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Abstract
Passive, continuous-wave mode-locked (CWML) operation of a 1.83 µm Tm:YLF laser is experimentally demonstrated for the first time, to the best of our knowledge. Two specially selected output couplers are used to realize this operation. Stability of the CWML laser is obtained with a commercial semiconductor saturable absorber mirror. The maximum average output power is 1.04 W with a pulse duration of 107 ps and repetition rate of 54.1 MHz. Further, a 0.1 mm fused-quartz Fabry-Perot etalon is used to tune the central wavelength of the stable CWML laser at 1827.2 nm, 1829.5 nm, 1831.9 nm, and 1833.5 nm.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Ultrashort pulse lasers that play a vital role in material processing, nonlinear optics, laser surgery, and so on can be achieved by mode-locking technique [1–4]. Tm3+ doped materials are widely used as the gain media for building mode-locked laser sources around 2 µm [5]. In recent years, mode-locked Tm3+ bulk lasers have been extensively studied [6–23]. Most of these lasers operated in the spectral range of 1.9-2.1 µm [6–17]. Some actively mode-locked Tm:YLF lasers have been demonstrated, and their pulse widths were more than 170 ps [15,16]. In 2018, an 1.91 µm passively mode-locked Tm:YLF laser with pulse duration of 31 ps was demonstrated [17]. The operation wavelength of 2.3 µm Tm:YLF laser has also been demonstrated in few studies [18,19]. However, the operation of these lasers at wavelength shorter than 1.9 µm has been rarely reported [20–23]. In 2010, a passively mode-locked Tm:GdLiF4 laser operating at 1886 nm with 47 mW output power and pulse width of 20 ps was demonstrated [20]. In 2011, a passively mode-locked Tm:YLF laser using a single-walled carbon nanotubes based saturable absorber was demonstrated, and 1.888 nm laser pulses with duration of 19 ps and 55 mW output were generated [21]. In 2017, a stable continuous-wave mode-locked (CWML) Tm:CYA laser at 1874 nm was reported, and maximum average output power of ∼0.6 W with pulse duration of 46 ps were obtained [22]. In 2019, a CWML Tm:CaF2 laser emitting at 1886 nm with 132 mW output power has been reported, and pulse duration with tens of picoseconds was obtained [23]. However, mode-locked Tm3+ bulk lasers at wavelengths shorter than 1.87 µm have not been reported yet.
Laser sources emitting around 1.8 µm have garnered considerable research attention due to their potential applications in gas detection, polymer welding, Cr2+ ion laser pumping, and infrared neural stimulation [4,24–28]. Cr:ZnSe and Cr:ZnS are two kinds of intensively investigated laser materials for the direct lasing of 2 to 3 µm wavelength band [26]. Compared with 1.88 µm or 1.9 µm wavelength, the absorption cross section of Cr:ZnSe at 1.83 µm or the one of Cr:ZnS at 1.83 µm is much larger [26], so 1.83 µm pump could result in more efficient absorption if a disk geometry is employed for the Cr:ZnSe or Cr:ZnS crystal. For neurology application, water absorption at 1.83 µm is less than at 1.88 µm or at 1.9 µm, which could enable greater optical penetration depth [27]. Several Tm3+-doped bulk lasers operating at wavelength shorter than 1.85 µm have been reported [29–34]. When the levels of Tm3+ ion (3F4→3H6) are used for lasing, the major challenge in building a short wavelength Tm3+ doped bulk laser is the nonnegligible reabsorption effect resulting from the quasi-three level scheme of Tm3+ at short wavelengths. There are several methods to shift laser operation wavelength, such as the usage of intra-cavity birefringent filters or etalon [33]. However, intra-cavity wavelength tuners tend to add some additional loss, which degrades the laser performance. By controlling the transmittance of the output coupler (OC) at the emission peaks of Tm:YLF, a highly efficient and high-power continuous-wave (CW) Tm:YLF laser operating around 1.83 µm has been demonstrated [34].
