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Abstract
We report on the first sub-100 fs mode-locked laser operation of a Tm3+-doped disordered calcium lithium tantalum gallium garnet (Tm:CLTGG) crystal. Soliton mode-locking was initiated and stabilized by a transmission-type single-walled carbon nanotube saturable absorber. Pulses as short as 69 fs were achieved at a central wavelength of 2010.4 nm with an average power of 28 mW at a pulse repetition rate of ∼87.7 MHz. In the sub-100 fs regime, the maximum average output power amounted to 103 mW.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Calcium niobium gallium disordered cubic garnets of the Ca3(Nb,Ga)5O12 type or shortly CNGG, including those co-doped with univalent alkali metal cations such as Na+(CNNGG) or Li+ (CLNGG), when doped with trivalent rare earth ions (RE3+ = Nd3+ or Yb3+), feature significant inhomogeneous spectral broadening induced by their intrinsic structure disorder while preserving moderate thermal conductivity [1]. They have been proven to be excellent gain media for generation of femtosecond pulses at ∼1 μm [2–7]. Recently, thulium (Tm3+) doped CNGG-type disordered garnets, such as Tm:CNGG, Tm:CNNGG and Tm:CLNGG, profiting from their broad, smooth, and flat gain profiles extending well above 2 μm, have emerged as extremely suitable materials for sub-100-fs pulse generation at ∼2 μm from passively mode-locked (ML) lasers [8]. The mode-locking results achieved with such crystals. are summarized in Table 1. An overview of gain media suitable for generation of sub-100 fs pulses around 2 μm can be found in the recent review paper [11].
Table 1. Sub-100 fs mode-locked thulium lasers based on disordered gallium garnets
Abbreviations: SA – saturable absorber, Pout – average output power, FWHM – full width at half maximum, Δτ – pulse duration, PRF – pulse repetition frequency, SWCNT – single-walled carbon-nanotube
Alternatively, another multicomponent garnet host, namely calcium tantalum gallium garnet (abbreviated: CTGG), has been explored with Nd3+ or Yb3+ doping for ultrashort pulse generation at ∼1 µm [12,13]. CTGG offers higher thermal conductivity as compared to CNGG which is attractive for power scalable laser operation [14]. Similar to CNGG, CTGG can be co-doped by univalent Li+ cations for local charge compensation and elimination of unwanted cationic vacancies; such a crystal is known as CLTGG. The Li+ cations further modify the multi-ligands around the active ions leading to additional inhomogeneous spectral broadening and expanding the potential for applications in ultrafast lasers [15].
Fig. 1. Room temperature spectroscopy of the Tm:CLTGG crystal: (a) absorption cross-section, σabs, spectrum for the 3H6 → 3H4 transition at ∼800 nm; (b) calculated gain cross-section spectra, σgain, at ∼2 μm, β is the inversion ratio for Tm3+ ions.
Very recently, a high quality 3.17 at.% Tm3+:CLTGG crystal was grown in the Institute of Chemical Materials, China Academy of Engineering Physics by the Czochralski method. It has a chemical formula of {Ca2.889Tm0.093□0.018}[Ta1.32Ga0.68](Ga2.367Ta0.204Li0.345□0.084)O12, here □ is the cationic vacancy. It was shown recently that Tm:CLTGG exhibits a relatively high thermal conductivity (4.33 Wm-1K-1 at room temperature [16]). The structure disorder of CLTGG results from a random distribution of Ta5+ and Ga3+ cations over octahedral and tetrahedral lattice sites. The dopant Tm3+ ions replace for Ca2+ cations. The inhomogeneous spectral broadening for the Tm3+ emission originates from the variation in the second coordination sphere composed by various cations (Ta5+, Ga3+, Li+). The spectral broadening is observable in the absorption and emission spectra of the Tm:CLTGG crystal. The absorption cross-section, σabs, spectrum for the 3H6 → 3H4 transition of Tm3+ ions is shown in Fig. 1(a). The maximum σabs is 3.1 × 10−21 cm2 at 795 nm. The absorption bandwidth (full width at half maximum, FWHM) is as large as 26.7 nm. This makes this new crystal attractive for pumping by high-power AlGaAs diode lasers at ∼800 nm. The calculated gain cross-section, σgain, spectra for the 3F4 ↔ 3H6 transition with different Tm inversion levels β are shown in Fig. 1(b), revealing very broad, smooth, and flat gain profiles, extending up to at least 2.2 μm owing to a strong electron-phonon (vibronic) interaction [17]. The measured luminescence lifetime of the upper laser level (3F4) is 4.93 ms.
