1 Introduction

Ultrafast lasers near 2 μm exhibit a rapidly growing trend over the last decades due to numerous potential applications, such as free space optical communication [1], laser surgery [2], as a seed source for chirped pulse amplifier (CPA) [3], and synchronous pumping of optical parametric oscillators (SPOPOs) to generate mid-infrared (mid-IR) lasers [4] or mid-IR frequency combs in the so-called “molecular fingerprint” spectroscopy region [5]. Rare earth Thulium (Tm3+) and Holmium (Ho3+) are two most used ions emitting in the 2 μm spectral region. In comparison, Tm3+ ion exhibits a broader emission band in the transition of 3H6 → 3F4 and its highest absorption peaks around 790 nm is well matched with the commercially available AlGaAs diode laser [6]. To date, various host materials including cubic sesquioxides (RE2O3 with RE = Sc, Lu or Y) [7], disordered CNGG-type garnets [8], monoclinic tungstates [9], and orthorhombic perovskite gadolinium scandate [10], doped with Tm3+ ions, have been exploited for femtosecond lasers to generate sub-100 fs pulses in the 2-μm spectral range. In combination with the in-band pump scheme at ~ 1.7 μm, average output power of the femtosecond pulses up to watts level have also been demonstrated since the reduced quantum defect [11, 12]. In additional to the broad gain spectra, a critical reason for choosing these hosts is their strong crystal field to shift the emission peak above 2 μm to avoid the structured water vapor absorption.

Tetragonal ABCO4-type oxide crystals (K2NiF4 type structure [13]), where A represents Ca or Sr, B represents Y or Gd, and C represents Al or Ga, with a disordered structure, are also promising candidates doped with rare-earth ions for ultrashort pulses generation. In fact, the shortest pulses from the mode-locked Yb-lasers in the 1-μm spectral range are obtained by employing CaGdAlO4 [14] and CaYAlO4 [15], respectively, both benefited from the relatively broad and flat gain spectra and the high thermal conductivity of hosts. The formation of broad emission spectra of rare-earth ions in such CaREAlO4 crystal is originated from the second coordination sphere of the rear-earth cations (RE3+ = Tm3+, Ho3+ and Yb3+) caused by the charge difference of different cations and the difference of cation-cation distances [16]. However, because the emission peak wavelength of Tm ions in ABCO4 crystals is slightly below 2 μm, the stability of the mode-locked laser is seriously affected by the water vapor absorption in the air, resulting in relatively longer pulse duration in the picosecond regime [17,18,19,20]. To avoid the structured water vapor absorption and thus achieving the stable mode-locking, two schemes have been used: (1) Co-doping with Ho3+-ion can shift the emission peak to a longer wavelength at ~ 2.1 μm, thus shortening the pulse duration to sub-100 fs with a Tm,Ho:CaYAlO4 crystal [21]. Since then even shorter pulses have been demonstrated by employing Tm,Ho:CALGO [22] and Tm,Ho:Ca(Gd, Lu)AlO4 [23] with co-emission of the Tm3+ and Ho3+ ions. (2) The alternative is the usage of a special output coupler which meanwhile plays a role of a cut-off filter with high transmission below 2 μm, thus forcing the oscillation wavelength of Tm:CaGdAlO4 laser beyond 2 μm to achieve stable mode locking with few hundreds femtosecond pulse duration [24].

Here, we used a simple, low-cost, home-made birefringent filter to controllably select the lasing wavelength for avoiding the influence of water vapor absorption, and thus realizing the stable mode-locking of such Tm-doped orthoaluminate crystal (Tm:CaYLuAlO4), delivering 288 fs pulses at 2037 nm. Moreover, wavelength tunability of the mode-locked femtosecond laser is demonstrated with such birefringent filter.

