This paper reports on a diode-pumped picosecond passively mode-locked Yb:SSO laser. Pulses as short as 2.3 ps with a repetition rate of 53 MHz were generated, without extra negative dispersion elements. The output power achieved 1.87 W at a pump power of 11.5 W. Continuous-wave operation and wavelength tuning were examined. The CW operation achieved 3.55 W output power with a slope efficiency of 44.5%; its tuning can cover the range of 1034.0-1089.7 nm.
© 2010 OSA
The amazing features of all solid-state passively mode-locked picosecond (ps) lasers, such as ultra-short pulse duration, high peak power, structure compactness, high efficiency and well stability make them widely used in science and technology. Combined with regenerative amplification technology, it has wide applications in fields such as industrial processing, high-precision micro-machining, laser ultra-fast detection, and biomedicine [1–4]. However, as production processes become gradually integrated, high-precision micro-machining has become one of its most important and interesting applications. Recent experiments and theoretical investigations have shown that laser pulses of 0.5-5 ps were ideal for many micro-machining applications of semiconductor and metal materials . Therefore, due to the outstanding properties and demand for modern industrial processing, it is very important to develop stable and reliable lasers with pulse durations from sub-picosecond to several picoseconds.
Generally speaking, present common picosecond laser systems are usually based on neodymium-doped crystals such as Nd:YAG or Nd:YVO4 [6,7]. However, the narrow line widths of these gain mediums restrict the pulse duration. Hence, in the past few years, Yb-doped crystals, which have broader emission bandwidth, have aroused extensive attention. Moreover, Yb-doped crystals have other advantageous laser properties such as low intrinsic quantum defect, elimination of up-conversion, excited-state absorption, cross-relaxation, and concentration quenching, which make them very attractive in developing ultrafast lasers. Many Yb-doped materials have been demonstrated in femtosecond mode-locked laser operations such as Yb:KGW, Yb:YSO, and Yb:LSO etc [8,9]. To date, the reported directly diode-pump passively mode-locked ps lasers based on Yb-doped crystals are Yb:CaF2, Yb:YAG, Yb:LuAG, Yb:LSO, and Yb:GSO [10–14]. The reported pulse durations are from 1 ps (Yb:CaF2 ) to 46 ps. The reported output power are from 360 mW to 3.5 W (Yb:YAG ). Meanwhile, the relatively small stimulated emission cross-section and long upper-state lifetime (usually several ms) of Yb3+ leads to a strong tendency to Q-switched mode-locking. Therefore, many efforts are still devoted to develop more potential crystals.
In this paper, we experimentally investigate, for the first time to our knowledge, a diode-pumped picosecond passively mode-locked Yb:SSO laser by using a semiconductor saturable absorption mirror (SESAM). Without extra negative dispersion elements, a stable continuous-wave mode-locked (CWML) train in a very low threshold output power of 96 mW (1% output coupler) was realized. Pulses durations as short as 2.3 ps with output power as high as 1.87 W were obtained. Additionally, the CW and tunable Yb:SSO lasers pumped by 978 nm LD were also examined. The tuning range is from 1034.0 to 1089.7 nm and the efficiency reached 44.5% for the CW operation.
2. Spectral properties of Yb:SSO
The Sc2SiO5 crystal has excellent thermo-mechanical characteristics such as high thermal conductivity (7.5 Wm−1K−1) and negative refractive index (−6.3 × 10−6 K−1), which make it a promising laser host for high-power laser operations. Its basic properties and growth method have been reported in detail in Ref. 15. The 5 × 6 × 3 mm3 5at. % Yb:Sc2SiO5 (Yb:SSO) sample used in our experiment was provided by the R&D Center for Laser and Opto-Electronic Materials of Shanghai Institute of Optics and Fine Mechanics (SIOM). The absorption and emission cross-section of Yb:SSO crystal at room temperature is shown in Fig. 1 . It can be observed from the illustration that, the absorption spectrum of Yb:SSO is composed of three bands at around 910, 956 and 976 nm. The absorption peak at around 976 nm has a peak absorption cross-section of 9.2 × 10−21 cm2 and an absorption line-width (FWHM) of 24 nm. This makes it very suitable for diode-pump operation. The emission spectrum mainly includes four bands centered around 1006, 1036, 1062, and 1087 nm, whose emission cross-sections are 2.6 × 10−21, 4.4 × 10−21, 3.8 × 10−21, and 1.0 × 10−21 cm2, respectively. Among the four emission bands, the line-width of emission band at 1006 nm is relatively high, but laser performance is inefficient due to the strong reabsorption losses. While the emission band at around 1087 nm is relatively low. The emission line-width (FWHM) at 1036 and 1062 nm was about 8 and 7 nm, respectively. The fluorescence time detected was 1.64 ms.
