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Abstract
In this paper, a laser diode (LD) pumped passive mode-locking Tm,Ho:GAGG laser based on a semiconductor saturable absorber mirror (SESAM) is reported. By adjusting the group delay dispersions inside the laser cavity and transmissions of the output couplers (OCs), a shortest pulse duration of 10.84 ps at 2089.9 nm is achieved, the average output power is 33.17 mW and the laser runs at a 83.01 MHz repetition rate. A maximum average output power of 66.43 mW is also obtained at 2089.9 nm with a pulse duration of 16.56 ps by using an OC of 3%. To the best of our knowledge, this is the first report on the mode-locking Tm,Ho:GAGG laser.
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1. Introduction
With the fast development of ultrafast optics, the ultrafast laser has been already turned out to be excellent tools for material micro-processing [1]. Especially, 2 micrometer ultrafast lasers also have other important applications in various fields such as laser lidar, laser surgery, environmental monitoring [2–4]. During the last few years, a rapid progress has been made in the field of direct generation of 2 μm ultrafast pulses based on Tm3+ or Ho3+ co-doped laser crystals. Together doped with Tm and Ho ions, the laser crystal features the merits of capable of pumping around the visible wavelength region but simultaneous with a natural emission slightly above 2 μm wavelength region, which can avoid the water vapor absorption and subsequently benefit for the generation of mode-locking ultrafast laser pulse. To date, laser pulses with short pulse durations below 100 fs have been already realized with Ti:sapphire laser pumping. For example, 87 fs laser pulses with a maximum output power of 27 mW at 2043nm are achieved from a Tm,Ho:CaYAlO4 laser [5]. Moreover, 73 fs laser pulses with 36 mW output power at 2061nm in a Tm,Ho:CNGG laser [6], 67 fs laser pulses with 123 mW output power at 2083nm in a Tm,Ho:CLNGG laser [7], 52 fs laser pulses with 376 mW output power at 2015nm in a Tm,Ho:CALGO laser [8], 46 fs laser pulses with 121 mW output power at 2033nm in a Tm,Ho:CaGdAlO4 laser [9] also have been reported. Compared with Ti:sapphire laser pumping source, LD holds the advantages of low-cost, compact and easy power scaling up. But the intrinsic weak brightness of LD result in a low efficiency and weak Kerr effect in a typical LD-pumped Tm,Ho co-doped laser, which makes the achieved laser pulse duration yielded from a LD-pumped Tm,Ho co-doped mode-locking laser mainly in the range of picosecond. Such as: 10.5 ps laser pulses with 151 mW output power at 2041nm is achieved in a SESAM mode-locking Tm,Ho:YVO4 laser [10], 5.2 ps laser pulses with 40 mW output power at 2051nm is obtained in a Graphene mode-locking Tm,Ho:LiYF4 laser [11]. Only recently, a femtosecond LD pumped mode-locking Tm,Ho:CLNGG laser based on SESAM is reported, which delivers a pulse duration of 212.8 fs at 2093.4 nm [12].
On the other hand, the laser performance of 2 μm mode-locking lasers strongly depend on the properties of the employed Tm,Ho doped laser crystals. The garnet crystals have been already turned out to be excellent host laser crystals for doping. With garnet crystals, 56.9 ps laser pulses with a maximum output power of 285 mW at 2091 nm and 2097 nm are achieved in a Tm,Ho:YAG laser [13], 38 ps laser pulses with a maximum average output power of 1.21 W at 2022.9 nm are obtained in a Tm:LuAG laser [14]. Gadolinium gallium garnet (GGG) is one kind of garnet crystal with similar advantages like high thermal conductivity, good mechanic properties, and large damage threshold. By substituting a part of Ga ions with Al ions in the GGG crystal, a novel garnet crystal Gd3Ga3Al2O12 garnet (GAGG) is developed. The disordered structure of GAGG can effectively broaden the laser spectral bandwidth but still maintain good properties of the garnet crystal [15,16]. To date, the mode-locking GAGG laser has been already realized in the spectral range of 1 μm wavelength region with Nd3+ or Yb3+ ions doping. In the year of 2010, a LD pumped mode-locking Nd:GAGG laser is reported with laser pulses of 3.7 ps at 1061.4 nm and 5.9 ps at 1062.7 nm [17]. Laser pulses of 643 fs at 1041 nm and 493 fs at 1035.5 nm are also achieved in mode-locking Yb:GAGG lasers [18,19]. In the 2 μm wavelength region, the fluorescence spectrum of the Tm,Ho:GAGG crystal is investigated by employing a pulsed LD for pumping, which shows a broad wavelength range from 1600 nm to 2170 nm [20]. A LD pumped continuous wave (cw) Tm,Ho:GAGG laser is also reported with a maximum average output power of 190 mW [21]. Other than that, there is no report on the laser performance of Tm,Ho:GAGG crystal in the 2 μm wavelength region.
