We report on a high-power passively mode-locked, cavity-dumped picosecond oscillator with dual Nd:GdVO4 crystal schemes. An average power of up to 19 W is obtained at 8.1 MHz, corresponding to a pulse energy of 2.35 μJ in continuous-wave mode-locking. With electro-optic cavity dumping, pulse energies of 16.22 μJ at 1 MHz and 34.7 μJ at 300 kHz are achieved, with pulse widths of 8.55 and 7.0 ps, respectively. These results demonstrate the potential of Nd:GdVO4 as a very promising candidate for high-power ultrafast laser.
© 2015 Optical Society of America
Picosecond laser systems with microjoule-regime pulse energies and Megahertz repetition rates have been applied in microscopy, material processing, and biomedicine [1–3 ] because of their compactness, high efficiency, and competitive price. Diode-pumped solid-state lasers based on Nd-doped materials that are passively mode-locked with semiconductor saturable absorber mirror (SESAM) [4, 5 ] have been used to obtain a reliable oscillator of picosecond pulses [6, 7 ]. However, pulse energy that directly originates from a mode-locked oscillator is relatively low, usually at nJ-level, which limits its applications in scientific and industrial areas.
Several mechanisms, such as fiber amplifier, multi-pass amplifier, and regenerative amplifier, are typically used to boost pulse energy. Certain noticeable results have been demonstrated by several groups. A rod-type fiber master oscillator power amplifier has been accomplished to deliver 3 ps, 3 μJ pulses with a repetition rate of 50 MHz . A 105 W, 120 MHz, corresponding to 0.9 μJ, 8.4 ps near-diffraction limited laser output from a double end-pumped Innoslab amplifier was presented by Lin et al. . At the maximum repetition rate of 200 kHz, a Nd:GdVO4 regenerative amplifier system that generated 6.8 ps, 13 W laser with a pulse energy of 65 μJ was achieved by Kleinbauer et al. . SESAM mode-locked thin-disk laser also has been proved to be an effective way which could deliver output powers exceeding 100 W and pulse energies in the several tens of micro-joules . Meanwhile, cavity dumping is an alternative method to efficiently generate high pulse energies, while maintaining comparable compactness to contain a single resonator. An impressive cavity-dumped oscillator based on Nd:YVO4 was demonstrated by Wegner et al. . The average power reached 7.8 W at 500 kHz and 10 W at 1 MHz, which corresponds to pulse energies of 15.6 and 10 μJ respectively with pulse widths of sub-10 ps. A passively mode-locked chirped-pulse Yb:KYW laser oscillator with cavity dumping, which generated a 7 µJ pulse energy at 1 MHz repetition rate, was presented by Palmer et al. . The 14 ps pulse duration was further compressed down to 416 fs with a peak power exceeding 12 MW.
As a Nd-doped gain material, Nd:GdVO4 has attracted considerable attention because of its excellent physical, optical, and mechanical properties [14–17 ]. Despite its stimulated emission cross-section at 1.06 um (7.6 × 10−19 cm2) is smaller than that of Nd:YVO4 (15.6 × 10−19 cm2), the thermal conductivity of Nd:GdVO4 (10.1 W m−1 K−1) is considerably higher, similar to that of Nd:YAG [18, 19 ]. Thus Nd:GdVO4 combines the advantages of Nd:YVO4 and Nd:YAG rendering the material more suitable for high-power diode-pumped solid-state lasers.
In this paper, a picosecond passively mode-locked and electro-optical cavity-dumped Nd:GdVO4 oscillator was demonstrated. A maximum continuous-wave (CW) and continuous-wave mode-locking (CWML) laser output power of 28 and 23 W were obtained with a dual-crystal configuration, respectively. To increase the output pulse energy of a laser, cavity length was prolonged using Herriott-style multi-pass cell (MPC) . A maximum output power of 19 W was achieved with a repetition rate of 8.1 MHz. During cavity-dumped operation, a maximum pulse energy reaching 34.7 μJ at 300 kHz and 16.22 μJ (16.22 W) at 1 MHz with pulse widths of sub-10 ps were obtained. Therefore, this laser can meet various requirements of scientific and industrial applications, such as high-precision material processing, at high repetition rates. To the best of our knowledge, it is the highest output pulse energy and highest average power reported from an ultrafast mode-locked oscillator with cavity dumping. Experimental results indicated that Nd:GdVO4 is suitably employed in high pulse energy and high average power ultrafast lasers.
