1. Introduction

Ultrafast lasers providing femtosecond or picosecond pulses have been widely applied in many fields, including industry, medicine and scientific research, owing to their simplicity, high stability and good beam quality [1–3 ]. In general, the output power and the efficiency of a laser system are ultimately determined by the properties of the gain medium. Therefore, great efforts have been made to improve the quality of laser materials and develop new high-quality laser crystals.

The Nd-doped crystals which have large absorption coefficient and stimulated emission cross section, relatively broad gain bandwidth and high Nd³⁺ doping level, have been proved to be good gain materials for compact diode-pumped ultrashort solid-state lasers [4, 5 ]. Among the crystal family, the most famous representative is Nd:YVO₄, which has been investigated in details and has become the most common medium with various applications [6, 7 ]. The impressive oscillators pumped by 888 nm based on Nd:YVO₄ were demonstrated by McDonagh et al [8, 9 ]. The TEM₀₀ output were achieved with continuous-wave (CW) power of 60 W and mode-locked laser of 56 W at 110 MHz with 33 ps pulse duration. Another analogous vanadate crystal, Nd:GdVO₄, has attracted great attention in the most recent period due to its well thermal properties. SESAM mode-locking of Nd:GdVO₄ crystal was first reported by Zhang et al in 2003 [10]. A CW output power of 46 W based on composite GdVO₄/Nd:GdVO₄ and double-end-pump configuration with the beam quality factor M² < 1.1 was obtained by Li et al [11].

Among these candidates, Nd:LuVO₄ has made dramatic progress in recent years [12–15 ]. Compared with the Nd:YVO₄ (𝜎_a = 6 × 10⁻¹⁹ cm², 𝜎_e = 13.5 × 10⁻¹⁹ cm²) and Nd:GdVO₄ (𝜎_a = 5.36 × 10⁻¹⁹ cm², 𝜎_e = 7.6 × 10⁻¹⁹ cm²), the Nd:LuVO₄ has an even larger absorption cross section (𝜎_a) at 808 nm and emission cross section (𝜎_e) at 1.06 um (𝜎_a = 6.9 × 10⁻¹⁹ cm², 𝜎_e = 14.6 × 10⁻¹⁹ cm²), and a relatively high thermal conductivity of 7.96 Wm⁻¹K⁻¹ and 9.77 Wm⁻¹K⁻¹ along a- and c-axis, respectively [16]. These excellent crystalline features indicate the potential use of Nd:LuVO₄ in the development of high power, ultrafast solid-state lasers. CW output powers of 12.55 W and 17.2 W with average slope efficiencies of 52.3% and 48.0% based on Nd:LuVO₄ crystal have been demonstrated in 2004 and 2011, respectively [17, 18 ]. Mode-locked lasers using GaAs and SESAM with average powers of 3.11 W and 8.6 W were investigated to generate pulse durations of 7.1 ps and 13.2 ps, respectively [19, 20 ].

In this study, a high power, few picosecond, passively mode-locked Nd:LuVO₄ oscillator with dual-crystal configuration was demonstrated. Under the absorbed pump power of 44 W, a maximum continuous-wave mode-locking (CWML) average power of 28 W was obtained at 58 MHz, and output power could reach up to 31.6 W in CW operation. By using a Herriott-style multi-pass cell (MPC) to prolong the cavity length [21, 22 ], a maximum output power of 24 W was achieved with a repetition rate of 12.1 MHz, corresponding to a pulse energy of almost 2 μJ. In addition, cavity dumping was employed, which is an efficient method for generating high pulse energies while avoiding complex amplifier schemes [23–25 ]. By inserting a KTP Pockels cell, the cavity-dumped Nd:LuVO₄ oscillator was accomplished with a frequency rate up to 1.5 MHz. The pulse energy was scaling up to 29.3 μJ (17.6 W) at 600 kHz, 20.0 μJ (20.0 W) at 1 MHz and 14.3 μJ (21.4 W) at 1.5 MHz by cavity dumping technique. The shortest pulse duration was measured to be 4 ps at 300 kHz with a maximum pulse energy of 40.7 μJ (12.2 W). To the best of our knowledge, it is the highest output pulse energy and highest average power reported from a Nd:LuVO₄ oscillator. Experimental results indicate that Nd:LuVO₄ is a very promising gain medium for high pulse energy and high average power ultrafast lasers.