In this letter, we report a LD pumped Tm,Ho:GAGG laser. In the cw operation regime, a maximum output power of 448 mW is achieved with a 3% transmission output coupler. By introducing a birefringent filter inside the laser cavity, the Tm,Ho:GAGG laser could operate from 2041 nm to 2118 nm, corresponding to a wavelength tunable range of 77 nm. In the SESAM mode-locking regime, a shortest pulse of 10.84 ps at 2089.9 nm is obtained, corresponding to an average output power of 33.17 mW. A maximum average output power of 66.43 mW is also obtained with a pulse duration of 16.56 ps. The experimental results show that the Tm,Ho:GAGG crystal is suitable for the laser operation around 2 μm wavelength region.
2. Experimental setup and results
The schematic diagram of the Tm,Ho:GAGG laser is an x-type cavity, as shown in Fig. 1. The pump source is a fiber-coupled LD (BWT, DS3-51412-K793DAERN-16.00W) with a maximum output power of 30 W. The stable center wavelength of the LD is 793 nm with width of 3.2 nm. The core diameter and numerical aperture of the coupled fiber is 105 μm and 0.22, respectively. The coupling system consists with two convex lenses with the radius of 50 mm. The pump beam is collimated into a 6.5 mm long, 5 mm×5 mm Tm,Ho:GAGG crystal (generated by the Institute of Crystal Materials at Shandong University) by a coupling system with a beam radius of 50 μm, which matches well with the calculated laser beam waist of the oscillating laser (39 μm). The doping concentration of the Tm3+ and Ho3+ ions is 5.2 at. % and 1.8 at. % in the Tm,Ho:GAGG crystal, respectively. Besides, the ratio of the Ga3+ ions and Al3+ ions is 3:2. The laser crystal is placed at the Brewster’s angle and mounted into a heatsink with water-cooled at 14 °C. The four-mirror laser cavity consists of a plane high-reflectivity mirror (M3), two concave mirrors with the radius of curvature of 75 mm (M1 and M2), and a plane output coupler (OC). In the mode-locking laser cavity, another additional concave mirror M4 with the radius of curvature of 50 mm is employed to focus the laser beam on to a SESAM. Two chirped mirrors (CM1 and CM2) are employed in one arm of the laser cavity for intracavity dispersion compensation. Apart from the OC, all of these cavity mirrors are high reflecting coated from 1800nm to 2200 nm and high transmitting coated from 750 nm to 850 nm. The employed OCs have different transmission (T) of 1%, 2% and 3% from 1800nm to 2200 nm.
Fig. 1. Experimental setup for cw and mode-locking Tm,Ho:GAGG laser. The birefringent filter (BRF) is employed for the wavelength tuning in the cw operation regime.
Initially, the cw operating performance of the Tm,Ho:GAGG laser is investigated with three different output couplers of T=1%, 2%, and 3%. The Tm,Ho:GAGG crystal absorbs about 50% of the total incident pump power under no lasing condition. In the cw laser operation, the output power scales up almost linearly with the absorbed pump power as shown in Fig. 2(a), indicating a good thermal conductivity and low thermal load of the Tm,Ho:GAGG crystal. Maximum output powers of 303 mW (T=1%), 400 mW (T=2%) and 448 mW (T=3%) are achieved with a maximum absorbed pump power of 3.5 W, corresponding to a slope efficient of 8%, 11%, 13%, respectively. The output wavelength of cw Tm,Ho:GAGG laser are always located around 2112 nm for different OCs.
Fig. 2. (a) Output power versus absorbed pump power with different OCs in cw operation. η denotes the slope efficiency with respect to the absorbed pump power. (b) Wavelength tuning using a BRF with an OC of T=1%.