2. Experimental setup
The experimental setup of cavity-dumped mode-locked Nd:GdVO4 laser was shown in Fig. 1 . Two fiber-coupled laser diodes at 880 nm with a core diameter of 400 um and a numerical aperture of 0.22, LD1 and LD2, were used as the pumping source which had a maximum output power of 24 W, respectively. Achromatic lenses were used to re-image and focus pump beams into the center of the crystal. A dual Nd:GdVO4 crystal configuration was adopted to achieve high power output. Two facets of crystals were coated with antireflection (AR) at 880 nm and 1063 nm. The crystals were wrapped in an indium foil and mounted in a water-cooled copper holder; the temperature was maintained at 15 °C.
Two dichromatic mirrors, M1 and M2, were coated with AR at 880 nm and high reflectivity at 1063 nm. The cavity for CW operation was terminated by M12 and M5, and M5 was used as an output coupler with a transmission of 11.7%. High-power CW operation was performed to confirm the appropriate alignment of the dual-crystal configuration. With M12 removed, thin-film polarizer (TFP) and SESAM were inserted into the cavity for short-cavity CWML operation with a cavity length of 2.92 m. To extend the cavity length, a MPC was inserted, and M5 was removed to the position of M9 for long-cavity CWML operation, (Fig. 1). It was an effective method for MPC to reduce the pulse repetition rate and improve the output pulse energy, while maintaining laser cavity modes [21, 22 ]. MPC consisted of a flat HR mirror (M6) and a concave mirror (M7), while the beam bounced 10 times between mirrors to return to its incident point and complete a closed ray path. The periodic length of MPC was 800 mm to obtain a total cavity length of approximately 18.5 m.
While in cavity-dumped operation, the output coupler was replaced by a HR mirror, M9. Cavity dumping was accomplished by combining a RTP Pockels cell, a TFP, and an appropriate setting on the delay-pulse generator.
3. Experimental results
In CW operation, a maximum output power of 28 W was obtained under an absorbed pump power of 42 W. For short-cavity CWML operation, a maximum output power of 23 W at a repetition rate of 51.3 MHz was obtained. As shown in Fig. 2 , it can be seen that both CW and short-cavity CWML output power increased linearly with the absorbed pump power, and the corresponding slope efficiencies was 68.9% and 58.4%, respectively. Both spatial quality of CW and short-cavity CWML output were measured to be almost diffraction limited with M2 values less than 1.1.
When a MPC was inserted, a maximum output power of 19 W was achieved in long-cavity CWML operation. Given the large number of reflections on MPC per round-trip that enlarge the optical losses, the output power decreased in comparison with that in CW operation under the same conditions. The steady oscilloscope trace of the mode-locked pulse train with repetition rates of 8.1 MHz was depicted in Fig. 3(a) , corresponding to a pulse energy of 2.35 μJ from a single oscillator. Output pulse duration was measured with an intensity autocorrelator (FR-103XL, Femtochrome Research, Inc). The pulse width of 8.94 ps was illustrated in Fig. 3(b), assuming the pulse had a sech2-shaped temporal intensity profile.