3.1 CW operation and wavelength tuning
Firstly, we research the laser property in CW operation. An experimental setup is presented schematically in Fig. 2 . A fiber-coupled diode laser was used as the pump source, which had a core-diameter of 200 μm and N.A. of 0.22, emitting at a wavelength of 978 nm at room temperature. A series of lens, which had an image ratio of 1:1, was used to focus the pump beam onto the crystal. The laser cavity consisted of a dichroic mirror M1, a highly reflective concave mirror M2 with radii of curvature of 300 mm, and a plane output coupler (OC). The laser cavity was designed to provide a mode size diameter of 180 μm inside the crystal. The 5 at. % Yb:SSO crystal, which was 5 × 6 × 3 mm3, was coated for anti-reflection at a lasing wavelength of 1030-1090 nm and a pump wavelength of 978 nm on both faces. And in order to suppress the influence of the Fabry-Pérot etalon effect, the sample was cut at a wedge angle of 2°. The crystal was wrapped with indium foil and mounted in a water-cooled copper block. The water temperature was maintained at 16 °C.
The performance of the laser in the CW operation is illustrated in Fig. 3(a) . Output couplers with different transmissions of 9% and 12% were used to get optimum output. And the highest output power was achieved with 12% OC. At an absorbed pump power of 9.3 W, the maximum output power of 3.55 W at 1062.3 nm was obtained, corresponding to a laser slope efficiency of 44.5%. The efficiency would be higher if the crystal was not cut at a wedge angle and a simple two-mirror cavity was used. Meanwhile, the output wavelength was dependent on the transmission of OC. With the increase of OC transmission, we found that the output wavelength moved to a shorter value. As shown in the experiment, the central wavelength with 9% and 12% OC were 1062.7 and 1062.3 nm, respectively. This is a usual effect in Yb-doped materials.
In wavelength tuning, a SF10 dispersive prism was introduced into the output arm of the laser cavity, as shown in Fig. 2. Inserting the prism increased the intracavity losses; thus, we chose the OC with transmission of 6% to obtain efficient tuning output, instead of using the optimum output transmission of 12% in the CW operation. The laser could be tuned from 1034.0 to 1089.7 nm, corresponding to a tunability range of 55.7 nm, as shown in Fig. 3(b). There is a relatively narrow emission peak occurs at around 1062.0 nm in the tuning curve of Yb:SSO, because of the strong emission cross-section around that wavelength. Thus, in the femtosecond mode-locked operation, effort must be exerted to avoid lasing at around 1062.0 nm in order to obtain ultra-short pulses. Further tuning on a shorter wavelength is limited by the coating of the input coupler, which makes it difficult to maintain both high reflection at this range and high antireflection at around 978 nm.
3.2 CWML operation
A five-mirror folded cavity was designed for the CWML operation, as shown in Fig. 4 . The input mirror M1 was a flat dichroic mirror, which was antireflection coated at 978 nm and high reflection at 1030-1090 nm. M2 and M3 were high-reflective concave mirrors, which both had radii of curvature of 500 mm. The OC was also a flat mirror. We used a SESAM with a saturation fluence of 70 μJ/cm2, relaxation time of 500 fs, and a maximum modulation depth of 1.2% at one end of the cavity to initiate and maintain the CWML pulse train. To optimize mode matching and to reduce the CWML threshold, the cavity was designed to provide a mode size radii of 90 μm inside the crystal and 50 μm on the SESAM, respectively. The round trip cavity length was about 2.8 m, giving a pulse frequency of 53 MHz.
