Continuous-wave (cw) mode-locking of a diode-pumped Nd:Lu0.5Gd0.5VO4 mixed crystal laser is reported for the first time to our knowledge with a simply compact three-mirror cavity. Stable pulses as short as 5.5 ps were generated at a repetition rate of 147 MHz. At the absorbed pump power of 16 W, a mode-locked laser with average output power of 5.31 W was obtained, giving an optical conversion efficiency of 33.2%, and a slope efficiency of 46.7%.
© 2009 Optical Society of America
High power ultrashort pulse lasers have wide applications in industry, military and scientific researches, and have been intensively investigated [1–7]. The performance of a mode-locked laser is ultimately determined by the property of the laser gain medium. Generally speaking, in order to achieve high efficiency and ultrashort pulse mode-locking, laser materials with large stimulated emission cross section and broad fluorescence linewidth are always preferred. The Nd-doped single vanadates, Nd:YVO4[3,4], Nd:GdVO4, and Nd:LuVO4, have been proved to be excellent laser crystals for the diode-pumped mode-locked lasers. They all have large absorption and stimulated emission cross sections, compared with Nd:YAG. In addition, the Nd:GdVO4 crystals also have a large thermal conductivity along the <110> crystal directions. Recently, it was found that by substituting a fraction of Gd ions with Y ions in Nd:GdVO4 crystal, a new mixed vanadate crystal Nd:YxGd1-xVO4 could be produced, and the mixed crystal had much broader fluorescence spectrum than either of the single crystals Nd:GdVO4 and Nd:YVO4. Passive mode-locking with a Nd:YxGd1-xVO4 mixed crystal has proved that the mixed crystal is a better candidate for ultrashort-pulse generation than the single ones. Among the three Nd-doped single vanadate crystals, the Nd:LuVO4 crystal has the highest stimulated emission cross-section and relatively broad florescence spectrum . It is anticipated that substituting a fraction of Gd ions with the Lu ions should produce mixed crystal with even more promising mode-locking properties. Indeed, preliminary studies have proven the excellent cw and Q-switching performance of the high-power diode-pumped Nd:LuxGd1-xVO4 mixed crystal lasers . In this letter, we report on the passive mode-locking performance of a high-power diode pumped Nd:Lu0.5Gd0.5VO4 mixed crystal laser. Mode-locked with a semiconductor saturable-absorber mirror (SESAM), stable pulses with duration of 5.5 ps and a repetition rate of 147 MHz have been achieved. A maximum output power of 5.31 W was obtained under an absorbed pump power of 16 W, which gives an optical conversion efficiency of 33.2%. Comparing with the mode-locking of the Nd:LuVO4 laser under the same experimental conditions, the mixed crystal laser produced the much shorter pulses.
The Nd:Lu0.5Gd0.5VO4 crystal is an isomorph of Nd:LuVO4 and Nd:GdVO4. Therefore, they have similar physical properties, e.g. they all posses the ZrSiO4 structure, and have the same central absorption and emission wavelength. However, due to the random distribution of the Lu and Gd ions neighboring the Nd ions, the inhomogeneous broadening of the spectrum lines is caused in the mixed crystal, which leads the broader absorption and fluorescence linewidths. Figure 1 shows the measured fluorescence spectrum of the mixed crystal. The full width at half-maximum (FWHM) of the fluorescence band centered at 1.063 μm is about 4 nm, which is much broader than those of the vanadate single crystals (~1–2 nm) and even Nd:YxGd1-xVO4 mixed crystal (>2 nm)  .
A schematic of the laser configuration is shown in Fig. 2. The pump source employed was a 50 W fiber-coupled LD array with the central wavelength at 806 nm. The core size of the fiber is 300 μm in radius with a numerical aperture of 0.22. The pump light was focused into the crystal by an imaging unit with a beam compression ratio of 1.8:1. The focal spot in the crystal had a size of about 170 μm in radius. A compact three-mirror cavity was used for the laser. The input mirror M1 was a flat mirror anti-reflection (AR) coated at 808 nm and high-reflection (HR) coated at 1.06 μm. M2 was a concave mirror with radium of curvature (ROC) of 500 mm and a transmission rate of 2% or 6% at 1.06 μm. Hence, the total OC was 4% and 12%, respectively. The lengths of the two arms of the cavity, L1 and L2, were 49.5 cm and 52.5 cm, respectively. Taking into account the measured thermal lens effect in the laser crystal, the mode radius of the laser beam in the SESAM was calculated to be about 110 μm. The Nd:Lu0.5Gd0.5VO4 crystal used was grown by the Czochralski method under a nitrogen atmosphere containing 2% oxygen (v/v) in an iridium crucible. A Lu composition value of x=0.5 was selected, because it makes the mixed crystal having the broadest spectrum linewidth . The crystal was cut along its a-axis with dimensions of 3×3×6 mm3. The 3×3 mm2 faces were polished and AR coated at 808 nm and 1.06 μm. The SESAMs used in the experiments were two commercial products (BATOP Optoelectronics, Germany). They had the respective absorbance (A) of 2% and 3% at 1.06 μm with modulation depth of 1.2% and 1.6%, and same saturation fluence of 70 μJ/cm2. The mode-locked pulse trains were monitored by a high-speed photoreceiver (1611-FS, New Focus, Inc) and an oscilloscope (OPO 7104, Tektronix Inc.), and the pulse duration was measured with a commercial autocorrelator (FR-103XL, Femtochrome Research, Inc.).
