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Abstract
A 785 nm diode-pumped nanosecond passively Q-switched (PQS) 2.3 µm Tm:YLF laser using a ReSe2-based saturable output coupler (SOC) was reported for the first time. The SOC combined the function of a passive Q-switch and output coupler, greatly reducing the additional insertion loss compared with a conventional separate structure. The modulation depth, saturation intensity, and nonsaturable loss for this ReSe2-based SOC were determined to be 1.3%, 1.7 GW/cm2, and 0.2%, respectively. A maximum average output power of 486 mw, a shortest pulse width of 716 ns, and a repetition rate of 5.0 kHz were obtained under the absorbed pump power of 7.21 W. The nanosecond pulses make the diode-pumped compact 2.3 µm thulium solid-state lasers more attractive for practical applications.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Middle infrared laser sources operating in the range of 2.3 µm are in great demand for a variety of applications. The 2.3 µm waveband corresponds to the weak absorption of water and strong absorption of N2O (2.28 µm), CO (2.36 µm) and CH4 (2.37 µm) according to the HITRAN database. Therefore, the 2.3 µm laser source can find many interesting applications in the detection of important trace gases [1,2]. In medical applications, human skin tissue exhibits a relative maximum in transmission extending from 2.1 to 2.4 µm and glucose has a characteristic absorption fingerprint in this spectral window. Thus, the 2.3 µm laser can be used in noninvasive blood glucose measurements [3–5]. Moreover, pump lasers operating above 2.1 µm are typically preferred in order to minimize absorption losses in ZnGeP2 mid-IR optical parametric oscillators [6,7]. Transition metal Cr2+ doped II-VI compounds represented by Cr:ZnS and Cr:ZnSe can produce laser emission covering 2.3 µm wavelength [8–10]. But the development of this type of laser is limited by crystal quality and cost-effective 1.5-2.0 µm pump sources. In addition, InP, GaSb and GaInAsSb/AlGaAsSb semiconductor laser sources emitting in 2.3 µm spectral region have been realized [11–14]. They usually exhibit quite low output powers of tens of milliwatts.
An alternative promising approach to producing laser emission around 2.3 µm wavelength is exploring the 3H4-3H5 transition of the Tm3+-doped gain media. As has been shown in previous studies, continuous-wave (CW) 2.3 µm Tm-doped solid-state lasers with different pumping sources have been realized [15,16]. As for the Q-switched operations with 2.3 µm in Tm-doped lasers, to the best of our knowledge only one article on PQS Tm:YLF laser at 2.3 µm has been reported so far. A Cr2+:ZnSe was used to passively Q-switch the Tm:YLF laser pumped by a 780 nm Ti:sapphire laser. Microsecond Q-switched pulses at 2309 nm with an average output power of 27 mW, a pulse width of 1.2 µs and a pulse repetition rate of 2.1 kHz were obtained [17]. The avenue to elevate the performance of 2.3 µm thulium laser in terms of high average output power, short pulse width, and convenience of application remains to be explored.
As a new two-dimensional (2D) transition metal chalcogenide, ReSe2 has the characteristics of low lattice symmetry and weak interlayer coupling. As a result of the distorted 1 T structure, the bandgap of ReSe2 shows weak dependence on the layer number [18]. These properties make ReSe2 exhibit different physical properties from other 2D transition metal chalcogenides. In comparison with conventional saturable absorbers (SAs), such as Cr:ZnSe and Cr:ZnS, these 2D-materials-based SAs have fascinating advantages of controllable modulation depth, ultrafast recovery time, and broadband saturable absorption, as well as easy fabrications. Using ReSe2 as SA, microsecond PQS lasers at wavelengths of 1.55 and 1.9 µm have been realized respectively [19,20]. Our group have experimentally demonstrated the nanosecond laser performance of ReSe2 PQS Tm solid-state lasers with broadband multiple wavelengths covering the 1.87 to 2.05 µm [21]. The potential of using ReSe2 to generate pulse operation at other attractive wavelengths remain to be further developed.
In this paper, we experimentally demonstrated the nanosecond laser performance of ReSe2 PQS 2.3 µm Tm:YLF laser for the first time. The ReSe2 was directly deposited on the surface of the output mirror, forming a compact configuration of saturable output coupler (SOC). The modulation depth, saturation intensity, and nonsaturable loss for the SOC were determined to be 1.3%, 1.7 GW/cm2, and 0.2%, respectively. A 785 nm LD was used to pump the Tm:YLF gain medium inside a linear cavity. In the CW operation, a maximum output power of 1.15 W with a slope efficiency of 19% was achieved. Nanosecond Q-switched pulses with a maximum average output power of 486 mW, a minimum pulse width of 716 ns, and a pulse repetition rate of 5.0 kHz were obtained. Due to the small insertion loss of SOC, the ratio between the achievable Q-switched average output power to the continuous-wave output power is up to 42.3%. The pulse width of 716 ns was the shortest in the 2.3 µm PQS Tm-doped solid-state lasers reported so far.