In this study, the passive CWML operation of Tm:YLF laser at 1.83 µm is demonstrated for the first time, to the best of our knowledge. Two specially selected OCs are used to realize this operation. Under the output coupling of 28%, stable 1.83-µm CWML laser is obtained with a maximum average output power of 0.4 W and corresponding pulse duration of 150 ps at a repetition rate of 54.1 MHz. Using a 0.1 mm Fabry-Perot (F-P) fused-quartz etalon, we have tuned the wavelength of this stable CWML laser to 1827.2 nm, 1829.5 nm, 1831.9 nm, and 1833.5 nm. Under the output coupling of 38%, a maximum average output power of 1.04 W with corresponding pulse duration of 107 ps is achieved. The mode-locked spectrum is centered at 1831.9 nm with a full width at half maximum (FWHM) of 0.067 nm.
2. Experimental setup
Figure 1 shows the experimental setup for obtaining the passively mode-locked Tm:YLF laser. The pump source is a fiber-coupled diode laser emitting at 793 nm. The coupling fiber has a core diameter of 105 µm and numerical aperture of 0.22. The pump beam is collimated by lens f1 with a focal length of 35 mm and then focused by lens f2 with a focal length of 80 mm. The spot diameter of the pump beam is calculated as ∼240 µm with the software WinABCD. A 4 mm × 4 mm × 7 mm a-cut Tm:YLF crystal with 3 at. % doping concentration is used as the gain medium with high transmission (HT) coating for 780-800 nm and 1800-2050 nm. The laser crystal is wrapped with indium foil and mounted in a copper heat sink, whose temperature is maintained at 18 °C by a water chiller.
Fig. 1. Schematic of the experimental setup for CWML Tm:YLF laser. Here, BS: beam splitter, LD: laser diode, OC: output coupler, M: mirror, F-P: Fabry-Perot etalon, and SESAM: semiconductor saturable absorber mirror.
The resonator comprises of three curved mirrors: three plane mirrors and one semiconductor saturable absorber mirror (SESAM). M1, M3, and M5 are flat mirrors with HT coating for 780-800 nm and high reflectivity (HR) coating for 1800-2050 nm. M2 and M4 are curved mirrors with a radius of curvature (ROC) of 500 mm, HT coating for 780-800 nm, and HR coating for 1800-2050 nm. OC is also a curved mirror with ROC = 200 mm. OC acts as a folded mirror, so the effective transmission is doubled. Two OCs are used in the experiment, and their transmittance at 1830 nm, 1880 nm, and 1908 nm are summarized in Table 1. For realizing high-efficiency operation at 1.83 µm, it is crucial to use an OC with high transmission at this wavelength. A commercial SESAM (Batop Gmbh, SAM-2000-2-10 ps) is used to initiate and stabilize the mode-locking. It has a working wavelength in the range of 1700-2150 nm, a modulation depth of 1.2%, a non-saturation loss of 0.8%, a saturation flux of 70 µJ/cm2, and a relaxation time of 10 ps. The physical length of the resonator is ∼2760 mm. Using the ABCD matrix propagation theory, we calculated the laser mode radius to be ∼105 µm in the laser crystal and ∼45 µm on the SESAM. In addition, a 0.1 mm F-P fused-quartz etalon is used to tune the central wavelength of the mode-locked Tm:YLF laser. Because the OC is a folding mirror, the laser output has two branches (1st branch and 2nd branch in Fig. 1). The laser beam in the 2nd branch is divided by a beam splitter (BS) for characterization. The pulse train of CWML laser is detected by the detection system, which consists of a high-speed detector (EOT5000, bandwidth: 12.5 GHz) and a high-speed digital oscilloscope (Tektronix DPO 71254C, bandwidth: 12.5 GHz, sample rate: 100 Gs/s). The laser spectrum is recorded by an optical spectrum analyzer (YOKOGAWA, AQ6375B).
