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We have investigated the lasing characteristics of Tm:LSO crystal in three operation regimes: continuous wave (CW), wavelength tunable and passive Q-switching based on graphene. In CW regime, a maximum output power of 0.65 W at 2054.9 nm with a slope efficiency of 21% was achieved. With a quartz plate, a broad wavelength tunable range of 145 nm was obtained, corresponding to a FWHM of 100 nm. By using a graphene saturable absorber mirror, the passively Q-switched Tm:LSO laser produced pulses with duration of 7.8 μs at 2030.8 nm under a repetition rate of 7.6 kHz, corresponding to pulse energy of 14.0 μJ.
© 2013 Optical Society of America
1. Introduction
In recent years, pulsed solid-state lasers emitting in 2 μm eye-safe region have attracted a lot due to their wide applications in medicine, ranging, laser LIDAR, environmental atmosphere monitoring and so on. Additionally, they can also be used as pump source for pumping optical parametric oscillators (OPOs) and solid-state lasers in middle-infrared region [1–5]. Generally, Q-switching methods provide simple ways to obtain nano- or microsecond pulses with high energy and peak power at 2 μm. Active Q-switching methods such as acousto-optical modulator had been employed before the suitable saturable absorber for 2 μm lasers came forth [6–9]. Compared with active Q-switching, the saturable absorber based passive Q-switching has advantages of compactness, simplicity and reliability, etc. Up to now, lots of materials have been employed as saturable absorbers for passively Q-switched 2 μm lasers, such as graphene [10], Cr:ZnS, Cr:ZnSe [11, 12], InGaAs/GaAs [13], PbS-doped glass [14] and etc. Among these saturable absorbers mentioned above, graphene was characterized with a broadband wavelength-insensitive absorption range due to the zero band gap between the conduction and valence band, which enables it absorb light from visible to the THz waveband in principle [15]. Moreover, its ultrafast recovery time [16], low saturation intensity, high damage threshold, and easy fabrication [17], make it become a promising saturable absorber suitable for pulsed lasers.
The radiations around 2 μm relying on the transitions from 3F4 to 3H6 level in trivalent lanthanide ions Tm3+ have presented the advantages of easy pumping with low-cost and powerful laser diodes around 795 nm [18]. However, due to the quasi-three-level scheme in Tm3+ laser system, the ground-state works as lower laser level, which can be significantly populated at room temperature resulting in high oscillation threshold and low efficiency [19]. In addition, such system also suffers from the thermal load, so a long-term effort has been paid to explore novel hosts providing high thermal conductivity and large stark splitting of ground-state levels. Recently, Tm doped silicate crystals become attractive mainly because such silicate crystals can provide large stark splitting in ground state introduced by the disordered structure. For example, a large ground state splitting of 1021 cm−1 was found in Tm:YSO crystal, and 691 cm−1 for Tm:SSO crystal. Moreover, their relatively high thermal conductivity (4.4 Wm−1K−1 for un-doped YSO host and 7.5 Wm−1K−1 for un-doped SSO host) are also attractive [20,21]. Another monoclinic biaxial silicate crystal, Lu2SiO5 (short for LSO), belonging to the oxyortho-silicate group [22], provides a strong lattice field which could increase the stark-splitting in the ground-state levels of the dopants [23]. In the case of the Tm ions doping, a stark splitting as large as about 1094 cm−1 have been found [24]. Moreover, the fluorescence lifetime of as long as 2.03 ms indicates a promising high energy storage capacity of Tm:LSO crystal [24]. Together with a moderate thermal conductivity of 5.3 Wm−1K−1 [25], Tm:LSO crystal is desirable for investigation of realizing pulsed 2 μm lasers . Yao et al. have reported a CW Tm:LSO laser with a maximum output power of 0.67 W at 2058.4 nm, corresponding to a slope efficiency of 21% [26]. Other than that, however, no more report was found on Tm:LSO lasers at 2 μm .
In this paper, for the first time as far as we know, we have investigated the detailed lasing characteristics of Tm:LSO crystal operating in CW, wavelength tunable and graphene based Q-switching regimes. In CW regime, a maximum output power of 0.65 W at 2054.9 nm was obtained, corresponding to a slope efficiency of 21%. By using a quartz plate as the wavelength selector, tunable Tm:LSO laser was realized, which produced a total wavelength tunable range of 145 nm from 1936.0 nm to 2081.9 nm. When a graphene saturable absorber mirror (SAM) was employed, a passively Q-switched Tm:LSO laser was realized, and pulses with duration of 7.8 μs under a repetition rate of 7.6 kHz at 2030.8 nm were obtained, corresponding to a maximum pulse energy of 14.0 μJ.
