Femtosecond lasers in the 2 μm spectral range have important applications in various fields, such as molecular ultrafast dynamics research, high-precision organic material processing, and free-space optical communication. They can also be utilized as a pump source for optical parametric oscillators to achieve mid-IR laser generation through frequency down-conversion, which is a current frontier hotspot in laser technology. Since the commercialization of semiconductor preparation processes and the rapid development of new low-dimensional materials, passively mode-locked solid-state lasers gradually became one of the main means to obtain femtosecond pulses at 2 μm from 1980. Currently, the main host materials that can produce sub-100 fs pulse output based on Tm³⁺- or Tm³⁺, Ho³⁺ co-doped laser materials are sesquioxides (Re₂O₃, where Re is Lu, Y, Sc, or their mixture), aluminates (CaReAlO₄, where Re is Y, Gd, or mixture), disordered CNGG-type garnets, and tungstate materials. The laser emission wavelength should be above 2 μm to avoid the structured water vapor air absorption and obtain ultrashort femtosecond pulses in the 2 μm spectral range, which can be realized by Tm/Ho co-doping or the strong lattice field effect of the host materials. Meanwhile, the spectral broadening effect of the host material on the active ions is also a key factor to achieve femtosecond pulses in this region. Therefore, it is necessary to explore new materials with flat and broadband gain spectra. In this study, we investigate the passively mode-locked performance of the new Tm∶GdScO₃ crystal with orthorhombic perovskite structure and a flat broadband (>450 nm) gain spectrum to demonstrate that it is an ideal laser material for achieving few-optical-cycle pulses in the 2 μm spectral range.

A standard astigmatically compensated X-shaped cavity is employed for the experiments (Fig. 1). To reduce the quantum loss and improve the laser slope efficiency, we adopt an Er-doped Raman fiber laser at 1700 nm for in-pumping and match well with the second absorption peak of the Tm∶GdScO₃ crystal. The maximum pump power is 5.2 W and the beam quality factor M² is 1.05. The pump light is focused on the crystal through a lens with a focal length of 75 mm and a beam radius of 22 μm. The gain medium is a Tm-doped GdScO₃ crystal with atomic fraction of 3% and dimensions of 3 mm×3 mm×6 mm. It is cut along the b-axis at Brewster's angle to enforce the laser polarization along the b-axis (E//a). To mitigate the thermal load in the crystal during the laser operation, we wrap the laser crystal with indium platinum and place it in a water-cooled fixture with a working temperature of 13 ℃. A semiconductor saturable absorber mirror (SESAM) is utilized as a saturable absorber (SA) to initiate and stabilize the mode-locking (ML). Chirped mirrors (CM1 and CM2) are introduced in the other arm of the cavity to compensate for the intracavity dispersion. The total physical cavity length is about 1.9 m. The laser beam radii in the sagittal and tangential planes on the crystal are 29 μm and 56 μm, respectively.

Initially, a 1% output coupler (OC) is employed for laser operation. At the maximum absorbed pump power, the laser delivers 0.74 W power in the continuous wave (CW) regime. With an optimized configuration for dispersion compensation of two beam bounces on CM1 and CM2, the physical cavity length amounts to 1.9 m, leading to a pulse repetition rate of 71.6 MHz. The mode-locked laser is self-starting and stable for hours. At an absorbed pump power of 3.14 W, it delivers an average output power of 89 mW, corresponding to a pulse energy of 1.24 nJ. The measured optical spectrum has a peak wavelength of 2052 nm and a full width at half maximum (FWHM) of 78 nm. The corresponding interferometric autocorrelation trace is shown in Fig. 2(b). The nearly perfect fits of the envelopes assuming a sech²-pulse profile and the expected 8∶1 peak-to-background ratio indicate chirp-free pulses. The deconvolved pulse duration (FWHM intensity) amounts to 63 fs. Single-pulse ML without any temporal satellites is confirmed by the measured intensity autocorrelation trace on a 15 ps-long time scale (Fig. 2).

With the same configuration, ML of the same Tm∶GdScO₃ crystal is thereafter investigated by the 0.5% OC. We obtain an average output power of 38 mW at an absorbed pump power of 3.26 W, with a single pulse energy of 0.53 nJ and a peak power of 7.7 kW. At this time, the central wavelength is located at 2034 nm with an FWHM of 80 nm. The hyperbolic secant curve fit shows a good sech²-type profile. Self-correlation measurements are performed to characterize the time-domain information of the mode-locked pulse, which is fitted with a sech² function. Meanwhile, a pulse width of 60 fs is obtained, with a corresponding TBP of 0.35, again close to the Fourier transition limit. Compared to the case of a 1% transmittance, the smaller laser intensity fails to excite the strong nonlinear effects of the medium, which may be the reason why the pulse width does not significantly shorten after reducing the transmittance (Fig. 3).

To characterize the stability of the mode-locked Tm∶GdScO₃ laser, we record radio frequency (RF) spectra of the shortest pulses on different span ranges. The fundamental beat note at 71.6 MHz exhibits an extinction ratio of more than 65 dBc above the noise level. The high contrast and near constant harmonic beat notes on a 1 GHz span range are evidence of stable ML operation. Furthermore, no Q switching behavior and multi-pulse instabilities are observed in the recorded uniform real-time pulse trains on different time scales (Fig. 4).

In summary, we report on a SESAM mode-locked Tm∶GdScO₃ crystal laser in-band pumped by a Raman fiber laser at 1700 nm. The flat broadband gain spectrum of the Tm∶GdScO₃ crystal is well utilized in the mode-locked laser operation, and an average output power of 38 mW is achieved for transform-limited 60 fs pulses at a repetition rate of 71.6 MHz, corresponding to a spectral bandwidth of 80 nm. Our results demonstrate that Tm∶GdScO₃ crystal is a promising candidate for generating few-optical-cycle pulses in the 2 μm spectral range. Thus, it has potential applications in scientific research such as molecular ultrafast dynamics and high-resolution molecular spectroscopy.