By inserting a birefringent filter into one arm of the cw laser cavity close to the mirror M3 at a Brewster’s angle, the wavelength tuning performance of the Tm,Ho:GAGG laser is investigated under the maximum absorbed pump power of 3.5 W. The BRF is a 2 mm thick quartz plate with its optical axis parallel to the surface of the BRF. Figure 2(b) shows the tuning performance obtained with the OC of T=1%. The output wavelength can be continuously tuned from 2041nm to 2118 nm with a tuning range of 77 nm, indicating the potential of the Tm,Ho:GAGG crystal in realizing ultrafast laser pulses.
In the mode-locking operation regime, the mirror M3 is replaced by the mirror M4 for focusing the oscillating laser to the SESAM. The SESAM (BATOP GmbH) used for initializing and maintaining mode-locking operation is placed near the focus of M4. The radius of the oscillating laser beam at the position of the SESAM can be varied over a range of 15 ∼24.5 μm by adjusting the distance between SESAM and mirror M4. The SESAM has a high reflectivity about 98% at 2090nm, a modulation depth of 1.2%, a non-saturation loss of 0.8%, and a saturation fluence of 70 μJ/cm2 from 1700nm to 2150 nm. The intracavity GDD introduced by the Tm,Ho:GAGG crystal and atmospheric air are calculated to be +330 fs2 and +15 fs2 at 2090nm, as shown in Fig. 3(a). Considering each cavity mirror has a positive GDD of +200 fs2, the total intracavity GDD for a round trip is calculated to be +1490 fs2. When operating the mode-locking Tm,Ho:GAGG laser in the positive GDD condition inside the laser cavity, we only get an unstable laser pulse train. In order to compensate the positive GDD, two chirped mirrors (CM1 and CM2) are inserted into the cavity with GDD of -600 fs2, resulting in total intracavity GDD of +290 fs2 for a round trip.
Fig. 3. (a) the GDD of the 6.5 cm long Tm,Ho:GAGG crystal [22] and the 180 cm long air path measured at 25 °C and 1.01×105 Pa [23]. (b) the beam quality parameter M2 of the mode-locking Tm,Ho:GAGG laser.
The performance of the mode-locking Tm,Ho:GAGG laser is investigated with an OC of T=1%. The small transmittance of the OC results in a small intracavity loss and low laser threshold of mode-locking. By gradual increasing pump power, the laser operation from cw regime to Q-switching mode-locking (QML) regime when the average output power exceeds 7.2 mW. Further increasing the pump power to the average output power of 12.9 mW, the laser goes into cw mode-locking regime, as shown in Fig. 4(a). As the pump power scaling up, the cw mode-locking operation can maintain to a maximum average output power of 34.1 mW, the corresponding energy density is 2.3 mJ/cm2 at the position of the SESAM. However, power rolling-over is observed when the absorbed power exceeds 3.0 W, indicating the occurrence of the thermal lens effect. The beam quality of the mode-locking laser is measured under the maximum output power via the knife-edge method with a convex lens with focal length of 15 cm. The measured M2 factor is around 1.14 in the horizontal direction and 1.18 in the vertical direction, as shown in Fig. 3(b).
Fig. 4. (a) the power characteristics of the mode-locking Tm,Ho:GAGG laser with OC of T=1%. (b) the laser spectrum. (c)the measured autocorrelation trace fitted by Gaussian equation. (d) The radio frequency spectra of the mode-locking Tm,Ho:GAGG laser.
Figure 4(b) shows the measured laser spectrum with a spectrometer (APE, waveScan USB) with a resolution of 0.4 nm. As the absorbed pump power increases from the laser threshold to the maximum, the center wavelength of the mode-locking laser is always located around 2089.9 nm with a FWHM of 0.7 nm (see Fig. 4(b)). Figure 5 gives typical mode-locking pulse trains recorded in time windows of 200 ns and 10 ms with an oscilloscope (Tektronix DPO 7104C), which indicate a good stability. At the maximum output power of 33.1 mW, the pulse duration of the mode-locking Tm,Ho:GAGG laser is measured with an autocorrelator (APE, pulse check 150). By assuming a Gaussian shape intensity distribution, a laser pulse duration of 10.84 ps is achieved, as shown in Fig. 4(c). Due to the low resolution of the employed spectrometer (0.4 nm), the measured spectrum bandwidth of 0.4 nm here is believed not accurate, so we omit to give the time bandwidth product here. To further evaluate the stability of the ML operation, the radio frequency (RF) spectrum is measured with a RF meter. Figure 4(d) shows the first beat note of the RF spectrum located at 83.01 MHz, which exactly agrees with the roundtrip time of the laser cavity. Moreover, the first beat note shows a high signal-to-noise ratio up to 60 dB without side peaks, indicating a stable cw mode-locking operation absence of any Q-switching instabilities. The inset of the Fig. 4(d) gives the RF spectrum in a wide span of 1 GHz, which shows no side peak between each beat note, indicating a single pulse mode-locking operation.