The promising spectroscopic and thermal properties of the Tm:CLTGG crystal motivated us to explore it for sub-100 fs pulse generation. In this work, we present the first soliton ML operation of a Tm:CLTGG laser delivering pulses as short as 69 fs at 2010.4 nm.
2. Experimental configuration
The continuous-wave (CW) and passively ML regimes of the Tm:CLTGG laser were investigated in an X-shaped astigmatically compensated linear cavity, as shown in Fig. 2. The uncoated crystal had an aperture of 3 mm × 3 mm and a thickness of 3.1 mm. It was mounted in a water-cooled Cu holder (coolant temperature: 12°C) and placed at Brewster’s angle between two dichroic folding mirrors, M1 and M2, both having a radius of curvature (RoC) of -100 mm. The pump source was a narrow-linewidth CW Ti:Sapphire laser tuned to 795 nm for matching the absorption maximum. The pump beam was focused by a spherical lens (focal length, f = 70 mm) into the laser crystal resulting in a beam waist radius of 30 μm and 65 μm in the sagittal and tangential planes, respectively. The measured single-pass pump absorption under lasing conditions slightly varied as a function of the output coupling and amounted to ∼40%, revealing a certain degree of ground-state bleaching. A Lyot filter (3.5-mm thick quartz plate) was inserted at Brewster’s angle close to the output coupler (OC) for wavelength tuning in the CW regime.
Fig. 2. Experimental arrangement of the CW and passively ML operation of Tm:CLTGG laser: L: spherical focusing lens (f = 70 mm); M1-M2: concave mirrors (RoC = -100 mm), M3: flat rear mirror used in the CW regime; DM1-DM4, dispersive mirrors; OC: output coupler; SWCNT-SA: single-walled carbon nanotube saturable absorber.
For ML operation, a transmission-type SWCNT-SA was used to initiate and stabilize the soliton pulse shaping. The intracavity group delay dispersion (GDD) was managed by several broadband dispersive mirrors (DMs) exhibiting different negative GDD per bounce (-250 fs2 and -125 fs2) and more than 300 nm reflectivity bandwidth from 1850 to 2150 nm.
3. Experimental results
3.1 Continuous-wave laser operation
The performance of the CW Tm:CLTGG laser was evaluated with a four-mirror cavity including a flat rear mirror M3 and a plane-wedged output coupler (OC) without SA, see Fig. 2. The maximum output power reached 294 mW with a 3% OC, corresponding to a slope efficiency of 26.9%, see Fig. 3(a). The laser wavelength exhibited a blue-shift with increasing OC transmission from 1995 to 1969 nm, see Fig. 3(b). The round-trip cavity loss δ and the intrinsic slope efficiency η0 which is related to the mode matching efficiency as well as the pump quantum efficiency were estimated with the Caird analysis by fitting the measured slope efficiency as a function of the OC reflectivity, ROC = 1 – TOC [18]. The corresponding fitting curve is shown in Fig. 3(c), giving the values of η0 = 56.7% and δ = 0.3%. These values indicate a well-optimized cavity mode matching and good quality of the laser crystal. With a 0.5% OC, a broad spectral tuning range of ∼286 nm (1799.4–2085.3 nm) was achieved in the CW regime at an absorbed pump power of 1.58 W, as shown in Fig. 3(d).
