Open Access
Add to BibSonomy
Abstract
Tm,Ho:CaYLuAlO4 (Tm,Ho:CALYLO) crystal has wide emission spectra both for π-polarization and σ-polarization, showing significant potential for the generation of ultrashort pulses. Here, a widely tunable and passively mode-locked laser operation based on Tm,Ho:CALYLO crystal under two polarizations was demonstrated for what we believe to be the first time ever. For π-polarization, a maximum output power of 1.52 W and a tuning range of 255.3 nm were achieved in the continuous wave (CW) regime. In the mode-locked regime, a pulse duration of 68 fs and an average output power of 228 mW were achieved upon GaSb-based semiconductor saturable absorber mirror (SESAM). As for σ-polarization, a broader tuning range of 267.1 nm was realized, leading to the shorter pulse duration of 58 fs at 79.7 MHz repetition rate.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Ultrafast lasers in the ∼2 µm region have been proven indispensable in many applications, such as surgery, high precision processing and pumping of mid-IR optical parametric oscillators (OPO) [1–4]. To date, ∼2 µm ultrafast solid-state lasers are typically achieved using thulium (Tm3+) and holmium (Ho3+) ions doped crystals. Ho3+ ions doped crystals with larger emission cross-section and longer emission wavelength (> 2 µm) are more suitable for the aforementioned applications [5]. However, the narrower emission spectrum width (full width at half maximum, FWHM) of Ho3+ ions limits the pulse duration of a single Ho-doped mode-locked laser [6], which can be solved by co-doping with Tm3+ ions [7]. Due to the overlapping emission spectra of Ho3+ and Tm3+ ions, the gain medium can combine the emission spectra of both two ions, thereby providing a wider emission spectrum for mode-locking and enabling the realization of ultrashort pulses [8].
Currently, sub-100 fs mode-locked Tm,Ho co-doped lasers are mainly based on disordered host materials [9,10], which offer wide emission spectra essential for the generation of ultrashort pulses. Among them, the performance of the tetragonal aluminate crystals CaLnAlO4 (Ln = Gd3+ [11,12], Y3+ [13,14]) with the K2NiF4-type disordered structure (space group I4/mmm) stands out prominently. The disorder in the material structure, arising from the random distribution of Ca3+ and Ln3+ ions within the same crystal lattice sites, leads to a significant inhomogeneous broadening of the emission bandwidth [15]. In recent years, several Tm,Ho co-doped CaLnAlO4 crystals have been widely applied in the generation of femtosecond pulses. In 2018, Zhao et al. reported a mode-locked Tm,Ho:CaYAlO4 laser with a pulse duration of 87 fs, marking the first instance of sub-100 fs pulses being achieved based on Tm,Ho co-doped crystal [16]. Subsequently, Wang et al. achieved even shorter pulses of 52 fs at 2015nm based on the Tm,Ho:CaGdAlO4 crystal, corresponding to a repetition rate of 80.45 MHz [17]. Broader emission spectra are prerequisite for obtaining shorter pulses in mode-locked crystals. One approach to enhance the broadband emission properties of CaLnAlO4 crystal is to leverage compositional disorder in CaLn1xLn21-xAlO4 solid solutions, a technique initially applied to Yb:CaLuxGd1-xAlO4 crystal [18]. And in the ∼2 µm region, pulses as short as 50 and 46 fs were generated by employing Tm,Ho:CaYGdAlO4 and Tm,Ho:Ca(Gd,Lu)AlO4 crystals [19,20]. Recently, we reported the polarized spectral properties based on Tm,Ho:CaY0.9Lu0.1AlO4 (Tm,Ho:CALYLO) crystal [21]. The wide emission spectra highlight its potential for application in widely tunable and mode-locked laser operations.
In this study, we present continuous wave (CW) and passively mode-locked laser operations of Tm,Ho:CALYLO crystal for π-polarization and σ-polarization. We reveal a tuning range of 255.3 nm and a pulse duration of 68 fs for the π-polarized laser, while achieving a broader tuning range of 267.1 nm and the shorter pulse duration of 58 fs for the σ-polarized laser.