Table 1. Transmission of the two OCs (TOC) at three wavelengths.
3. Results and discussion
The CW laser performance is preliminarily characterized by replacing the SESAM with a HR mirror. The absorption efficiency of pump light is ∼30% at pump power of 16 W. The absorption efficiency could be improved a lot if a 681 nm LD is used as pump source [35]. For OC1, the obtained maximum output power is 0.76 W at the absorbed pump power of 3.3 W and the slope efficiency is 42%. The output laser is linearly polarized due to YLF is a uniaxis crystal. For OC2, the obtained maximum output power is 1.51 W at the absorbed pump power of 4.8 W and the slope efficiency is 59%. If the pump power continues to increase, the CW output power becomes unstable, which can be attributed to the thermal lensing effect. The central wavelengths for OC1 and OC2 are 1832.16 nm and 1832.18 nm, respectively. Based on the analysis in [34], it is expected that the laser wavelength is 1880 nm if OC2 is used as an end mirror, but it can shift to 1830 nm if OC2 is used as a folding mirror because the transmission at 1880 nm is increased by 76%. We experimentally prove this conjecture by directly reflecting the laser beam through OC2.
When OC1 is used, stable CWML operation is achieved. Figure 2(a) shows the dependence of the CWML output power on the absorbed pump power. The slope efficiency of CWML is 32% with respect to the absorbed pump power. Initially, the laser operates in the CW state. When the absorbed pump power increases to 2.13 W, Q-switched mode locking (ML) is observed. Stable CWML operation is realized when the absorbed pump power increases to 2.48 W. The maximum output power of 0.4 W is obtained at the absorbed pump power of 2.98 W. The mode-locked state becomes unstable and pulse splitting is onset if the pump power continues to increase. As shown in Fig. 2(b), the pulse duration of CWML laser is measured as 150 ps by the high-speed oscilloscope. The fastest response time of the detection system is evaluated as ∼73 ps using a commercial ultra-fast laser with a pulse width of 10 ps, which is shown in the inset of Fig. 2(b). The pulse trains recorded with two different time scales are shown in Fig. 2(c), which manifest the stability of the CWML operation. The stability of CWML operation is also evaluated by analyzing the radio-frequency (RF) spectrum, as shown in Fig. 2(d). It can be seen that the RF spectrum has a signal-noise ratio of ∼53 dB for the fundamental beat note. The harmonic beat notes indicate stable ML without any Q-switching. The pulse repetition rate of ∼54.1 MHz is in agreement with the laser cavity length of 2.76 m. The pulse energy and peak power at the output power of 0.4 W is calculated to be 3.7 nJ and 24 W, respectively.
Fig. 2. CWML results obtained using OC1. (a) Output power versus absorbed pump power. (b) Pulse duration of CWML laser (inset: fastest response of the detection system). (c) Pulse trains recorded at time scales of 20 ns and 31.25 µs. (d) RF spectrum of CWML laser (inset: RF spectrum recorded from 0 Hz to 1 GHz).
The CWML laser spectrum is shown in Fig. 3(a), which is centered at 1831.36 nm with FWHM of 0.07 nm. According to the equation $\Delta \upsilon \ast \Delta \tau \textrm{ = }0.441$ for a Gaussian pulse, where $\Delta \tau$ is the pulse width and $\Delta \upsilon$ is the linewidth of the mode-locked laser spectrum, the minimum pulse width is theoretically calculated as 70 ps. The wavelength tuning of the CWML laser is also investigated using the 0.1 mm F-P fused-quartz etalon when OC1 is used. The stable CWML operation at different wavelengths are obtained. As shown in Fig. 3(b), when the absorbed pump power is 2.93 W, the output power at 1827.2 nm, 1829.5 nm, 1831.9 nm, and 1833.5 nm is 0.1 W, 0.14 W, 0.19 W, and 0.19 W, respectively. We used a software PCModeWin to calculate the atmosphere transmission which is shown in the following Fig. 4 [36]. The setting conditions of the software are 1976 US standard, Rural-Vis = 23 km, Observer Height = 10 m., Path Length = 5 m. The blue triangles denote the oscillating wavelengths at 1827.2 nm, 1829.5 nm, 1831.9 nm, and 1833.5 nm, respectively. Obviously, the atmosphere absorption lines are a main reason for the discrete wavelengths.