4. Results and discussion
In the mode-locked operation, OC with different transmissions (1%, 5%, 9% and 13%) were used to get the optimum output. The dependence of the laser output power versus incident pump power in both CW and CWML regimes is illustrated in Fig. 5 . In this illustration, the hollow data represent that the laser work in CW regime, while the solid data represent that the laser work in stable CWML regime. On increasing pump power, the laser turned from CW to CWML state directly; no Q-switched mode-locked state was detected. The threshold output power for CWML with different OC was 96 mW for 1%, 300 mW for 5%, 580 mW for 9% and 498 mW for 13%. The maximum output power was 1.87 W at a pump power of 11.5 W with a 13% OC, corresponding to a slope efficiency of 22.4%. On keep increasing the pump power, double pulsing phenomenon was detected by the autocorrelator. When the OC with bigger transmission (such as 15%, 20%) was used in the cavity, the laser could not operate in the mode-locked regime because the intra-cavity energy was hard to reach the threshold.
The output pulse duration was measured with a rapid scanning autocorrelator (FR-103XL, Femotochrome. Research, Inc.). The measured value kept to be 2-3 ps with different OC transmissions. For the same OC coupler, the pulse width showed an inverse proportionality versus intracavity pulse energy, which is common for passively mode locked laser . We could obtain pulses as short as 2.3 ps (FWHM) with single pulsing at an output power of 1.87 W, assuming the pulses had a sech2-shaped temporal intensity profile. The corresponding autocorrelation trace of single pulsing is shown in Fig. 6(a) . The mode-locked pulse spectral profile is shown in Fig. 6(b), which was measured by an optical spectrum analyzer with a resolution of 0.2 nm (USB2000, Ocean Optics, Inc.). The spectral profile has a FWHM of 3 nm, centered at 1067.0 nm. The time-bandwidth product of the mode-locked pulses was calculated as 1.83, which is 5.8 times larger than the transform-limited value of 0.315 for pulses of hyperbolic sec shape. Higher output could be achieved, but the laser would operate in double pulsing regime, and the autocorrelation trace of double pulsing is shown in Fig. 7 .
Based on the theory of C. Hönninger et al. , the threshold of stable CW passively mode-locking laser with SESAM is given by Ep2>Esat,LEsat,AΔR, where Esat,L is the saturation energy of the gain, Esat,A denotes the saturation energy of absorber, and ΔR denotes the maximum modulation depth of the SESAM. In our experiment, the threshold for CWML was relatively small, as shown in Fig. 5. Take output coupler with transmission of 1% for instance, the threshold output power for CWML was 96 mW, corresponding to the intra-cavity single-pulse energy of 0.184 μJ, which is almost one fourth of the theoretical value of 0.64 μJ. This indicated that the threshold energy of CWML operation could be several times lower than the theoretical one for passively mode locked laser.
Generally, laser with cavity design of gain-at-the-end (GE) could produce a shorter pulse width than the one that with a gain-in-the-middle (GM) design due to the spatial hole burning effect . For a common GE Nd:GdYVO4 mode-locked laser, it could produced a pulse duration as short as 4 ps . Therefore, the result, that a GM Yb:SSO mode-locked laser could produce a pulse width as low as 2.3 ps with an output power as high as 1.87 W, is due to the broad gain bandwidth of the material. This showed great potential of the crystal in making high-power stable sub-picosecond lasers.
We have demonstrated, for the first time to our knowledge, a diode-pumped Yb:SSO laser at picosecond passively mode-locked operation with a SESAM. The laser pulses duration of 2.3 ps was obtained with an average output power of 1.87 W and a repetition rate of 53 MHz. Its large emission cross-section and wide emission spectra range confirm that this crystal is also suitable for developing high efficiency all-solid-state femtosecond laser. The experimental result proved that Yb:SSO is a very promising crystal for diode-pumped all-solid-state ultra-fast lasers, and pulse duration as short as femtosecond could be hopefully obtained in a further research.
The authors acknowledge support from the National Science Foundation of China under grant (No. 60921004 and No.60778036), and the National Basic Research Program of China (Grant No. 2006CB806000).
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