Fig. 3. Continuous-wave laser performance of the Tm:CLTGG laser: (a) input-output dependences, η – slope efficiency; (b) laser spectra; (c) evaluation of the cavity loss using the Caird approach; (d) tuning curve for a 0.5% OC in the CW regime.
3.2 Soliton mode-locked operation
For ML operation, a transmission-type SWCNT-SA was employed for initiating and stabilizing the intracavity soliton pulse shaping in the Tm:CLTGG laser. Purified SWCNTs synthesized by the dried arc-discharge method were used. The SWCNT/PMMA film was deposited on a 1-mm thick uncoated quartz substrate by spin coating [17]. The individual SWCNTs were uniformly distributed in the film and randomly oriented. The saturable absorption is due to the first fundamental transition of semiconducting carbon nanotubes (E11). Due to a certain distribution of nanotube diameters (1.5–2.2 nm), the SA exhibited a broad absorption band spanning from 1.8 to 2.1 μm. The modulation depth (for normal incidence), saturation fluence, non-saturable loss and recovery time were measured at ∼2 μm to be < 0.5%, ∼10 μJ/cm2, ∼1% and ∼1 ps, respectively [19]. The SWCNT-SA was oriented at Brewster’s angle and was placed in the second beam waist created by two curved DMs, DM1 (RoC = -100 mm, GDD = -125 fs2) and DM2 (RoC = -50 mm, GDD = -125 fs2). The beam radius at the SWCNT-SA was estimated by using the ABCD matrix formalism to be 101 and 146 μm in the sagittal and tangential planes, respectively.
Initially, two additional flat DMs, DM3 (GDD = -250 fs2) and DM4 (GDD = -125 fs2), were implemented in the other cavity arm for GDD management in the ML operation. Stable and self-starting soliton mode-locking operation was obtained with a 1.5% OC and a total round-trip negative GDD of -1125 fs2 provided by the DMs. The laser spectrum had almost perfect sech2-shaped profile with a central wavelength of 1996 nm and emission bandwidth (FWHM) of 45 nm indicating soliton ML operation, see Fig. 4(a). The average output power amounted to 103 mW at an absorbed pump power of 1.68 W. The pulse repetition rate was ∼87.7 MHz. The measured second harmonic generation (SHG) based intensity autocorrelation trace could be well fitted by a sech2-shaped temporal pulse profile giving a pulse duration (FWHM) of 97 fs, as shown in Fig. 4(b). The corresponding time-bandwidth product (TBP) of 0.328 was only slightly above the theoretical value for sech2-shaped pulses (0.315).
Fig. 4. (a) Spectrum of the soliton ML Tm:CLTGG laser with TOC = 1.5% and (b) the corresponding SHG-based intensity autocorrelation trace.
The far-field beam profiles of the Tm:CLTGG laser for CW and soliton mode-locking regimes were measured with an IR camera (WinCamD, DataRay) placed at 1.4 m from the OC, see Fig. 5. In the CW regime, the beam profile exhibited strong interference fringes due to reflections from the front and rear surfaces of the camera neutral density filter. The measured beam diameter was 2.71 mm × 3.01 mm, as shown in Fig. 5(a). When soliton mode-locking was initiated by the SWCNT-SA, the interference fringes disappeared, see Fig. 5(b). The almost unchanged beam diameter of 2.81 mm × 3.12 mm indicates that the dominant mode-locking mechanism is soliton pulse shaping stabilized by the SWCNT-SA. The ML laser generated a nearly diffraction limited beam, as indicated by a beam quality factor (M2) < 1.05.
Fig. 5. Measured far-field beam profiles of the Tm:CLTGG laser (TOC = 1.5%): (a) CW regime; (b) soliton mode-locking regime.
The pulse duration was shortened by using lower output coupling at the expense of the average output power. An optimum performance for a 0.5% OC was achieved with all DMs having GDD = -125 fs2, i.e., with a total round-trip GDD of -875 fs2. The fitting of the measured spectrum and the autocorrelation trace yielded a pulse duration of 81 fs and a spectral bandwidth (FWHM) of 53.8 nm corresponding to a central wavelength of 2002.2 nm and an average power of 48 mW. The resulting TBP was 0.326, see Figs. 6(a) and 6(b).