2. Experimental configuration
The schematic of CW and passively mode-locked Tm,Ho:CALYLO laser is shown in Fig. 1. The pump source is a Raman-shifted Er:fiber laser with a beam quality factor (M2) of ∼1.05, which emits up to 5.2 W at 1.7 µm. An anti-reflection-coated lens with a focal length of 75 mm was used to focus the pump beam onto the sample with a focusing radius of 22 µm. The gain material is an a-cut Tm,Ho:CALYLO crystal with dimensions of 3 × 3 × 3 mm3, which was placed in the cavity at Brewster’s angle. To avoid thermal effects, it was mounted in a copper holder water-cooled at a temperature of 13°C. Under CW operation, the X-type resonator was composed of M1, M2 (flat concave mirror, R = 100 mm), M3 (flat mirror) and plane-wedged output couplers (OCs) with transmissions of 0.5%, 1%, 1.5%, 3%, 5% respectively. A 2-mm-thick quartz plate acting as a Lyot filter (LF) was inserted close to the OC at Brewster’s angle for wavelength tuning.
Fig. 1. Schematic of the CW and SESAM mode-locked Tm,Ho:CALYLO laser. RM, reflective mirror; Lens, focusing lens; M1-M4, cavity mirrors; CM1 and CM2, chirped mirrors; OC, output coupler.
For the mode-locking operation, a GaSb-based semiconductor saturable absorber mirror (SESAM) was employed for initiating and stabilizing the mode-locking, which contains two 8.5 nm thick InGaAsSb quantum wells and a 50 nm cap layer [22]. To ensure the required fluence for SESAM saturation, a second beam waist of ∼61 µm was created by M4 (plano-concave mirror, R = -100 mm). Additionally, two plane-chirped mirrors (CM1, CM2) offering group delay dispersion (GDD) of -125 fs2 per bounce were inserted into the cavity for balancing the frequency chirped induced by self-phase modulation (SPM) effect.
3. Result and discussion
Initially, the performance of CW Tm,Ho:CALYLO laser was investigated with a four-mirror laser cavity. Under laser operation, the absorption efficiency of the Tm,Ho:CALYLO crystal with TOC = 3% was determined to be 73% for π-polarization and 65% for σ-polarization. Figures 2(a) and 2(d) demonstrate the relationship between output power and absorbed pump power for different OC transmittances (0.5%, 1%, 1.5%, 3%, 5%). For π-polarization, a maximum output power of 1.52 W was achieved at 2081.4 nm (TOC = 3%), corresponding to a slope efficiency of 51.9% and a threshold absorbed pump power of 0.19 W. While for σ-polarization, that of 1.31 W was achieved at 2044.5 nm, corresponding to a slope efficiency of 50.6% and a threshold absorbed pump power of 0.32 W. No thermal saturation phenomenon was observed during CW laser operation, indicating the potential for power scaling. The laser emission spectra at different OCs were recorded and shown in Figs. 2(b) and 2(e). As the transmittance of the OCs decreases, the laser wavelength undergoes a redshift, ranging from 2078.8 nm (TOC = 5%) to 2100.2 nm (TOC = 0.5%) for π-polarization and from 2044.5 nm (TOC = 5%) to 2094nm (TOC = 0.5%) for σ-polarization. This phenomenon is a typical characteristic of quasi-three-level laser systems and attributed to the enhanced reabsorption effect [23].
Fig. 2. CW and wavelength tuning performance of the Tm,Ho:CALYLO laser. CW output power versus absorbed pump power with different OCs for (a) π-polarization and (d) σ-polarization. Optical spectra with different OCs for (b) π-polarization and (e) σ-polarization. Wavelength tuning curves with TOC = 0.5% for (c) π-polarization and (f) σ-polarization.
By inserting the LF in the CW regime, wavelength tunability of the Tm,Ho:CALYLO laser was studied with TOC = 0.5%, as shown in the Figs. 2(c) and 2(f). A tuning range of 255.3 nm spanning from 1894.7 to 2150 nm (π-polarization) and a broader tuning range of 267.1 nm spanning from 1887 to 2154.1 nm (σ-polarization) were achieved. Notably, the tuning range for σ-polarization is the widest range of Tm,Ho co-doped lasers that have been reported. The wide and flat tuning curves of Tm,Ho:CALYLO crystal laser underscore its potential for generating ultrashort pulses.