2. Lasing characteristics of Tm:LSO crystal
2.1 CW operation
The schematic of the diode-pumped Tm:LSO laser is shown in Fig. 1. A fiber-coupled diode laser was used as the pump source with a maximum output power of 50 W at 790 nm at 20 °C. The fiber core with a numerical aperture of 0.22 was 100 µm in diameter. A 1:1 imaging module was employed to focus the pump light into Tm:LSO crystal. The 3 × 3 × 5 mm3 Tm:LSO crystal was grown by the Czochralski technique with 4 at.% Tm3+ ions doped. Both surfaces of the Tm:LSO crystal were antireflection coated from 750 to 850 nm (reflectivity < 2%) and 1930-2230 nm (reflectivity < 0.8%). Its absorption coefficient was calculated to be 3.3 cm−1 at 790 nm. The laser crystal was wrapped in indium foil and mounted in a copper block cooled to 12°C by water.
The CW operation of diode-pumped Tm:LSO laser was investigated first by using a X-type cavity consisting of mirrors M1, M2, M4 and OC1. M1 was an input mirror with antireflection coated from 750 to 850 nm (reflectivity < 2%) and high reflectivity coated (reflectivity> 99.9%) from 2010 to 2100 nm, M2 was a concave mirror with radius of 75 mm and high reflectivity coated (reflectivity> 99.9%) from 2010 to 2100 nm, M4 was a flat mirror also with high reflectivity coated (reflectivity> 99.9%) from 2010 to 2100 nm. In CW regime, four output couplers (OCs) with different transmissions of 0.5%, 1%, 2% and 3% were employed for comparisons. A laser power meter (MAX 500AD, Coherent, USA) was used to measure the average output power and a laser spectrometer with a resolution bandwidth of 0.4 nm was employed for measuring the output spectra (APE WaveScan, APE Inc.). The threshold absorbed pump powers were 0.37 W, 0.42 W, 0.47 W and 0.65 W for OCs of T = 0.5%, 1%, 2% and 3%, respectively. The power performance and output spectra characteristics for the CW Tm:LSO lasers with different OCs are shown in Fig. 2 and Fig. 3, respectively. In Fig. 2, the average output powers are plotted as a function of the absorbed pump power, which increased linearly with the augment of absorbed pump powers. A maximum average output power of 0.65 W at 2054.9 nm with a slope efficiency of 21% was achieved by using OC of T = 1%. The maximum average output powers of 0.43 W at 2053.7 nm, 0.58 W at 2056.1 nm and 0.45 W at 2053.1 nm were obtained with OCs of T = 0.5%, T = 2% and T = 3%, corresponding to slope efficiencies of 14.5%, 20.5% and 16.8%, respectively.
Fig. 2 Average output power versus absorbed pump power in CW regime for different OCs of T = 0.5%, 1%, 2% and 3%, respectively.
2.2 CW wavelength tunable operation
In the wavelength tunable experiment, the employed laser cavity was the same with the CW operation, except inserting a 2 mm thick birefringent quartz plate at Brewster's angle before the output coupler OC1. Since the CW output results indicated that the OC of T = 1% could give a high efficiency, the wavelength tunable lasing characteristics of the Tm:LSO crystal was studied with OC of T = 1% at the maximum absorbed pump power of 3.4 W. By rotating the quartz plate and aligning the OC carefully, the output wavelengths could be tuned from 1936.0 nm to 2081.9 nm with a total wavelength tunable range over 145 nm, corresponding to a FWHM of 100 nm. The output powers versus output wavelengths are shown in Fig. 4, from which relatively high efficiencies could be obtained from 1960 nm to 2060 nm. A maximum output power of 201.2 mW was obtained at 2036.7nm.
Fig. 4 Wavelength tunability of CW Tm:LSO laser at the absorbed pump power of 3.4 W with OC of T = 1%.