Fig. 3. (a) Spectrum of the CWML laser, and (b) tunability of the CWML laser with the F-P etalon when OC1 is used.
Fig. 4. The wavelength of CWML laser and the calculated corresponding water absorption lines in atmosphere [36].
For obtaining higher output power from the CWML laser, OC2 is used. Figure 5(a) shows the dependence of the CWML output power on the absorbed pump power. The slope efficiency of CWML is 49% with respect to the absorbed pump power. The threshold pump power is elevated to 2.5 W. Stable CWML operation is realized when the absorbed pump power reaches 3.79 W. The maximum output power of 1.04 W is obtained at the absorbed pump power of 4.49 W. Similar to the trend for OC1, pulse splitting is observed when pump power is above 4.5 W. As shown in Fig. 5(b), the pulse duration of CWML laser is 107 ps. The narrower pulse duration obtained with OC2 should be attributed to the higher pulse energy.
Fig. 5. CWML results for OC2. (a) Output power versus absorbed pump power in the CWML regime. (b) Single CWML laser pulse (Inset: Corresponding spectrum of the CWML pulse). (c) Pulse trains recorded with time scales of 20 ns and 31.25 µs. (d) RF spectrum of CWML laser (Inset: RF spectrum recorded from 0 Hz to 1 GHz).
The inset of Fig. 5(b) shows the corresponding spectrum for the CWML operation, where the central wavelength is 1831.36 nm and FWHM is 0.067 nm. We try inserting a fused quartz plate (5 mm thick, GDD=−68 fs2/mm) into the laser cavity, but no shorter pulse can be obtained. The pulse duration is limited by the narrow spectral width which is caused by high transmission of the used OCs and the atmosphere absorption lines around 1.83 µm. The pulse trains recorded with two different time scales are shown in Fig. 5(c). The stability of the CWML operation is also evaluated by analyzing the RF spectrum, as shown in Fig. 5(d). It can be seen that the RF spectrum has a signal-noise ratio of ∼55 dBm for the fundamental beat note. The harmonic beat notes indicate stable mode-locking without any Q-switching. The pulse energy and peak power at the output power of 1.04 W is calculated to be 10.3 nJ and 96 W, respectively.
4. Conclusion
We demonstrated the stable CWML operation of a Tm:YLF laser at 1.83 µm for the first time, to the best of our knowledge. Pulses with duration of 107 ps and 150 ps with a repetition rate of 54.1 MHz were achieved. Under the output coupling of 38%, the stable CWML laser had a maximum average output power of 1.04 W. In addition, the wavelength tunability of the CWML laser was realized using a 0.1 mm F-P fused-quartz etalon. The operation wavelength of 1.83 µm is the shortest wavelength achieved among the CWML Tm-doped bulk lasers reported until now. Among CWML Tm-doped bulk lasers with wavelength short than 1.9 µm, the average power of 1.04 W of this work is the highest. The absorption of water vapor at 1.83 µm is much less than that at 1.9 µm, which makes the 1.83 µm CWML laser more suitable for some potential applications.
Funding
National Natural Science Foundation of China (61405126, 61704112, 61775146); Shenzhen Science and Technology Project (JCYJ 20160427105041864, JCYJ20170817094438146, ZDSYS201707271014468); Educational Commission of Guangdong Province (2016KCXTD006).
Disclosures
The authors declare no conflicts of interest.
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