3. Results and discussions
Figure 3 shows the variation of the average output power of the laser with the absorbed pump power, by using the SESAM with A=3%. With an OC of 2%, the laser had a threshold of 2.5 W. Under an absorbed pump power of 11.4 W, the average output power was 2.1 W, which gives an optical conversion efficiency of 19%. The slope efficiency of the laser was 25%. In order to avoid too strong saturation of the SESAM by the intracavity power with the OC of 2%, the pump power was limited to be lower than 11.4 W. We then used the OC with transmission of 6% to replace that with OC=2%. The average output power as high as 5.31 W was obtained with a pump power of 16 W, corresponding the optical conversion efficiency of 33.2%. When the pump power was further increased, the average power exhibited clear tendency of power saturation caused by the thermal lens effect induced cavity instability. The threshold was 4.38 W and slope efficiency was as high as 46.7%.
Stable cw mode-locking was always obtained when the intracavity power was larger than about 20 W. Figure 4 shows a typical cw mode-locked pulse train in 10 ns per division time scale. The inset of this figure shows the pulse train in 10 μs per division, which also indicated that the relaxation oscillations were well suppressed. The repetition period of the pulses was 6.8 ns, which corresponds to the cavity round trip time and gives the pulse repetition rate of 147 MHz. The pulse-to-pulse intensity fluctuation was estimated to be less than 2%. Based on the measured average power and pulse repetition rate, we estimated that the maximum mode-locked pulse energy was about 36 nJ. Figure 5 shows a typical autocorrelation trace of the mode-locked pulses. From this figure, it can be found that the laser was clean mode-locked. The FWHM of the autocorrelation was about 8.6 ps. If a hyperbolic secant (sech2) pulse profile was assumed, the pulse duration of the mode-locked pulses was τ=5.5 ps. The spectrum of the mode-locked pulses is shown in the right inset of Fig. 5, which has a FWHM of 0.32 nm. Therefore, the time-bandwidth product of the pulses is 0.46, which is close to the value of a transform-limited sech2 pulse. The a bit larger value indicated that the output pulses were chirped. The mode-locking of Nd:Lu0.5Gd0.5VO4 was also studied with A=2%. The left inset in the figure shows the autocorrelation trace of the mode-locked pulses with the A=2% SESAM, where the FWHM of the mode-locked pulse achieved was 16.4 ps, corresponding τ=10.6 ps if a sech2 profile was taken. Stable mode-locked pulses with duration of about τ=13.2 ps have also been obtained from a Nd:LuVO4 laser under the same experimental condition with A=3% . It is owing to its broader fluorescence linewidth that the Nd:Lu0.5Gd0.5VO4 laser produced the shorter pulses.
Compared with the previous results with Nd:YxGd1-xVO4, we have obtained the mode-locking with comparable pulse width but much larger average output power, by using Nd:Lu0.5Gd0.5VO4. Recently, a passively mode-locked Nd:YVO4 laser with an average output power of 56 W and pulse width of 33 ps was reported . Compared to that the Nd:Lu0.5Gd0.5VO4 crystal has the similar thermal properties , and much broader gain linewidth than those of the Nd:YVO4 crystal, we believe that comparable, and even better in terms of the mode-locked pulse duration, mode-locking performance of the laser with the Nd:Lu0.5Gd0.5VO4 laser could be obtained. Giving the broad fluorescence linewidth and the large stimulated emission cross-section of the mixed crystal, we proposed that subpicosecond mode-locked pulses should be feasible to achieve.
In conclusion, passive mode-locking of a diode pumped Nd:Lu0.5Gd0.5VO4 laser was demonstrated with a SESAM. Stable cw mode-locked pulses with 5.5 ps pulse duration and 147 MHz repetition rate have been obtained. A clean mode-locked laser with maximum average output power of 5.31 W was achieved with a slope efficiency of 46.7%. Compared with the Nd doped vanadate crystals, Nd:Lu0.5Gd0.5VO4 is a better candidate for ultrashort pulse laser, owing to its broad gain spectrum. Giving the large fluorescence linewidth and stimulated emission-cross section of the crystal, we believe it could be a promising material for the diode pumped ultrashort-pulse lasers.
This work is supported by the Natural Science Foundation of Shandong Province under Grand No Y2004F01, the National Natural Science Foundation of China under Grand No 50590401, 60508010, 50721002, and the National Basic Research Program of China under Grand 2004CB619002.
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