2. Experimental setup

A schematic of the laser configuration was shown in Fig. 1 . The pumped sources employed were two fiber-coupled laser diodes at 880 nm which had the maximum output power of 26.5 W. The core size of the fibers had a 400 μm diameter with a numerical aperture of 0.22. The pump beams were imaged and focused into the center of the a-cut Nd:LuVO₄ crystals by the coupling optics systems. A dual-crystal end-pumped configuration was adopted to achieve a high power output. Two facets of the crystals were coated with antireflection (AR) at 880 nm and 1.06 μm. The crystals were wrapped with a thin layer of indium foil and mounted in a water-cooled copper holder, maintaining a constant temperature of 15 °C during laser operation to dissipate the deposited heat efficiently.

 

Fig. 1 Schematic of the laser cavity.

Download Full Size | PDF

M1 and M2, were two input mirrors with an AR coating at 880 nm and high reflectivity at 1.06 μm. M3 and M10 were concave mirrors with radii of curvature of 600 and 500 mm, respectively (Fig. 1). The CW operation was accomplished by M11 and M5. The flat mirror M5 with a transmission of 11.7% at the wavelength of 1.06 μm was used as an output coupler. For short-cavity CWML operation, the mirror M11 was removed, thin-film polarizer (TFP) and SESAM were inserted with a cavity length of 2.58 m. Then as shown in Fig. 1, a MPC was used to extend the cavity length, and M5 was relocated to the position of M9 for long-cavity CWML operation. The MPC consisted of a flat HR mirror (M6) and a concave mirror (M7) in order to obtain a total cavity length of approximately 12.4 m. In cavity-dumped operation, the output coupler was replaced by a HR mirror (M9). Finally, cavity dumping was fulfilled by combining a KTP Pockels cell, a TFP, and suitable settings on the delay-pulse generator.

3. Experimental results and discussions

The CW operation of Nd:LuVO₄ laser with a wavelength of 1.06 um was realized firstly in order to confirm the appropriate alignment of the dual-crystal configuration. The dependence of output power on the absorbed power was given in Fig. 2(a) . A maximum output power of 31.6 W was obtained from a-cut Nd:LuVO₄ at an absorbed pump power of 44 W. The corresponding slope efficiency was measured to be 74.4% with the output coupler of T = 11.7%. Next, the SESAM was inserted to act as a resonator mirror for short-cavity CWML operation. Under the same pump power and transmission, a maximum output power of 28 W at a repetition rate of 58 MHz was obtained with a corresponding slope efficiency of 65.4%. Compared with the CW operation, the average output power of the short-cavity CWML laser was slightly decreased as a result of the loss introduced by SESAM.

 

Fig. 2 (a) CW and short-cavity CWML output power versus the absorbed pump power; (b) Pulse train of the mode-locked laser at 12.1 MHz.

Download Full Size | PDF

In long-cavity CWML operation, the cavity length was prolonged through the use of MPC. A maximum output power of 24 W was extracted from the mode-locked resonator without suffering q-switched instability. Due to the optical losses enlarged by reflections on MPC, the output power decreased farther under the same experimental conditions. The mode-locked pulse train was detected by an oscilloscope via a fast response photodetector, as depicted in Fig. 2(b). The cavity repetition rate was measured to be 12.1 MHz, corresponding to a pulse energy of almost 2 μJ from the oscillator. The mode-locked pulse duration was measured with an intensity autocorrelator (FR-103XL, Femtochrome Research, Inc). By the fitting of a sech²-shaped profile, the pulse width was measured to be 5.5 ps.