We also investigate the laser performance of the Tm,Ho:GAGG mode-locking laser with different OCs. Using OC of T=2%, a maximum average output power of 41.5 mW is achieved, the corresponding energy density is calculated to be 1.9 mJ/cm2 at the position of the SESAM, as shown in Fig. 6(a). The laser spectrum is centered at 2089.5 nm with a bandwidth of 0.6 nm, as shown in Fig. 6(b). Under the maximum average output power, the pulse duration is measured to be 17.52 ps, see in Fig. 6(c). With OC of T=3%, the Tm,Ho:GAGG mode-locking laser delivers a maximum average output power of 66.4 mW at 2089.9 nm with a spectral bandwidth of 0.4 nm, as shown in Fig. 6(d) and Fig. 6(e). The energy density at the position of the SESAM is calculated to be 1.5 mJ/cm2. The corresponding pulse duration is measured to be 16.56 ps, see in Fig. 6(f). As the transmission of the OC increasing from 1% to 3%, the intracavity loss is also increased, which narrows the stable mode-locking operation range (as shown in Fig. 6(a) and Fig. 6(d)). When the absorbed pump power exceeds 3 W, similar power rolling-over is also observed for each OC.
Fig. 6. (a) and (d) The output power characteristics, (b) and (e) the laser spectrum, (c) and (f) the autocorrelation trace of the mode-locking Tm,Ho:GAGG laser. (Upper three are for the OC of T=2% and below three are for the OC of T=3%).
In order to explore the influence of intracavity GDD on the mode-locking operation, chirped mirror pairs with different amounts of GDD (-2000 fs2, -1300 fs2, -300 fs2) are introduced into the laser cavity with OC of T=1%. The total intracavity GDDs for a round trip are calculated to be -2510 fs2, -1110 fs2, and +890 fs2 at 2090nm by considering the influence of laser crystal and atmospheric air. The shortest laser pulse duration of 16.43 ps with a spectral bandwidth of 0.4 nm is achieved with an intracavity GDD of -2510 fs2 (see Fig. 7(a) and (b)). Moreover, laser pulse durations of 17.10 ps and 19.19 ps are also obtained with total intracavity GDD of -1110 fs2 and +890 fs2, respectively (see Fig. 7(c) to (f)). In each case, the laser spectrum is always located around 2090nm only with a little difference between each other. Moreover, the narrow laser spectrum with sub-1 nm bandwidth also makes the laser pulse duration could be hardly further shortened by the introduced negative intracavity GDD.
Fig. 7. (a), (c) and (e) the laser spectrum, (b), (d) and (f) the autocorrelation trace with different group delay dispersion compensated in the cavity with OC of T=1%. The total GDDs for a round trip are (a), (b) -2510 fs2; (c), (d) -1110 fs2, (e), (f) +890 fs2.
3. Conclusion
In conclusion, for the first time, we report the laser performance of a LD-pumped mode-locking Tm,Ho:GAGG laser. In the cw operation regime, a maximum output power of 448 mW is achieved with a slope efficiency of 13% at 2112 nm. The emission wavelength could be tuned from 2041nm to 2118 nm with a birefringent filter inserted into the laser cavity. In mode-locking operation regime, by using OC of T=1%, the laser yields a shortest pulse duration of 10.84 ps at 2089.9 nm with an average output power of 33.17 mW and a repetition frequency of 83.01 MHz. With OC of T=3%, a maximum average output power of 66.4 mW is obtained with a laser pulse duration of 16.56 ps at 2089.9 nm. The experimental results show that the Tm,Ho:GAGG crystal is suitable for the generation of ultrafast lasers around 2 μm wavelength region.
Funding
Natural Science Foundation of Shandong Province (ZR2020QF096, ZR2021QE281); National Natural Science Foundation of China (61775119, 62005144, 62175128); Qilu Young Scholar Program of Shandong University; Taishan Scholar Foundation of Shandong Province (tsqn201812010); High-level Talent Cultivation Funds of State Key Laboratory of Crystal Materials of Shandong University; Funds of Basic Research Operations of Shandong University (2020GN079).
Disclosures
The authors declare no conflicts of interest.
Data availability
Date underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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