2.3 Q-switching operation
The characteristic of Q-switched Tm:LSO lasers was investigated by using a graphene SAM. The SAM was fabricated by transferring high-quality chemical-vapor-deposited graphene on a 2 μm high reflectivity plane mirror, which was the same process with the description in Ref [27]. In Ref [27], a passively Q-switched Tm:CLNGG laser was also realized by using such a graphene SAM, where pulses with duration of 9 µs under a repetition rate of 5.8 kHz were produced. In our experiment, two layers of graphene were transferred on a high reflectivity mirror. The passively Q-switched Tm:LSO laser characteristics were studied in a five mirror cavity formed by mirrors M1, M2, M3, OC2 and graphene SAM. M3 was a concave mirror with a radius of 50 mm and high reflectivity coated (reflectivity> 99.9%) from 2010 to 2100 nm. According to ABCD matrix theory, the calculated beam radius on graphene SAM was 23 × 24 μm2. The employed OC2 had a transmission of 1% for obtaining high efficiency and intracavity power intensity. By aligning the cavity mirrors and graphene SAM carefully, passive Q-switching operation was achieved as soon as the absorbed pump power exceeded 1.5 W. The average output powers as a function of absorbed pump power for passively Q-switched Tm:LSO laser are shown in Fig. 5. As can be seen, a maximum average output power of 106.3 mW was achieved at an absorbed pump power of 3.4 W, corresponding to a slope efficiency of 5.7%.
The output pulse trains were recorded by a digital oscilloscope (1 GHz bandwidth, Tektronix DPO 7102, USA) and a fast InGaAs photodetector with a rise time of 35 ps, (EOT, ET-5000, USA).The relationships between pulse repetition rates and pulse durations on the absorbed pump powers were recorded as shown in Fig. 6. From Fig. 6, we can see that the pulse repetition rates increased from 3.5 kHz to 7.6 kHz and the pulse durations decreased from 20 μs to 7.8 μs as the absorbed pumped power increased from threshold to 3.4 W. The long pulse duration in microsecond region was due to a low modulation depth of the 2 layer graphene and a long cavity in our experiment.
Fig. 6 The repetition rate and pulse duration versus absorbed pump power from Q-switched Tm:LSO laser.
With the measured average output powers and the repetition rates given in Fig. 6, the pulse energies could be calculated as shown in Fig. 7. A maximum pulse energy of 14.0 μJ was achieved at absorbed pump power of 3.4 W. The temporal profiles of pulse trains with repetition rate of 7.6 kHz and a single 7.8 μs pulse are shown in Fig. 8, from which it can be observed that the pulse to pulse fluctuation is less than 5%, indicating a nice Q-switching stability.
Fig. 8 The temporal profiles of the pulse trains from passively Q-switched Tm:LSO laser with OC of T = 1%.
The emission wavelength of the graphene passively Q-switched Tm:LSO laser was located at 2030.8 nm without variation when the absorbed pump power increased from threshold to 3.4 W, which is shown in Fig. 9. However, this emission wavelength was slightly blue shifted compared with 2054.9 nm in CW running regime. We attribute the spectral blue shifting to the increased inversion rate introduced by the insertion losses of the graphene SAM in a typical three-level laser system [28].
3. Conclusion
In this paper, the characteristic of CW, wavelength tunable and graphene passively Q-switched Tm:LSO lasers were demonstrated for the first time. In CW regime, the Tm:LSO laser produced a maximum average output power of 0.65 W at 2054.9 nm with a slope efficiency of 21%. With a quartz plate as wavelength selector, the Tm:LSO laser could be wavelength tuned within a total wavelength range of 145 nm from 1936.0 nm to 2081.9 nm, corresponding to a FWHM of 100 nm. In passive Q-switching regime, the Tm:LSO laser yielded 7.8 μs pulses at 2030.8 nm under a repetition rate of 7.6 kHz, corresponding to a pulse energy of 14.0 μJ, indicating that Tm:LSO crystal is a potential candidate for producing high energy pulses at 2 μm.
Acknowledgments
The work was supported by National Natural Science Foundation of China (61008024, 61205145, 61078031, 60908030, 60938001), Research Award Fund for Outstanding Middle-aged and Young Scientist of Shandong Province (BS2011DX022), Independent Innovation Foundation of Shandong University, IIFSDU (2012JC025), and Innovation Project of Shanghai Institute of Ceramics (Y04ZC5150G). The authors also acknowledge the support from Prof. Thomas Dekorsy in Konstanz University, Germany.
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