1. Introduction

The widespread applications of ultrashort pulses in science and technology have promoted rapid development of ultrafast laser sources in the last decades. In the laser material research field, a lot of work has been done to explore new solid-state laser media for the generation of ultrashort sub-100 fs or even sub-50 fs pulses [1–20]. Among them, disordered crystals have attracted more and more attention due to their good spectroscopic and thermal properties [1, 3, 7,9, 10, 12–20]. Recent experimental research with disorder crystals has even demonstrated multi-watt high power sub-100 fs pulses, showing that disordered crystals could be excellent ultrafast laser gain hosts [17–20]. In disordered crystals, the cation ions could randomly distribute in the lattice sites, resulting in considerable inhomogeneous broadening and/or splitting of the spectrum of the active ions. Thus, disordered crystal based laser gain media generally could provide broader and flatter emission spectra than those of the constituting single crystal based gain media, which benefits the ultrashort pulse generation. Meanwhile, compared to the laser glasses that have even broader spectral bandwidth, disordered crystal gain media exhibit higher thermal conductivity. To date, various Yb³⁺-doped disordered crystals have been developed in the 1µm wavelength region. And outstanding results with pulse duration below 50 fs have been successfully obtained in Yb³⁺-doped disordered crystals such as the Yb:CaGdAlO₄ [3, 10, 13, 20], Yb:Ca₄YO(BO₃)₃ [7, 9], Yb:CaYAlO₄ [14, 21], and Yb:NaY(WO₄)₂ [15].

In recent years, Yb³⁺-doped calcium niobium gallium garnet (CNGG) type disordered crystals have been widely investigated, and ultrashort mode-locked pulses with pulse duration of 73 fs, 55 fs, 389 fs was obtained from a Yb:CNGG [22], Yb:CLNGG [23], and Yb:CTGG [24] disordered crystal laser, respectively. CNGG is an isotropic crystal, in which Ca²⁺ occupy dodecahedral sites, and Nb⁵⁺, Ga³⁺ have a random distribution in octahedral and tetrahedral sites, leading to inhomogeneous spectral broadening of Yb³⁺. Very recently, a novel Na⁺ modified Yb:CNGG (Yb,Na:CNGG) disordered garnet crystal was reported [25,26]. It has been found that Na⁺ incorporation into CNGG could greatly influence the crystal growth and optimize the crystal properties, such as decreasing the crystal growth temperature, promoting Yb³⁺ doping, and enlarging the spectral bandwidth of the Yb³⁺ ions. Therefore, it is expected that the new Yb,Na:CNGG disordered crystals could be a potential laser material for ultrashort pulse generation.

In this letter, we report on a diode-pumped passively mode-locked Yb,Na:CNGG soliton laser for the first time. The CW and wavelength tuning performances of the laser were also experimentally investigated. A CW emission slope efficiency of 68.5% and a wavelength tuning range of 45 nm was obtained. With a SESAM as the mode locker, soliton pulses as short as 62 fs and 45 fs were obtained without and with external pulse compression, respectively. The soliton pulses have a spectral bandwidth (FWHM) of 26.8 nm centered at the wavelength of 1061 nm. To the best of our knowledge, this is the first demonstration of sub-50fs pulse generation in Yb,Na:CNGG disordered crystal lasers, and it is also the shortest pulses ever obtained from the Yb-doped CNGG type disordered crystal lasers.

