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Diode-pumped Yb:LuAG laser has been passively mode locked with a SESAM for the first time to the authors’ knowledge. The pulses as short as 7.63ps were generated, without negative dispersion elements. The output power achieved 610mW at pump of 11.2W with repetition rate of 86MHz. The continuous-wave operation and wavelength tuning pumped at 940nm were also examined, and its tuning can cover the range of 1033nm–1079nm.
©2009 Optical Society of America
1. Introduction
Owing to the only and efficient 2F5/2→2F7/2 stimulated radiation around 1 µm between the only two spin orbit 4f13 configuration states, Yb3+ ion has a low intrinsic quantum defect, a weak thermal load, and doesn’t suffer from the luminescence quenching, upcoversion and excited-state absorption. In addition, the radiative lifetime of Yb3+-doped crystals is substantially longer than that of Nd3+-doped ones (approximately 3–4 times longer), which permits greater energy-storage efficiency with diode-pumped laser systems. Therefore, Yb3+-based laser systems have been expected to be the most potential alternatives to the Nd3+-based ones in the near-IR spectral range. Besides that, the wider emission spectrum makes it more attractive for development of diode pumped ultrafast laser systems. Recently, a variety of interesting results have been reported for cw or mode-locked operations based on the diode pumped Yb3+-doped crystals, such as garnet Yb:YAG [1,2], vanadate Yb:YVO4 [3], tungstates Yb:KYW and Yb:KGW [4–6], oxyorthosilicates Yb:YSO and Yb:GYSO [7,8]. Among these crystals, Yb: YAG is very attractive because of the well known thermomechanical properties of YAG. However, the measured peak effective emission cross section for Yb:YAG is relatively modest compared to other hosts. Hence, a similarly robust host material with more favorable emission cross section is desired. We identified a different garnet host material Lu3Al5O12 (LuAG) here. Yb:LuAG crystal has a large effective emission peak cross section compared to that of the Yb:YAG crystal [9]. Additionally, Yb:LuAG has better mechanical properties, such as high thermal conductivity, compared to that of Yb:YAG crystal [10]. So that, Yb:LuAG offers high potential as a laser gain medium for generating high-average-power and ultra-short pulse output with high efficiency.
The first laser experiment of Yb:LuAG crystal was reported in mid-1970’s under Xe lamp pumped [11]. Then optical properties of Yb:LuAG and the first 970nm LD-pumped Yb:LuAG lasers were reported at room temperature [9]. In 1999, Tadashi Kasamasu and his colleagues investigated the temperature-dependent laser performance of Yb:LuAG crystals under 970 nm LD pumping [12]. After that, growth, spectroscopic, and laser properties of Yb:LuAG crystals doped with various Yb3+ concentrations were investigated [10]. More recently, the experimental investigations of LD end-pumped passively Q-switched Yb:LuAG microchip lasers were demonstrated. Laser pulses with pulse energy of 19µJ and a pulse width of 610ps at the repetition rate of 12.8 kHz were achieved [13]. The results showed that Yb:LuAG would be suitable for ultrashort pulse generation.
In this paper, we report, for the first time to our knowledge, on the experimental investigation of LD-pumped passively continuous wave mode-locked (CWML) Yb:LuAG laser by using a semiconductor saturable absorption mirror (SESAM), to get the pulse duration as short as 7.63ps. Meanwhile, the continuous wave (CW) and tunable Yb:LuAG lasers pumped by 940nm LD were also examined.
2. Spectral properties of Yb:LuAG
The 5×6×3.5mm3 10at. % Yb:LuAG sample, provided by the R&D Center for Laser and Opto-Electronic Materials of Shanghai Institute of Optics and Fine Mechanics(SIOM), was grown by Czochraski method and polished with parallel end faces. The absorption efficiency of the crystal at 940nm was 95%. The unpolarized absorption and emission spectra at room temperature compared with 10at. %Yb:YAG [14] was illustrated in Fig. 1. Apparently, these two crystals had largely the same profiles. The absorption spectrum was recorded by Lambda 900 Spectrometer (Perkin Elmer Inc. With a resolution of 1nm), consisting of four bands around 918, 938, 970 and 1030nm. The broad absorption bandwidth centered at 938nm promised the high absorption efficiency of the ~940nm diode-pumped operation. In other side, its higher absorption at 1030nm band would lead to a higher threshold at 1030nm. The IR luminescence spectrum was obtained with a spectrofluorometer (Fluorolog-3, Jobin Yvon, Edision, USA. With a resolution of 0.5nm) equipped with a Hamamatsu R928 photomultiplier tube, excited by a 940 nm continuous wave diode laser. It’s composed of four emission bands around 969, 1006, 1030 and 1046nm. The 1030nm and 1046nm bands were some narrower, comparing with Yb:YAG. According to the absorption and emission spectra, we could approximately deduce the energy-level diagram of Yb3+in the LuAG host as shown in Fig. 2. The absorption peak of 969 nm belongs to the zero-line transition between the lowest levels of 2 F 7/2 and 2 F 5/2 manifolds. The ration of the partition function measured 0.88. The peak emission cross section at 1031nm was 2.373×10-20cm2 calculated by Reciprocity Method [15], which was about 25% higher than that of 10at. % Yb:YAG [9]. Although the maximum emission appeared at 1030nm, it could be more efficient at the secondary peak of 1046nm because of the huge reabsorption losses at 1030nm. Moreover, the 1046nm peak had a broader spectrum range, which promised to obtain a shorter pulse. According to our measurement, the radiative lifetime was 0.988ms and the thermal conductivity was 8Wm-1K-1
Fig. 1. The unpolarized absorption and fluorescence spectra of the 10at%Yb: LuAG sample together with those of 10at%Yb: YAG.