2 Experimental setup and CW laser operation

Figure 1 shows the schematic setup of the continuous-wave (CW) and passively mode-locked Tm (4 at.%):CaYLuAlO4 (Tm:CALYLO) laser. An uncoated Tm:CALYLO crystal with dimensions of 3 × 3 × 5 mm3 was Brewster's angle cut to enforce the linear laser polarization along its crystallographic c-axis (E//c). A 1700 nm Raman fiber laser with a M2 factor of ~ 1.05 was employed as the pump source. The collimated pump beam was focused into the crystal sample with a waist radius of 22 μm by using an aspheric lens with a focal length of f = 75 mm. The sample, tightly wrapped in indium foil, was mounted in a water-cooled copper holder with a temperature setting at 12 ℃ to mitigate the accumulated heat. We firstly investigated the performance of CW and wavelength tunable lasers utilizing a standard astigmaticlly compensated X-cavity consisted of two plane-concave mirrors (M1 and M2, curvature of radius of ROC =  − 100 mm), a flat rear reflector M3, and a plane-wedged output coupler with transmission of 0.5%, 1%, 1.5%, 5% or 10%. Using the ABCD formalism, the laser beam radius on the crystal sample was calculated to be 29 μm × 60 μm in the sagittal and the tangential planes, respectively. For the mode-locking operation, the flat rear mirror M3 was substituted by a plane-concave mirror M4 (ROC =  − 100 mm) which was used to form a second beam waist on the saturable absorber (SA) with a beam radius of 86 μm. The SA was a GaSb-based SESAM contained two InGaAsSb quantum wells (8.5-nm thickness) and a 50-nm cap layer with a high linear reflectivity of ∼ 97% at 2080 nm for starting and stabilizing the mode-locking [25]. To compensate the intracavity dispersion, two plane parallel chirped mirrors, CM1 and CM2, providing group delay dispersion (GDD) of − 125 fs2 and − 1000 fs2 per bounce, respectively, were employed. To realize the wavelength-tunable laser operation and stable mode locking in the femtosecond regime, a home-made birefringent filter was used as wavelength selector. The birefringent filter was an uncoated quartz plate with a thickness of 3 mm and a diameter of 30 mm, the angler between its crystal axis and the surface normal was 24 degree. For the wavelength tuning both in the CW and mode locking regime, the plate was inserted in the cavity at a Brewster’s angle.

Fig.1
figure 1

Schematic of the CW and SESAM mode-locked Tm:CALYLO laser in-band pumped by a Raman fiber laser at 1700 nm. M1-M4 cavity mirrors, CM1-CM2 chirped mirrors, OC output couplers, BF birefringent filter. Inset Tm:CALYLO crystal sample and the used birefringent filter

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Initially, we studied the CW and wavelength-tunable performances of the Tm:CALYLO laser with different OCs. The single-pass absorption measured under lasing conditions was 90% at the highest incident pump power of 4.02 W. As shown in Fig. 2a, the maximum CW output power was 2.01 W with TOC = 10%, corresponding to a slope efficiency of 66.5% with respect to the absorbed pump power. Figure 2b shows the measured optical spectra, an obvious red-shift from 1950.7 to 1994.4 nm was observed with decreasing the OC transmission, which was attributed to the enhanced reabsorption effect in such quasi-three-level Tm-system [26]. The total round-trip passive cavity losses (δ) and the intrinsic slope efficiency (η0) were thereafter estimated by using the Caird analysis [27]. As demonstrated in Fig. 2c, the measured slope efficiencies as a function of the – ln(ROC) yielded a δ = 0.64% and η0 = 70%. By inserting the 3-mm thick birefringent filter in the cavity, CW tuning performance of the Tm:CALYLO laser under an absorbed pump power of 3.34 W was investigated and shown in Fig. 2d. A broad tuning range of 315.9 nm was achieved from 1817.1 to 2133 nm, which is much broader than the previously reported Tm:CALGO laser (254 nm [24]). The broad and smooth tuning range is an indication of that the Tm:CALYLO crystal is a promising candidate for supporting ultrashort pulse generation.

Fig. 2
figure 2

CW and wavelength tunable performances of the Tm:CALYLO laser. a Output power with respect to the absorbed pump power (a) and the optical spectra b at different OCs; c Caird plot of the laser slope efficiency versus OC reflectivity, i.e., –ln(ROC); and d the CW tuning curve at 3.34 W absorbed pump power with TOC = 1%. η slope efficiency

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3 Mode locking of the Tm:CALYLO laser

Mode locking of the Tm:CALYLO laser was realized by replacing the flat rear mirror M3 with a plane-concave mirror M4 and the SESAM. Without the birefringent filter, the mode locking was very unstable and exhibited a multiple-peak structure on the optical spectrum. As shown in Fig. 3, this multiple-peak structure is exactly located in the water vapor absorption region. We have tested different OCs and carefully optimized the dispersion with different reflective bounces on the chirped mirrors, however, stable mode locking was still not realized and the intensity autocorrelation trace in the time domain was incomplete. So, the birefringent filter was thereafter employed to select the lasing wavelength to avoid the water vapor absorption.