2. Experimental setup

Figure 1 shows a schematic of the mode-locked laser setup. The Yb,Na:CNGG disordered crystal used in the experiment was grown by the conventional Czochralski method with a Yb³⁺ concentration of 10 at.%. A 3-mm-long, 3 mm × 3 mm Brewster-angle cut sample was mounted on a copper block heat sink, and the two end faces of the sample were laser-grade polished to minimize the transmission losses for the p polarization. The copper block heat sink was maintained at a temperature of ~20.0 °C by circulating water, which removes the generated heat during the laser operation. The pump source was a two-section distributed Bragg-reflector tapered diode-laser (DBR-TDL) (Eagleyard Photonics GmbH) with a maximum output power of 6 W around 980 nm. The full cavity length of the DBR-TDL is 6 mm with an aperture angle of 6°. It has a good beam quality with the beam propagation factor of M²_1/e² ≤ 1.5. The pump light was focused to about 35 μm in diameter at the position of crystal after first collimated by a home-made laser beam collimating system (BCS) and then focused by a spherical lens (F1) with 100-mm focal length. The BCS was made of a micro cylindrical lens (F1) with a focal length of about 0.5 mm, an aspheric lens (F2) with a focal length of about 3.5 mm, a plano-concave lens with a focal length of −25 mm, and a plano-convex lens with a focal length of 100 mm. The laser had a standard X-folded cavity as shown in Fig. 1. The three folding concave mirrors M1, M2, and M3 had the same radius of curvature (ROC) of −100 mm, and were all dichroic coated that have high reflection for the laser wavelength and anti-reflection for the pump wavelength. The lengths of M1 to OC and M2 to M3 were about 68 cm and 59 cm, respectively. Based on the ABCD matrix, a laser beam waist radius in the range of 20-25 µm could be formed at the position of the Yb,Na:CNGG crystal. A SESAM (BATOP GmBH) designed to operate at a central wavelength of 1064 nm was used to initiate and stabilize the mode locking. The SESAM has a modulation depth of 0.6%, non-saturable loss of 0.4%, a saturation fluence of 70 µJ/cm², and a relaxation time of ~1.0 ps.

 

Fig. 1 The schematic of the mode-locked Yb,Na:CNGG soliton laser. BCS: Beam collimating system; F1: Focus lens; M1, M2, and M3: Folding mirrors; OC: Output coupler.

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3. Continuous-wave and wavelength tuning experiments

Initially we investigated the CW performance of the laser with three different output couplers (OCs) of 0.8%, 2.4%, and 5.0% output coupling, while no prisms were inserted in the cavity and the SESAM was replaced with a highly-reflective plane mirror. Figure 2(a) shows the relationships between the absorbed pump power and the output power of the lasers under the CW operation. A maximum output power of 301 mW, 611mW, and 734 mW were obtained under the coupler transmission of 0.8%, 2.4%, and 5.0%, respectively, and the corresponding slope efficiency was 25.8%, 54.0%, and 68.5%. The CW operation with the highest efficiency was achieved with the coupler of 5.0% output coupling. To explore the potential of ultrashort pulse generation of the Yb,Na:CNGG disordered crystal, we further studied the wavelength tuning performance of the laser by inserting a SF10 prism as a wavelength tuning element in the cavity in the arm of OC. The 0.8% OC was used in this experiment. The tuning characteristic of the laser is shown in Fig. 2(b). A wavelength tuning range of 45 nm, from 1029 nm to 1074 nm, was obtained, indicating that the Yb,Na:CNGG disordered crystal could be a promising gain material to generate ultrashort pulses.

 

Fig. 2 (a) Output power versus absorbed pump power with different OCs in CW operation regimes and (b) wavelength tuning results under CW operation with a 0.8% OC.

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4. Mode-locked soliton laser operation

The passive mode locking performance of the laser was then experimentally studied. To reduce the intracavity loss and achieve ultrashort pulses, the output coupler with a transmission of 0.8% was used. Furthermore, an SF10 prism pair was inserted in the cavity in the arm containing the OC to compensate the group-delay dispersion (GDD). The intracavity prism pair has a tip-to-tip separation of ~38 cm. Initially, the laser operated in the CW regime. As the absorbed pump power was increased to 1.16 W, self-started mode locking was then obtained. It is worth noting that different stable CW mode locking regimes where the mode locked pulses have different pulse widths and spectra could be obtained in our laser as we carefully adjusted the laser cavity alignment. After optimization, mode-locked pulses as short as 62 fs were obtained directly from the oscillator. Figure 3(a) shows the autocorrelation trace of the mode locked pulses measured with a commercial autocorrelator (APE, PulseCheck 50). The trace can be well fitted assuming a sech²-pulse shape. Figure 3(b) shows the corresponding mode-locked pulse spectrum in linear (blue solid line) and logarithmic (red solid line) coordinates. The spectra were centered at 1061 nm and had a bandwidth (FWHM) of 26.8 nm measured by an optical spectrum analyzer (ANDO, AQ6315B) with a resolution of 0.1 nm. The time–bandwidth product of the mode-locked pulses is about 0.443, which is much larger than the Fourier transform-limit of 0.315 for the sech²-shaped pulses, indicating there was large residual chirp in the mode-locked pulses. We tried to optimize the intracavity prism pair induced GDD to eliminate the chirp by adjusting the insertion length of the prisms. However, pulse splitting appeared as we decreased the total negative dispersion of the cavity close to the zero GDD.

