan, L. Z. Zhang, W. Zhao, G. F. Wang, and Z. B. Lin, “Growth, structure, spectral and laser properties of Yb3+:LaMgB5O10-a new laser material,” CrystEngComm 17, 7392–7397 (2015).
10. A. Ellens, H. Andres, T. Heerdt, A. Meijerink, and G. Blasse, “Spectral-line-broadening study of the trivalent lanthanide-ion series. I. Line broadening as a probe of the electron-phonon coupling strength,” Phys. Rev. B 55, 173–179 (1997).
11. A. Ellens, H. Andres, M. L. H. Ter Heerdt, R. T. Weh, A. Meijerink, and G. Blasse, “Spectral-line-broadening study of the trivalent lanthanide-ion series. II. The variation of the electron-phonon coupling strength,” Phys. Rev. B 55, 180–186 (1997).
12. L. D. DeLoach, S. A. Payne, L. L. Chase, L. K. Smith, W. L. Kway, and W. F. Krupke, “Evaluation of absorption and emission properties of Yb3+-doped crystals for laser applications,” IEEE J. Quantum Electron. 29, 1179–1191 (1993).
13. S. Chénais, F. Druon, F. Balembois, P. Georges, A. Brenier, and G. Boulon, “Diode-pumped Yb:GGG laser comparison with Yb:YAG,” Opt. Mater. 22, 99–106 (2003).
14. N. V. Kuleshov, A. A. Lagatsky, A. V. Podlipensky, V. P. Mikhailov, and G. Huber, “Pulsed laser operation of Y b-dope d KY(WO4)2 and KGd(WO4)2.,” Opt. Lett. 22(17), 1317–1319 (1997). [PubMed]
15. F. Druon, S. Chénais, P. Raybaut, F. Balembois, P. Georges, R. Gaumé, G. Aka, B. Viana, S. Mohr, and D. Kopf, “Diode-pumped Yb:Sr3Y(BO3)4 femtosecond laser,” Opt. Lett. 27(3), 197–199 (2002). [PubMed]
16. F. Pirzio, S. D. Cafiso, M. Kemnitzer, A. Guandalini, F. Kienle, S. Veronesi, M. Tonelli, J. Aus der Au, and A. Agnesi, “Sub-50-fs widely tunable Yb:CaYAlO4 laser pumped by 400-mW single-mode fiber-coupled laser diode,” Opt. Express 23(8), 9790–9795 (2015). [PubMed]
17. J. Petit, P. Goldner, and B. Viana, “Laser emission with low quantum defect in Yb: CaGdAlO4.,” Opt. Lett. 30(11), 1345–1347 (2005). [PubMed]
18. S. Chénais, F. Druon, F. Balembois, G. Lucas-Leclin, P. Georges, A. Brun, M. Zavelani-Rossi, F. Augé, J. P. Chambaret, G. Aka, and D. Vivien, “Multiwatt, tunable, diode-pumped CW Yb:GdCOB laser,” Appl. Phys. B 72, 389–393 (2001).
19. H. D. Jiang, J. Y. Wang, H. J. Zhang, X. B. Hu, B. Teng, C. Q. Zhang, and P. Wang, “Spectroscopic properties of Yb-doped GdCa4O(BO3)3 crystal,” Chem. Phys. Lett. 357, 15–19 (2002).
20. J. L. Xu, X. L. Li, J. L. He, X. P. Hao, Y. Z. Wu, Y. Yang, and K. J. Yang, “Performance of large-area few-layer graphene saturable absorber in femtosecond bulk laser,” Appl. Phys. Lett. 99, 261107 (2011).
21. P. H. Tang, X. Q. Zhang, C. J. Zhao, Y. Wang, H. Zhang, D. Y. Shen, S. C. Wen, D. Y. Tang, and D. Y. Fan, “Topological insulator Bi2Te3 saturable absorber for the passive Q-switching operation of an in-band pumped 1645-nm Er:YAG ceramic laser,” IEEE Photonics J. 5, 1500707 (2013).
22. J. Ma, S. Lu, Z. Guo, X. Xu, H. Zhang, D. Tang, and D. Fan, “Few-layer black phosphorus based saturable absorber mirror for pulsed solid-state lasers,” Opt. Express 23(17), 22643–22648 (2015). [PubMed]
23. J. Ma, H. Huang, K. Ning, X. Xu, G. Xie, L. Qian, K. P. Loh, and D. Tang, “Generation of 30 fs pulses from a diode-pumped graphene mode-locked Yb:CaYAlO4 laser,” Opt. Lett. 41(5), 890–893 (2016). [PubMed]
24. X. Su, Y. Wang, B. Zhang, R. Zhao, K. Yang, J. He, Q. Hu, Z. Jia, and X. Tao, “Femtosecond solid-state laser based on a few-layered black phosphorus saturable absorber,” Opt. Lett. 41(9), 1945–1948 (2016). [PubMed]