3. Experiments
3.1 CW operation and wavelength tuning of Yb:LuAG
In a preliminary experiment, the laser performance of the 10at. % Yb:LuAG crystal was investigated with a stable three-mirror folded cavity for CW operation and wavelength tuning. The pump source was a fiber-coupled diode laser with the core-diameter of 200µm and N.A. of 0.22, emitting at the wavelength of 938nm at room temperature. Pump beam was focused by a series of lens, and the pump spot on the crystal was about 160µm. The uncoated sample was wrapped with indium foil and mounted in a water-cooled copper block. The water temperature was maintained at 14°C to prevent thermal fracture. In the case of cw laser operation, output couplers with different transmissions (3%, 9%, and 15%) were used to obtain the optimum output. The dependence of the laser outputs on the absorbed pump power is illustrated in Fig. 3(a). At absorbed pump power of 12.1W, maximum laser output power of 4.6W at 1050nm was obtained with a 15% transmission output coupler, and the slope efficiency arrived at 42.7%. The efficiency would be higher if the crystal was coated with proper anti-reflection film and a two-mirror short cavity was used as the laser cavity. The wavelength tuning for Yb:LuAG was fulfilled by inserting an SF10 dispersive prism in the collimated arm of the laser cavity. The wavelength could be tuned from 1033nm to 1079nm, as shown in Fig. 3(b). When we tuned the angle of prism for shorter wavelength output, the laser oscillated at its central peak, 1050nm. So, the shorter wavelength tuning was stopped at 1033nm. We think that the main reason is the reabsorption peak at 1030nm. The output bandwidth of 46nm is comparable with the result of ref.10, but the tuning range is some different. The main reason might be the different bandwidth of cavity mirrors.
Fig. 3. (a) The continuous wave output power dependence on absorbed pump. (b) The wavelength tuning curve of the Yb:LuAG crystal
3.2 Continuous wave mode locking of Yb:LuAG
In the passively mode-locked operation, we employed a folded cavity, as shown in Fig. 4.
Fig. 4. Setup of the mode-locked Yb:LuAG laser: SESAM-semiconductor saturable absorber mirror; M1-input coupler;M2, M3-folding mirrors; OC-output coupler.
The laser cavity consisted of a SESAM and four mirrors: an input flat mirror M1 with high transmission at 938 nm and high reflection in a broad band from 1015 to 1075nm, two folded concave mirrors M2 (R=500 mm) and M3 (ROC=100mm) with high reflection in a broad band from 1015 to 1075nm, and an output coupler (OC) flat mirror with a transmission of 1.5%. The total cavity length was added up to 1703mm. The SESAM used in our experiment was produced by BATOP GmbH (Germany), with the central wavelength at 1064nm, saturation fluence of 70µJ/cm2, maximum modulation depth of 1.2%, and the relaxation time of 20ps. Based on our calculation, the laser cavity was designed to get a beam diameter of 136µm on the crystal and 106µm on the SESAM, respectively.
4. Results and Discussion
The SESAM mode locked lasers based on Yb-doped materials are easily apt to the Q-switched mode-locked regime because of their long excited-state lifetime. So we must carefully choose the proper parameters of the cavity and SESAM. CW mode-locking threshold value is determined by E2p>Esat,LEsat,AΔR [16], where Esat,A and Esat,L denote the saturation energy of the absorber and gain medium, respectively, and ΔR is the modulation depth of the SESAM device. It is not difficult to calculate that the CW mode-locking threshold of our laser is 2.3nJ. In the experiment, we could detect the CW mode locked pulse with energy output above 2.5nJ. However, the operating mode was not so stable that it might skip into Q-switched mode locking when some disturbances of the cavity happened. With increasing of the output energy, the laser tended to operate at CWML mode more stably. When the pulse energy reached 7nJ, the CWML operated stably for hours even there were some disturbances, such as hitting the table or cavity mirrors softly. We can find that the experiment and the theoretical calculation agree well. In our experiment, the CW mode-locked pulse was detected by a fast response opto-electronic diode, and the pulse train is shown in Fig. 5. The repetition frequency of the CWL pulse is 86MHz.
A 1.5% output coupler was used in the mode locking experiment. At pump power of 11.2W (The maximum pump we could use), a maximum average output power of 610 mW was obtained. The CML pulses were centered at 1046 nm with a spectral full-width of half-maximum (FWHM) about 4.0 nm. The spectral profile of the CML output pulses is shown in Fig. 6. A 3% output coupler was used to get a higher output power, but it was difficult to get the laser mode locked because it was very hard to achieve the CWML threshold.
The output pulse duration was measured with a rapid scanning autocorrelator (FR-103XL, Femotochrome. Research, Inc.). Assuming the pulse had a Gaussian-shaped temporal intensity profile, we obtained pulses as short as 7.63ps (FWHM) when the output power was 610mW. The corresponding autocorrelation trace is shown in Fig. 7.
5. Conclusion
We have demonstrated, for the first time to our knowledge, a diode-pumped Yb:LuAG laser at mode locked operation with a SESAM. The laser pulses of 7.63ps duration were obtained with an average output power of 610mW and a repetition rate of 86MHz. 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. Further experiments (compensating the group delay dispersion and further improving cavity design and laser crystal quality) are under research.
Acknowledgments
The authors acknowledge support from the National Science Foundation of China under grant (No.60578052), and the National Basic Research Program of China (Grant No. 2006CB806000).
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