The cavity dumping was accomplished with a RTP Pockels cell inserted into the cavity, which has a repetition rate tuning from 300 kHz to 1 MHz. The output pulse energy and pulse width (sech2-fit) as a function of the repetition rates were showed in Fig. 4(a) . For a suitable setup of the electric pulse delay and width of Pockels cell, an average power of 16.22 W at the rate of 1 MHz was obtained, corresponding to a pulse energy of 16.22 μJ. The pulse duration was measured to be 8.55 ps, which was almost the same as that in long-cavity CWML operation. The maximum pulse energy of 34.7 μJ was achieved at a dumping rate of 300 kHz with a pulse width of 7.0 ps. Pulse energy increased with reducing repetition rate from 1 MHz to 300 kHz, meanwhile the pulse width slightly decreased. The short pulse duration along with increasing pulse energy during cavity-dumped operation was in accordance with the occurrences when output coupling decreased in mode-locked laser . The output signal of cavity-dumped mode-locked laser was recorded with a fast photodiode at 1 MHz. The standard deviations of the fluctuations in average output power at 1 MHz over 30 minutes was <1.2% and the pulse-to-pulse ratio was measured to be ~1:100.
4. Simulation and discussions
To explain and optimize our cavity-dumped mode-locked laser, a simple model was investigated based on the recurrence relation developed in [23, 24 ]. Two coupled rate equations were used to represent the evolution of mode-locked laser, from which an experimentally confirmed criterion was derived to achieve CWML without Q-switching instabilities. The dynamics of a SESAM mode-locked laser was described by intra-cavity pulse energy and the round-trip gain on a coarse-grained time scale, t.
Here, Ep denoted the intra-cavity pulse energy and Tr was the cavity round-trip time. The quantities g, l were round-trip gain and loss, respectively. g0 was the small-signal gain per round trip, tu and Esat,l were the upper state lifetime and the saturation energy of the laser medium, respectively. The loss introduced by SESAM in pulse energy per round-trip was described by [23, 25 ] as,
Ans represented the non-saturable loss of SESAM, ΔR was the modulation depth and Ea was the saturation energy of the absorber. Here, and . These equations were integrated by fourth-order Runge-Kutta method . It was feasible that the cavity-dumped operation was regarded as a periodic loss modulation at an integer multiple of the cavity round-trip period [27, 28 ]. The parameters used for simulation were summarized in Table 1 .
The images shown in Figs. 5(a) and 5(b) illustrated the experimental establishment process of the mode-locking pulse train in the cavity-dumping regime at the rates of 1 MHz and 300 kHz, respectively. It can be seen that the pulse train remained stable and unaffected by cavity-dumping because of a suitable setup of the electric pulse delay and high voltage of the Pockels cell. The corresponding numerical simulated evolution of the intra-cavity pulse energy were illustrated in Figs. 5(c) and 5(d) at 1 MHz and 300 kHz, respectively. The well accordance between experimental results and theoretical values indicate the validity of the model to describe the dynamics of the cavity-dumped SESAM mode-locked laser system.
In conclusion, a dual Nd:GdVO4 crystal SESAM passively mode-locked with cavity dumping picosecond oscillator has been demonstrated. Maximum CW and CWML laser output power of 28 and 23 W were obtained under an absorbed pump power of 42 W, respectively. With a MPC inserted into the cavity, an output power of 19 W was achieved, mode-locking at 8.1 MHz, with a pulse width of 8.94 ps. For cavity-dumped operation, we obtained a maximum pulse energy of 34.7 μJ at 300 kHz, and 16.22 μJ (16.22 W) at 1 MHz, with pulse widths of 7.0 and 8.55 ps, respectively. To the best of our knowledge, it was the highest pulse energy and highest average power reported from picosecond cavity-dumped mode-locked oscillator. Meanwhile, a theoretical model was investigated and agreed well with the experiment. This work demonstrated the potential of Nd:GdVO4 as a very promising candidate for high pulse energy and high average power ultrafast laser. It can be anticipated that, in further experiment, through higher reflectivity of MPC, appropriate pump power and suitable dumping rates, it is feasible to obtain hundred micro-joules at MHz repetition rates through the cavity-dumped oscillator.
The authors acknowledge the support of the National Science Foundation of China (Grant No. 60921004 and 61378030), and the National Basic Research Program of China (Grant No. 2011CB808101).
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