Dual-wavelength ultrafast lasers are gaining more and more attention, owing to their exceptional properties of synchronous emission of different-color beating signals with ultrahigh beating frequency up to terahertz (THz) caused by optical beating. Such lasers are highly required for applications needing very fast acquisition time such as fluid dynamic measurements^[1]. Additionally, pulsed lasers with dual-wavelength output also play a vital role in generating THz waves, which have become a hot topic due to their unique applications including interstellar and intragalactic observation, T-ray imaging, and broad-band secure communications^[2,3]. Employing Ti3+-doped sapphire and Nd3+-doped crystals as gain media, dual-wavelength synchronously mode-locked lasers operating at ∼0.8μm and ∼1μm have been widely demonstrated. Beating pulses with several hundred femtosecond pulse duration and THz-level ultrahigh beating frequency have been experimentally generated^[4–12]. Compared with the near-infrared laser emissions, ∼2μm lasers are more striking due to their characteristics of eye safety and special wavelength, which makes it gain numerous free space applications including coherent laser radar, gas sensing, medical surgery, and pumping of optical parametric oscillators (OPOs)^[13,14]. However, very few reports on dual-wavelength ultrafast pulsed lasers at 2μm have been found. In 2015 and 2017, ultrafast beating pulses with THz-level beating frequency were delivered from dual-wavelength mode-locked Tm:CaYAlO4 (Tm:CYA) and Tm,Y:CaF2 lasers, respectively^[15,16]. Tm:CYA and Tm,Y:CaF2 were disordered crystals, in which the fluorescence spectra were inhomogeneously broadened and therefore beneficial for synchronous dual-wavelength operation. However, the disordered structure has detrimental impact on the thermal property of the crystals, as described in Ref. [17]; thus, mass disorder would increase the thermal resistivity. For example, in our previous work based on Tm,Y:CaF2, a low damage threshold of several kilowatts per centimeter (kW/cm²) was observed^[18], which meant the broad emission spectra were at the cost of decreasing the thermal conductivity. So much more efforts should still be paid on exploring novel dual-wavelength ultrafast 2μm laser systems, especially based on the crystals with excellent mechanical and thermal properties.

A Tm-doped YAlO3 (Tm:YAP) crystal, a widely investigated gain material, has many favorable features including structural anisotropy, long Y-O bonds, and natural birefringence, which makes it free of the harmful thermally induced depolarization phenomenon^[19–21]. Compared with the Tm:Y3Al5O12 (Tm:YAG) crystal, the Tm:YAP crystal not only has similar mechanical properties, but twice the emission cross section^[21]. In recent years, continuous wave (CW), Q-switching, and single-wavelength mode-locking (ML) lasing operations of the Tm:YAP crystals have been experimentally demonstrated^[21–27]. As can be seen from the fluorescence spectrum of the Tm:YAP crystal in Ref. [28], there are multiple emission peaks located in the 2μm spectral region originating from its anisotropic structure, which benefits the generation of dual- or multi-wavelength emissions. In 2016, dual-wavelength operation was experimentally realized in a black phosphorus passively Q-switched Tm:YAP laser^[24], well verifying the ability of the Tm:YAP crystal in generating dual-wavelength laser radiations. Considering the advantages of the Tm:YAP crystal compared with the disordered crystals mentioned above, a synchronous dual-wavelength ultrafast laser using the Tm:YAP crystal is also expected. However, there are no reports on the dual-wavelength ultrafast laser operations based on the Tm:YAP crystal, to the best of our knowledge.

In this Letter, a diode-pumped dual-wavelength synchronously mode-locked Tm:YAP laser at 1985.6 and 1989 nm was experimentally demonstrated. The autocorrelation trace was recorded, and the beating signals were observed, which were found to have an ultrahigh beating frequency of 0.26 THz and full width at half-maximum (FWHM) duration of 2 ps. The results indicated a potential way to obtain efficient THz radiation by further using a suitable difference frequency generation (DFG) method.

The schematic setup for the diode-pumped mode-locked Tm:YAP laser was depicted in Fig. 1. A laser diode emitting at 793 nm was employed as a pump source and fiber coupled with a core diameter of 200μm and a numerical aperture of 0.22. Through a 1:1 imaging module, the pump light was focused into the Tm:YAP crystal. A 1 at. %-doped b-cut Tm:YAP crystal was enclosed with indium foil and mounted in a water-cooled copper heat sink with constant temperature of 8°C to efficiently remove the accumulated heat inside the crystal induced by the quantum defect and imperfect absorbance of pump light. M1 was an input flat mirror anti-reflection (AR) coated at the pump wavelength and high-reflection (HR) coated at the laser wavelength. The crystal was placed as close as possible to M1, while maintaining good mode-matching condition between the pump light and laser beam. This intentional configuration design was to enhance the spatial hole burning effect inside the Tm:YAP crystal, which would be helpful for forming the population grating, thus inhomogeneously broadening the gain bandwidth^[29] and generating short laser pulses. M2 and M5 were concave mirrors with curvature radii of 500 and 200 mm, respectively. M3 and M4 were flat Gires–Tournois interferometer (GTI) mirrors with group delay dispersion (GDD) of −300fs2 at ∼2μm for dispersion compensation. A flat mirror with low transmission rate of 1% at ∼2μm was employed as the output coupler. Due to the dual-channel setup, the total output coupling efficiency was 2%. The semiconductor saturable absorber mirror (SESAM) was placed at the end of the cavity to take advantage of the second beam waist formed by mirror M5. The laser spot radii on the Tm:YAP crystal and SESAM were calculated to be 88μm/85μm and 33μm/33μm in tangential and sagittal planes, respectively. The SESAM (Batop Inc, Germany) had a modulation depth of 1%, a recovery time of 10 ps, a saturation fluence of 80μJ/cm2, and a non-saturable loss of 1.2%.