 

Fig. 3 (a) Autocorrelation trace and (b) the corresponding optical spectra of the mode-locked pulses.

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To eliminate the large residual chirp of the output pulses, an additional external compressor made of a SF10 prism pair with a tip-to-tip distance of ~41 cm in a single-pass configuration, as shown in Fig. 1, was employed. To accurately control the amount of prisms induced intracavity GDD, each of the prisms was mounted on a precision translation stage. After passing through the prism pair, the mode-locked pulses were reflected into the autocorrelator by a plane high reflective mirror. The measured average output power of the mode-locked pulses before autocorrelator was about 30 mW with an absorbed pump power of 1.43W. By carefully optimizing the insertion length of the prisms, the shortest pulse obtained was 45 fs if a sech²-shape pulse profile was assumed, as shown in Fig. 4. With the same spectral bandwidth, the corresponding time-bandwidth product of the pulses was calculated as about 0.321, which is very close to that of the Fourier transform-limited pulses.

 

Fig. 4 Intensity autocorrelation trace of the mode-locked pulses after passing the external prism pair compressor.

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Figure 5 shows typical CW mode-locked pulse trains of the 45 fs pulses in the 10 nanosecond and 10 millisecond time scales. The mode-locked pulse train was measured with a high-speed detector (New Focus, 1611) and displayed on a 1 GHz bandwidth digital oscilloscope (Tektronix, DPO7104). We note that the mode-locked state was very stable in the experiment. No any sign of Q-switched mode locking was observed. The pulse repetition rate was about 104 MHz, corresponding to the laser cavity length of ~1.44 m. After the narrowest mode locked pulses were obtained, if the pump power was further increased, the mode-locked pulses would suddenly split into two broader pulses and consequently the spectral bandwidth of the mode-locked pulses became narrower, corresponding to the broader pulse duration. This phenomenon is exactly the same as the soliton pulse splitting observed in the passively mode-locked fiber lasers [27], which is known as a typical characteristic of the dissipative soliton operation of passively mode locked lasers whose dynamics is governed by the extended complex Ginzburg-Landau equation. Moreover, we note that a spectral sideband also appeared in the mode locked pulse spectrum at around 1120 nm, as can be clearly observed on the logarithmic spectrum shown in Fig. 3(b), which is another clear signature of the soliton operation of the mode-locked solid state lasers [28].

 

Fig. 5 Typical CW mode-locked pulse trains in nanosecond and millisecond time scales.

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5. Conclusion

In conclusion, we have experimentally studied the CW, wavelength tuning, and femtosecond mode-locking performances of a novel Yb,Na:CNGG disordered crystal laser. Slope efficiency as high as 68.5% and a wavelength tuning range of 45 nm was obtained on the CW operation of the laser. Stable CW mode-locking of the laser was achieved with a SESAM as the mode locker, and the mode-locked pulses were dissipative solitons. They had a pulse width as short as 62 fs directly from the oscillator and were centered at 1061 nm. The chirped soliton pulses were further compressed to 45 fs by an external compressor. To the best of our knowledge, this is the first demonstration of soliton operation of a Yb,Na:CNGG laser, and the shortest and first sub-50 fs pulses ever obtained from the Yb-doped CNGG type disordered crystals.