2. Preparation and characterization of the ReSe2 SA
The ReSe2 SA was prepared by a liquid phase exfoliation method. First, ReSe2 bulk with purity of 99.99% was ground into powder. Then the ReSe2 powder was mixed with alcohol followed by 5-hrs sonication. After that, the as-prepared ReSe2 solution was centrifuged at 2500 rmp for 10 min. In order to efficiently integrate the SA with the laser resonator, the ReSe2 solution was directly deposited on one surface of the plano-plane output mirror, forming a compact configuration of SOC to reduce the insertion losses. Finally, the as-prepared sample was placed under an infrared oven lamp to dry. Thus, an ReSe2-based SOC was prepared successfully. An atomic force microscopy (AFM) was taken using a Nanosurf Naio AFM, as shown in Fig. 1. The AFM image showed the thickness of the ReSe2 sheets was about 0.4 µm. The absorption spectrum of ReSe2 ranging from 400 to 2400 nm was measured, as shown by Fig. 1. The broadband absorption potential make ReSe2 ideal SAs for generating pulses from the visible to mid-infrared region.
The nonlinear optical properties of ReSe2-based SOC were characterized using an open aperture Z-scan method based on an optical parametric amplifier (OPA) pumped by a 1030 nm laser with a pulse duration of 120 fs. The emitting wavelength of the OPA was fixed at 2.3 µm, and the laser beam was focused to 50 µm at the focus position within the sample. As shown in Fig. 2(a), the transmittance as a function of the sample position exhibits sharp and narrow peaks, showing that the transmittance of the SOC increased with the excited intensity due to the saturable absorption effect of ReSe2. Figure 2(b) shows the nonlinear transmittance curve versus the excited intensity which is translated from the open aperture Z-scan curve as shown in Fig. 2(a). The corresponding data can be fitted by T = 1 − ΔR·exp (−I/Is) − Tns, where ΔR is the modulation depth, Is is the saturation intensity, Tns is the nonsaturable loss, and I is the exciting intensity. By fitting the curve, the modulation depth, saturation intensity, and nonsaturable loss were determined to be 1.3%, 1.7 GW/cm2, and 0.2%, respectively.
3. Experimental setup
In order to achieve the 2.3 µm lasing in Tm3+ single-doped lasers, it is necessary to use the host material with low doping concentration and low phonon energy [22]. Low doping concentration can weaken the concentration-dependent cross relaxation process in Tm3+ ions (3H6 + 3H4→3F4 + 3F4), which is in contrast to the high concentrations required for efficient 2 µm laser operation. Low phonon energy helps to reduce the probability of nonradiative transition, ensuring relatively good fluorescence quantum efficiency from the upper level 3H4. The fluoride crystals present a lower phonon energy compared to conventional oxide materials, and also have the advantages of reasonably high thermal conductivity, good mechanical hardness, and high chemical stability in comparison with other low phonon energy materials such as sulfides and chlorides. Based on the above considerations, Tm:YLF with a doping concentration of 1.5 at.% has been used to be an efficient Tm-doped crystal for 2.3 µm laser. The unpolarized fluorescence spectrum of the used 1.5 at.% a-cut Tm:YLF was first measured by a spectrofluorometer (Edinburgh Instruments, FS980). Figure 3 shows the smooth emission spectrum peaked at 2308 nm with a FWHM width of 60 nm. The broad fluorescence spectrum should be caused by the high multiplicity of crystal-field split Stark levels coupled with significant phonon broadening in the crystal. The fluorescence lifetime of the 3H4 manifold for this 1.5 at.% Tm:YLF was measured to be approximately 860 µs. The longer upper-level lifetime benefits from the lower multi-phonon relaxation rate of YLF crystal and reduced concentration quenching. It can enable the 2.3 µm Tm:YLF laser to have a lower room temperature lasing threshold under the CW LD pumping.
In comparison with the significant complexity and relative low output power of Ti:sapphire laser, the commercially available high power 0.78-0.8 µm AlGaAs LD provide the possibility as pumping source for developing compact high power 2.3 µm thulium solid-state lasers [16]. The experimental arrangement for the LD end-pumped PQS 2.3 µm Tm:YLF laser is shown schematically in Fig. 4. The pump source was a fiber-coupled 785 nm CW LD with a core diameter of 400 µm and numerical aperture of 0.22. Its radiation was coupled into the laser crystal by a 1:1 focusing optical system. The 33 mm long laser resonator comprised an input mirror M1, a Tm:YLF crystal, and a ReSe2-based output coupler M2. M1 was a plano-concave mirror with 300 mm radius of curvature. According to the measured fluorescence spectrum, the coatings of M1 was designed to be antireflection coated at 785 nm on one surface, high-reflection coated at 2250-2380 nm (R > 99.8%) and high-transmission coated at 785 nm (T > 98%) on the other surface. The output coupler M2 was a flat mirror coated with T = 1.5% transmission at 2250-2380 nm. The 1.5 at. %, 4 × 4 × 8 mm3, and a-cut Tm:YLF crystal was wrapped with indium foil and mounted in copper block cooled by water at a temperature of 16°C. Both light-passing ends of the Tm:YLF were antireflection coated at 760-810 nm and 2250-2380 nm.