Firstly, with an HR flat mirror substituting SESAM, the CW characteristics of the Tm:YAP laser were investigated. The threshold absorbed pump power was measured to be 1.1 W. The average output powers increased almost linearly with the absorbed pump powers, as shown in Fig. 2, corresponding to a slope efficiency of 21.7%. However, at high pump level (beyond 4.5 W of absorbed pump power), the average output powers somehow showed saturation tendency. In comparison with that, no obvious saturation tendency was observed in the later CWML operation. Considering the same pump level for CW and CWML operations, the saturation tendency only happening in the CW regime was not mainly attributed to the excess heat generated inside the Tm:YAP crystal, but the gain saturation effect induced by the faster decrease of the ground state population intensity in the CW regime than that in the CWML case, which was not only due to the low Tm3+ ions doping concentration of 1 at. % but the low intracavity loss of CW operation under high pump levels. Substituting the HR end mirror with SESAM, the threshold absorbed pump power increased to 2.66 W, and the laser ran into the Q-switched ML (QML) regime directly. Under the absorbed pump power of 5 W, a total maximum output power of 166 mW from the two output directions was obtained. To prevent the SESAM from being damaged, the pump power was not enhanced further, which subsequently limited the maximum output power.

With a fast InGaAs photodetector (EOT, ET-5000, USA) and digital oscilloscope (1 GHz bandwidth, Tektronix DPO 7102, USA), temporal pulse trains from the mode-locked Tm:YAP laser were monitored and shown in Fig. 3. The pulse stability can be confirmed by the smooth profile in scale of 500μs/div and the negligible time jitter of the discrete laser pulses in the scale of 20 ns/div.

Figure 4 shows the first beat note of the radio frequency (RF) spectrum at 85 MHz with a resolution bandwidth of 50 kHz. The single beat note with an extinction ratio of 52 dBc confirmed the clear CWML operation without Q-switching instability. In addition, no spurious modulation was found in the 1 GHz wide-span RF measurement, which indicated the single-pulse operation of the mode-locked laser, as shown in the inset of Fig. 4. However, the fast decline of the beat note amplitudes was observed, to which we attributed two aspects: one is the loss of the employed RG-58-type coaxial cable for connecting the photodetector and RF analyzer, since the larger loss for higher frequencies in such types of cables would reduce the bandwidth, and the other one may come from the saturation effect in the photodetector, which would cause the bandwidth reduction of the photodetector. The single-pulse operation that could be confirmed either by the oscillagram (Fig. 3) or RF spectrum (Fig. 4) strongly evidenced the temporal synchronization of the two sets of mode-locked pulse trains with different colors.

Using a laser spectrometer (APE WaveScan, APE Inc.) with a resolution of 0.4 nm and a second-harmonic generation (SHG)-based autocorrelator (APE GmbH, Pulse Check 150), the output spectrum and autocorrelation trace have been measured and shown in Fig. 5. By assuming a sech2 pulse shape, the pulse duration was measured to be about 16.8 ps. Moreover, the optical beating pattern of the two-color laser pulses could be clearly observed from the autocorrelation trace. The interval period T between the beating signals was found to be 3.8 ps, which is consistent with the theoretical value of 3.87 ps determined by Δν×T=1, considering the frequency difference Δν of 0.258 THz between the two output spectral peaks of 1985.6 and 1989 nm. In addition, the FWHM duration of the beating signal was found to be 2 ps. The GTI mirrors would be crucial for the realization of synchronous dual-wavelength ML, which requires the balances of dispersion and nonlinearity, gain and loss, as well as saturable absorption of the SESAM^[30].

In conclusion, a dual-wavelength synchronously mode-locked Tm:YAP laser operating at 1985.6 and 1989 nm has been experimentally demonstrated with a maximum output power of 166 mW and a pulse repetition rate of 85 MHz. The pulse duration was measured to be 16.8 ps by assuming a sech2 pulse shape. Frequency beating was observed from the autocorrelation trace, and the beating signals showed an interval of 3.8 ps and an FWHM duration of 2 ps, corresponding to an ultrahigh beating frequency of about 0.26 THz, which agrees well with the frequency difference of the emitted two spectral peaks. To the best of our knowledge, this is the first experimental demonstration of a dual-wavelength mode-locked 2μm laser based on a kind of homogeneously broadened gain medium, which has paved a novel potential way of generating THz radiation by a suitable DFG method.