The 2-μm spectral region is critical for applications in precision machining, biomedical surgery (tissue absorption peaks), atmospheric sensing (molecular vibrational bands), and as a driver for high-harmonic generation (HHG) toward soft X-rays. Ultrashort pulse generation (<50 fs) in this band remains challenging for garnet-family crystals due to thermal effects from conventional ~0.8-μm pumping and limited gain bandwidth. Prior garnet-based lasers achieved only >50-fs pulses. Tm,Ho∶CNGG—a disordered calcium niobium gallium garnet—exhibits broad, homogeneous-free emission spectra owing to structural cation disorder (Nb⁵⁺/Ga³⁺ site randomness) and co-doping-induced inhomogeneous broadening, theoretically enabling few-cycle pulses. This work pioneers resonant in-band pumping at 1.7 μm to minimize quantum defects and thermal load while systematically exploring continuous-wave (CW), tunable, and mode-locking capabilities of Tm,Ho∶CNGG laser.

The experimental setup employed an X-folded cavity with astigmatism compensation (Fig. 2) pumped by a 1.7-μm Raman fiber laser focused to a 22-μm radius spot on a water-cooled (13 ℃) 3 mm×3 mm×6 mm Tm,Ho∶CNGG crystal. For CW and tuning studies, a 1.65-m-long cavity utilized two dichroic mirrors (R_OC=-100 mm), a plane mirror (M3), and output couplers. Mode-locking was achieved using a semiconductor saturable absorber mirror (SESAM: 97% reflectivity, 1.5%?2% modulation depth) combined with Kerr-lens mode-locking; the SESAM replaced M3 with a second focus (beam radius ～65 μm), while two chirped mirrors (total GDD ～-125 fs2) compensated dispersion in the extended ～2 m cavity. Pulse characterization involved power meters, optical spectrum analyzers, autocorrelators, SHG-FROG, and stability diagnostics (oscilloscope, RF spectrum analyzer), with extra-cavity dispersion managed using a ZnSe plate (+730 fs2) and fused silica (-240 fs2).

Stable mode-locked operation at T_OC=1.5% yielded 42-fs pulses (6 optical cycles) at 2091 nm with a 113-nm FWHM bandwidth (Fig. 5)—the shortest pulse duration from any garnet crystal. SHG-FROG verified the pulse characteristics (<0.008 retrieval error, Fig. 8), confirming sech2 pulse profiles and linear spectral phase. The average output power was 105 mW (1.37-nJ pulse energy) at 76.6 MHz. Stability tests showed RMS power fluctuation <1.4% over 30 min [Fig. 9(a)], clean pulse trains [Fig. 9(b)], and RF spectra with >70-dB SNR [Fig. 9(c),(d)].

This study employs disordered Tm,Ho∶CNGG garnet crystal as the gain medium to investigate its continuous tuning and mode-locked laser performance. In continuous-wave operation, a tuning range from 1865 nm to 2132 nm was achieved, with a tuning bandwidth of 267 nm. In mode-locked operation, by balancing the group delay dispersion in the cavity, a 42-fs laser (corresponding to 6 optical cycles) was obtained at a central wavelength of 2091 nm with an output-coupler transmission of 1.5%, featuring a FWHM spectral bandwidth of 113 nm. This represents the shortest pulse width achieved based on garnet-series crystals, demonstrating the importance of the spectral inhomogeneous broadening induced by the structural disorder and co-doping mechanism of Tm,Ho∶CNGG crystals for broadband mode-locked spectrum generation, as well as the advantages of resonant pumping technology in few-cycle pulse generation. To achieve even shorter mode-locked pulses, Kerr media with high nonlinear refractive indices could be inserted into the cavity in the future to enhance the self-phase modulation effect, enabling nonlinear spectral broadening. Combined with dispersion management, this approach could further facilitate the generation of few-optical-cycle pulses.