Fig. 3
figure 3

Optical spectrum of the SESAM mode-locked Tm:CALYLO laser without birefringent filter in the cavity (TOC = 1.5%) and the atmospheric transmittance at normal conditions for a path length of 1 m (HITRAN database, USA model, high latitude, summer, H = 0)

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After inserting the birefringent filter in the cavity, the optimized configuration for dispersion management was two beam bounces on the chirped mirrors (CM1 and CM2), thus giving a total physical cavity length of around 2 m. By modulating the emission wavelength above 2 μm, stable mode locking could be easily realized. The shortest pulse was obtained at a central wavelength of 2037 nm with TOC = 1.5%, as can be seen in Fig. 4a, the spectral profile was well-fitted with a sech2 function, giving a spectral FHWM of 18.1 nm. In this case, the average output power of 166 mW was achieved at a pulse repetition rate of ~ 77.6 MHz, yielding a pulse energy of ~ 2.1 nJ. Figure 4b shows the corresponding intensity autocorrelation traces, the pulse duration amounted to 288 fs by assuming a sech2 intensity profile, giving a time-bandwidth product (TBP) of 0.38. See the inset of Fig. 2b, the single pulse operation was confirmed by the autocorrelation trace recorded on a long time scale of 15 ps. As summarized in Fig. 4c, the present work represents, to the best of our knowledge, the shortest pulse ever reported with such single Tm3+-doped ABCO4-type crystals, indicating that the birefringent filter is an effect way to realize stable mode locking of such Tm-laser with the lasing wavelength in the water vapor absorption region [17,18,19,20, 24, 28].

Fig. 4
figure 4

a Optical spectrum and b intensity autocorrelation trace of the SESAM mode-locked Tm:CALYLO laser for TOC = 1.5%; c the summary of the mode-locked bulk lasers based on Tm-doped ABCO4-type crystals. Inset in b: the corresponding long-scale (± 7.5 ps) intensity autocorrelation trace

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The stability of the mode-locked Tm:CALYLO laser was characterized by employing a radio frequency (RF) spectra analyzer, a fast photodiode (> 10 GHz) and a digital oscilloscope. Figure 5a shows the steady-state pulse train recorded in the different frequency span ranges. The fundamental beat note at ~ 77.6 MHz shows a high extinction ratio of > 60 dBc above the noise level. The uniform harmonic beat notes on a 1-GHz span range with a resolution bandwidth (RBW) of 300 kHz, see the inset of Fig. 5a, indicates stable and clean mode-locked pulses without any undesired modulations. Furthermore, the real-time pulse trains were recorded on the timescale of 10 ns or 10 ms, as shown in the Fig. 5b, the uniform pulse train again confirmed the stable mode locking without multi-pulse instabilities or Q-switching envelope modulations.

Fig. 5
figure 5

a Radio frequency spectra of the SESAM mode-locked Tm:CALYLO laser in 200-kHz and 1-GHz span ranges, b the corresponding typical pulse train on a nanosecond (10 ns) and millisecond timescale (10 ms). RBW: resolution bandwidth

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Finally, wavelength tunability of the mode-locked femtosecond laser was studied by rotating the birefringent filter around its surface normal. As show in Fig. 6a, the wavelength tuning rang of nearly 50 nm from 2031.8 to 2081.1 nm was realized, with the spectral FWHM in the between 6.5 and 10 nm. The highest average output power of 390 mW was obtained at a central wavelength of 2031.8 nm with a spectral FHWM of 8.4 nm. Figures 6b shows the corresponding intensity autocorrelation traces, the pulse duration amounted to 586 fs, yielding a TBP of 0.37. The longest central wavelength of the mode-locked laser was at 2081.1 nm, corresponding to a spectral FHWM of 10 nm and a pulse duration of 520 fs [see Fig. 6c]. From the recorded long-scale autocorrelation traces, see the insets of Fig. 6b and c, both cases exhibited steady-state mode-locking with single pulse operation.

Fig. 6
figure 6

a Wavelength tunability of the SESAM mode-locked Tm:CALYLO laser with TOC = 1.5%; b and c intensity autocorrelation traces of the femtosecond pulses at 2031.8 nm and 2081.1 nm, respectively. Insets in b and c: the corresponding long-scale (± 7.5 ps) intensity autocorrelation traces

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4 Conclusions

Summarizing, we have studied the performances of CW and mode-locked Tm:CALYLO laser in-band pumped by using a Raman fiber laser at 1700 nm. In the CW regime, a broad wavelength tuning range of 315.9 nm was achieved by using a birefringent filter, indicating its potential for generation of ultrashort pulses. However, stable mode locking of the Tm:CALYLO laser was not realized in the femtosecond regime because the laser emission wavelength was located in the water vapor absorption region. Therefore, a home-made 3-mm-thick birefringent filter was employed to select the emission wavelength above 2 μm, and thus forming a stable mode locking with a shortest pulse duration of 288 fs at 2037 nm. Moreover, wavelength tunable femtosecond laser from 2031.8 to 2081.1 nm was also realized simply by rotating the birefringent filter around its surface normal. The results indicated that, with a birefringent filter, stable mode-locked femtosecond lasers can be achieved with such Tm-bulk ABCO4-type oxide crystals which exhibit an emission peak in the water vapor absorption region.