By inserting the SESAM and CMs into the cavity, the stable mode-locked operations were successfully achieved for both polarizations using 1.5% OC. For π-polarization, an average output power of 228 mW was achieved at 79.2 MHz repetition rate, corresponding to a pulse energy of ∼2.9 nJ. The average energy fluence on the SESAM was estimated to be ∼1.65 mJ/cm2. Figure 3(a) displays the optical spectrum with a FWHM of 70.8 nm centered at 2074nm, and the corresponding intensity autocorrelation trace is shown in Fig. 3(b). The pulse duration is extracted to be 68 fs from sech2 pulse shape fitting, yielding a time-bandwidth product (TBP) of 0.336. For σ-polarization, the wider and smoother tuning range proves the potential benefits for realizing shorter pulses. Under the pump power to 4.72 W, an average output power of 178 mW at a repetition rate of 79.7 MHz was realized, equivalent to a single pulse energy of 2.2 nJ. The laser spectrum with a FWHM of 79.2 nm and a peak wavelength of 2054nm is shown in Fig. 4(a). Figure 4(b) demonstrates the intensity autocorrelation trace, and the pulse duration amounted to 63 fs by assuming a sech2-intensity profile, corresponding to a TBP of 0.355. The large TBP value means that the pulse duration could be further shortened by external linear chirp compensation. Therefore, a 3.5-mm-thick ZnSe polycrystalline plate with a GDD of + 700 fs2 was positioned outside the cavity. After chirp compensation, the pulse was compressed to 58 fs with a TBP of 0.327, close to the Fourier transform limit of 0.315. The corresponding intensity autocorrelation trace is shown in Fig. 4(c).
Fig. 3. (a) Optical spectrum and (b) autocorrelation trace of the SESAM mode-locked Tm,Ho:CALYLO laser for π-polarization.
Fig. 4. (a) Optical spectrum, (b) autocorrelation trace and (c) autocorrelation trace after extra-cavity compression of the SESAM mode-locked Tm,Ho:CALYLO laser for σ-polarization.
To characterize the stability of the SESAM mode-locked operation, the radio frequency (RF) spectra were measured in different span ranges. Figure 5(a) demonstrates the high signal-to-noise ratio of 52 dB recorded in a 300 kHz span range with the resolution bandwidth (RBW) of 100 Hz, and the uniform harmonic beat notes in a 1.5 GHz span range are shown in Fig. 5(b). They both reveal highly stable mode-locked operation of Tm,Ho:CALYLO laser. Furthermore, the real-time pulse trains were recorded on millisecond and nanosecond scales, as shown in Fig. 5(c). The uniform pulse trains without Q-switching behavior present a further evidence for stable CW mode-locking.
Fig. 5. RF spectra of the SESAM mode-locked Tm,Ho:CALYLO laser for σ-polarization in (a) a 300-kHz and (b) a 1.5-GHz span range. (c) Corresponding pulse trains recorded by a fast photodiode on nanosecond and millisecond (inset) time scales.
4. Conclusion
In conclusion, we demonstrate CW and the first mode-locked operations of a Tm,Ho:CALYLO laser in-band pumped by a 1.7 µm Raman fiber laser. In the CW regime, the maximum output power of 1.52 W and 1.31 W without thermal saturation were achieved for π-polarization and σ-polarization, corresponding to the tuning ranges of 255.3 nm and 267.1 nm, respectively. In the mode-locked regime, stable operations were achieved by employing SESAM and CMs for both polarizations, with the average output power of 228 mW and 178 mW, and the pulse durations of 68 fs and 58 fs (after extra-cavity compression) for π-polarization and σ-polarization, respectively. The wide emission spectra and the emission wavelength above 2 µm of Tm,Ho:CALYLO crystal empower the ultrashort pulse generation. These results prove the promising potential of the Tm,Ho:CALYLO crystal for few-optical-cycle pulse generation at ∼2 µm.
Funding
National Natural Science Foundation of China (62175133); Project of Taishan Scholar (2021-175).
Disclosures
The authors declare no conflicts of interest.
Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
References
1. S. Amini-Nik, D. Kraemer, M. L. Cowan, et al., “Ultrafast Mid-IR Laser Scalpel: protein signals of the fundamental limits to minimally invasive surgery,” PLoS One 5(9), e13053 (2010). [CrossRef]
2. R. R. Gattass and E. Mazur, “Femtosecond laser micromachining in transparent materials,” Nat. Photonics 2(4), 219–225 (2008). [CrossRef]
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. J. Liu, F. Yang, J. Lu, et al., “High output mode-locked laser empowered by defect regulation in 2D Bi2O2Se saturable absorber,” Nat. Commun. 13(1), 3855 (2022). [CrossRef]
5. S. A. Payne, L. L. Chase, L. K. Smith, et al., “Infrared cross-section measurements for crystals doped with Er3+, Tm3+, and Ho3+,” IEEE J. Quantum Electron. 28(11), 2619–2630 (1992). [CrossRef]
6. W. Yao, Y. Wang, S. Ahmed, et al., “Low-noise, 2-W average power, 112-fs Kerr-lens mode-locked Ho:CALGO laser at 2.1 µm,” Opt. Lett. 48(11), 2801–2804 (2023). [CrossRef]
7. K. Eremeev, P. Loiko, A. Benayad, et al., “Efficient continuous-wave Tm,Ho:CaF2 laser at 2.1 µm,” Opt. Lett. 48(7), 1730–1733 (2023). [CrossRef]
8. Z. Pan, P. Loiko, Y. Wang, et al., “Disordered Tm3+,Ho3+-codoped CNGG garnet crystal: Towards efficient laser materials for ultrashort pulse generation at ∼2 µm,” J. Alloys Compd. 853, 157100 (2021). [CrossRef]
9. Z. Pan, L. Wang, J. E. Bae, et al., “SWCNT-SA mode-locked Tm,Ho:LCLNGG laser,” Opt. Express 29(24), 40323–41332 (2021). [CrossRef]
10. Y. Zhao, Y. Wang, W. Chen, et al., “67-fs pulse generation from a mode-locked Tm,Ho:CLNGG laser at 2083nm,” Opt. Express 27(3), 1922–1928 (2019). [CrossRef]
11. J. Q. Di, X. D. Xu, C. T. Xia, et al., “Crystal growth, polarized spectra, and laser performance of Yb:CaGdAlO4 crystal,” Laser Phys. 26(4), 045803 (2016). [CrossRef]
12. J. Di, X. Xu, C. Xia, et al., “Growth and spectra properties of Tm, Ho doped and Tm, Ho co-doped CaGdAlO4 crystals,” J. Lumin. 155, 101–107 (2014). [CrossRef]
13. Z. Gao, J. Zhu, J. Wang, et al., “Generation of 33 fs pulses directly from a Kerr-lens mode-locked Yb:CaYAlO4 laser,” Photonics Res. 3(6), 335–338 (2015). [CrossRef]
14. S.-D. Liu, L.-L. Dong, Y. Xu, et al., “Femtosecond pulse generation with an a-cut Nd:CaYAlO4 disordered crystal,” Appl. Opt. 55(27), 7659–7662 (2016). [CrossRef]
15. P. Wang, K. Li, Y. Jin, et al., “Spectral properties and high-efficiency broadband laser operation of Tm:CaY0.9Gd0.1AlO4 crystal,” Opt. Laser Technol. 161, 109217 (2023). [CrossRef]
16. Y. Zhao, Y. Wang, X. Zhang, et al., “87 fs mode-locked Tm,Ho:CaYAlO4 laser at ∼2043nm,” Opt. Lett. 43(4), 915–918 (2018). [CrossRef]
17. Y. Wang, P. Loiko, Y. Zhao, et al., “Polarized spectroscopy and SESAM mode-locking of Tm,Ho:CALGO,” Opt. Express 30(5), 7883–7893 (2022). [CrossRef]
18. Q. Hu, X. Su, Y. Wang, et al., “Spectroscopic properties and ultrafast performance of Yb:CaLuxGd1-xAlO4,” Laser Phys. Lett. 14(4), 045809 (2017). [CrossRef]
19. H. Ding, J. Liu, Y. Wang, et al., “Mode-locking of a Tm,Ho:CALYGO laser delivering 50 fs pulses at 2.08 µm,” Opt. Lett. 48(23), 6267–6270 (2023). [CrossRef]
20. L. Wang, W. Chen, Y. Zhao, et al., “Sub-50 fs pulse generation from a SESAM mode-locked Tm,Ho-codoped calcium aluminate laser,” Opt. Lett. 46(11), 2642–2645 (2021). [CrossRef]
21. P. Wang, Y. Jin, J. Lv, et al., “Spectroscopy and efficient laser performances of a Tm,Ho:CaY0.9Lu0.1AlO4 crystal,” Opt. Lett. 48(17), 4544–4547 (2023). [CrossRef]
22. J. Paajaste, S. Suomalainen, A. Härkönen, et al., “Absorption recovery dynamics in 2 µm GaSb-based SESAMs,” J. Phys. D: Appl. Phys. 47(6), 065102 (2014). [CrossRef]
23. H. Zhao and A. Major, “Dynamic characterization of intracavity losses in broadband quasi-three-level lasers,” Opt. Express 22(22), 26651–26658 (2014). [CrossRef]