During cavity-dumped operation, the laser was fitted with a KTP Pockels cell which has a repetition rate tuning from 300 kHz to 1.5 MHz. The rise and fall times of the Pockels cell were both less than 5 ns. Cavity dumping of a single pulse was then obtained with a suitable setup of the electric pulse delay and high voltage on the Pockels cell. The experimental establishment process of the mode-locking pulse train in the cavity-dumped regime at rates of 600 kHz and 1.5 MHz were illustrated in Figs. 3(a) and 3(b) , respectively. It was feasible to model the dumped process along with a periodic loss modulation at an integer multiple of the cavity round-trip period [26–28 ]. The parameters used for simulation were summarized in Table 1 . The corresponding numerical simulated evolutions of the intra-cavity pulse energy at 600 kHz and 1.5 MHz were illustrated in Figs. 3(c) and 3(d), respectively. It can be seen that the pulse train remained stable, and unaffected by cavity-dumping. Meanwhile, the experimental and theoretical results maintain good accordance for comparison.

 

Fig. 3 Experimentally formed internal pulse trains with cavity-dumping at repetition rates of: (a) 600 kHz and (b) 1.5 MHz; the corresponding numerical simulations of the intra-cavity pulse energy at: (c) 600 kHz and (d) 1.5 MHz.

Download Full Size | PDF

Table 1. Key parameter values for simulation

View Table

Figure 4(a) depicted the cavity-dumped power and pulse width (sech²-fit) versus the dumped repetition rates. The maximum pulse energy of 40.7 μJ was achieved at a dumped rate of 300 kHz and the corresponding pulse width of 4.0 ps was illustrated in Fig. 4(b), assuming the pulse had a sech²-shaped temporal intensity profile. The average power of 17.6 W and 20 W were obtained at 600 kHz and 1 MHz, with the pulse energy of 29.3 μJ and 20 μJ, respectively. At the highest rate of 1.5 MHz, an output pulse energy of 14.3 μJ was achieved with a suitable setup of the Pockels cell. The pulse duration was measured to be slightly longer with a sech²-fit of 5.3 ps. The enlargement of pulse widths was in accordance with the observations when output coupling increased in a mode-locked laser [24].

 

Fig. 4 (a) Cavity-dumped power and pulse duration (sech²-fit) versus dumped repetition rates; (b) the measured autocorrelation trace at 300 kHz, assuming a sech²-shaped pulse.

Download Full Size | PDF

As shown in Fig. 4(a), the cavity-dumped power increased exponentially at relatively low dumped rate. But when the rate was beyond 1 MHz, the output power began to increase slowly. The reason for that is the higher dumped ratio resulting in a lower number of round-trips gain in the cavity, while the total cavity length remained constant. As shown in Figs. 3(a) and 3(b), the number of round-trips gain is 20 times at the dumped ratio of 600 kHz, and it changes to be just 8 times at 1.5 MHz. Moreover, the increasing dumped rate is equivalent to an increased cavity loss, resulting in a lower intra-cavity pulse energy [23]. It can be anticipated that, the cavity-dumped power will increase slower and the corresponding output pulse energy will be lower with the rate up to 2 MHz, or even higher dumped rate.

4. Conclusion

In conclusion, a SESAM passively mode-locked Nd:LuVO₄ oscillator with cavity dumping has been demonstrated. Under an absorbed pump power of 44 W, maximum CW and CWML laser average power of 31.6 W and 28 W were obtained, respectively. With a MPC inserted into the cavity, an output power of 24 W was achieved, mode-locking at 12.1 MHz, with a pulse width of 5.5 ps. Pulse energies of 40.7 μJ (12.2 W) at 300 kHz, 29.3 μJ (17.6 W) at 600 kHz, 20 μJ (20 W) at 1 MHz and 14.3 μJ (21.4 W) at 1.5 MHz were presented in cavity-dumped operation. Meanwhile, the shortest pulse duration of 4 ps was obtained at 300 kHz with a sech²-fit. To the best of our knowledge, this is the highest pulse energy and highest average power yet reported from a Nd:LuVO₄ oscillator. This study confirms that Nd:LuVO₄ has great potential for developing diode-pumped high pulse energy lasers. Pulse energies based on this material could be employed in various fields, such as materials processing, biological imaging and nonlinear spectroscopy.