4. Experimental results and discussions
The CW laser operation was investigated firstly. Figure 5(a) shows the input-output characteristics of the Tm:YLF laser in CW operation. The absorbed pump power was measured under lasing conditions as follows. The pump power transmitted through the input mirror M1 was first measured, which was denoted as the incident pump power. Then the residual pump light passing through the output coupler M2 was separated from the laser radiation by a bandpass filter. Further considering the partial reflection by the output coupler M2, the pump absorption efficiency under lasing conditions was determined to be 32% with respect to the incident pump power. A maximum output power of 1.15 W was obtained with a slope efficiency of 19%, as shown in Fig. 5(a). The CW output power of 1.15 W is the highest reported so far for the 2.3 µm thulium lasers. The output power was linearly increased, and showed no output power saturation, indicating that further power scaling should be achieved by increasing the pump power. The output laser beam was measured to be π polarized. By using a laser beam profiling system (NanoScan by PHOTOH, Inc), the M2 factor were fitted to be 1.1 in A1 direction and 1.6 in A2 direction at the maximum average output power. The laser beam profile and power intensity distribution are shown in Fig. 5(b). Our results demonstrate that the LD pumping is effective for the power scaling of 2.3 µm thulium solid-state lasers with high optical conversion efficiency.
Fig. 5. (a) Output power versus absorbed pump power under CW and PQS operation. (b) The three-dimensional beam profile and power intensity distribution.
By inserting the ReSe2-based SOC into the cavity, typical PQS operation was realized. There was no damage to the absorber during the experiment. Compared to the 1.2 W threshold pump power in CW operation, the pumping threshold for the ReSe2-based Q-switching was increased to 2.2 W. The dependence of the average output power on the absorbed pump power is also shown in Fig. 5(a). A maximum average output power of 486 mW was achieved with a slope efficiency of 9%. The ratio between the achievable Q-switched average output power to the CW output power is up to 42.3%, which can be attributed to the small insertion loss of ReSe2-based SOC. The optical spectrum of the PQS Tm3+:YLF laser was further measured by an optical spectrum analyzer (Yokogawa, AQ6375), as shown in Fig. 6(a). Owing to the broadband gain of Tm:YLF, typical multi-wavelengths emissions around 2306 nm appear in the PQS operation. The lasing spectrum was similar with that in the CW operation. Figure 6(b) depicts the pulse width (full width at half maximum) and pulse repetition rate versus the absorbed pump power. The pulse width varied from 3.6 µs to 716 ns, while the pulse repetition rate presented the variations of 0.9-5.0 kHz with increasing the pump power. Figure 7 shows the temporal waveform of the 716 ns pulse and stable pulse train of the 5.0 kHz repetition rate. The pulse-to-pulse intensity instability in the pulse trains was <6%. The instability should be mainly attributed to heating of the SA by the non-absorbed pump radiation. Compared to the previous report in Ref. [17], the laser performance of the PQS 2.3 µm Tm:YLF laser in terms of average output power and pulse width has been greatly improved. Such LD pumped nanosecond solid-state 2.3 µm laser sources should be interesting to pump optical para metric oscillators for mid-infrared generation.
Fig. 6. (a) Lasing spectrum of Tm:YLF CW and PQS laser. (b) The pulse width and pulse repetition rate versus the absorbed pump power.
Fig. 7. Typical pulse train and single pulse with the pulse width of 716 ns at the repetition rate of 5.0 kHz.
5. Conclusions
In conclusion, the experimental results demonstrated the possibility of generating nanosecond pulses by using a ReSe2-based SOC in the Tm:YLF laser at 2.3 µm. Due to the small insertion loss of ReSe2-based SOC, the ratio between the achievable Q-switched average output power to the continuous-wave output power is up to 42.3%. Nanosecond PQS operation was further realized, generating pulses with an average output power of 486 mW, a pulse width of 716 ns, and a pulse repetition rate of 5.0 kHz.
Funding
National Natural Science Foundation of China (NSFC) (61875077, 61605068, U1730119, 61911530131); Applied Basic Research Programs of Xuzhou (KC17085); The Natural Science Foundation of the Jiangsu Higher Education Institutions of China (18KJA510001); The Priority Academic Program Development of Jiangsu Higher Education Institutions.
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