Fig. 6. (a) Optical spectrum and (b) SHG-based intensity autocorrelation trace of the soliton ML Tm:CLTGG laser for TOC = 0.5%.
The shortest pulse duration was obtained by using an output coupling of 0.2% OC. It was easy to achieve and stabilize mode-locking due to the cavity design ensuring strong bleaching of the SA. The corresponding optical spectrum of the Tm:CLTGG laser is shown in Fig. 7(a). As expected for a quasi-three-level laser, the central wavelength experienced a further red-shift to 2010.4 nm. The spectral FWHM increased to 67 nm. Pulses as short as 69 fs were obtained after external linear chirp compensation with a 3-mm thick ZnS ceramic plate, see Fig. 7(b), for unchanged intracavity GDD management, corresponding to a TBP of 0.342. The average output power amounted to 28 mW at 87.7 MHz for an absorbed power of 1.7 W. Stable ML operation was maintained for hours without degradation of the output power and Q-switching instabilities.
Fig. 7. (a) Optical spectrum and (b) SHG-based intensity autocorrelation trace of the soliton ML Tm:CLTGG laser for TOC = 0.2%.
The steady-state pulse train of the soliton ML Tm:CLTGG laser was characterized by a radio-frequency spectrum analyzer. A sharp peak of the fundamental beat note centered at ∼87.7 MHz exhibited high signal-to-noise ratio above 70 dBc, as shown in Fig. 8(a). The latter together with the uniform harmonic beat notes seen on 1-GHz span in Fig. 8(b) indicates very stable soliton ML operation without any spurious modulation from Q-switching or multiple-pulse instabilities.
Fig. 8. Radio-frequency (RF) spectra of the soliton ML Tm:CLTGG laser: (a) fundamental beat note at ∼87.7 MHz measured with a resolution bandwidth (RBW) of 270 Hz, and (b) harmonics in a 1 GHz span, measured with a RBW of 150 kHz.
4. Conclusion
In conclusion, we present a detailed investigation of a Tm:CLTGG laser in the CW regime and, for the first time, to the best of our knowledge, in the soliton mode-locked regime accessing the sub-100 fs time domain. A transmission-type SWCNT-SA was utilized for initiating and stabilizing the ML operation. Nearly transform limited soliton-like pulses as short as 69 fs were achieved at 2010.4 nm with an average output power of 28 mW and a pulse repetition rate of ∼87.7 MHz. In terms of pulse duration, these results are better than the previously reported ones for ML lasers based on Tm:CNGG-type disordered garnets. As compared to these crystals, the main advantage of Tm:CLTGG is its higher thermal conductivity with weak dependence on the Tm doping, together with similar or even slightly superior inhomogeneous spectral broadening of the emission band around 2 μm. The broadband absorption and emission properties of Tm3+ in the disordered CLTGG crystal, the good thermal properties of this material, as well as the results of the present work, i.e., the broad tunability range and the generation of sub-100-fs pulses, indicate the potential of Tm:CLTGG for the design of ultrafast (few optical cycle) power-scalable oscillators emitting at ∼2 μm, e.g., pumped by commercial high-power diode lasers at ∼800 nm.
Funding
National Natural Science Foundation of China (61975208, 51761135115, 61850410533, 62075090, 52032009, 52072351); Sino-German Scientist Cooperation and Exchanges Mobility Program (M-0040); Foundation of President of China Academy of Engineering Physics (YZJJLX2018005); Key Laboratory of Optoelectronic Materials Chemistry and Physics, Chinese Academy of Sciences (2008DP173016); National Research Foundation of Korea (NRF-2020R1A4A2002828); Foundation of State Key Laboratory of Crystal Materials, Shandong University (KF2001).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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