High-Power Acoustic-Optical Q-Switched 1.83 µm Tm-Doped Bulk Laser

Abstract

We report on a high-power acoustic-optical (AO) Q-switched Tm:YLF laser operating at ~1.83 μm by controlling the transmittance of the output coupler. Under the continuous-wave (CW) operation, the maximum output power of 13 W is achieved, and the slope efficiency is up to 32.7%. With a YAG etalon inserted into the cavity, the linewidth is compressed below 0.5 nm with a maximum output power of 12.2 W. In the Q-switched state, the maximum pulsed output power of 10.32 W is achieved with a pulse duration of 150 ns when the repetition rate is 15 kHz. And the maximum pulsed energy of 1.13 mJ is generated with a duration of 131 ns at 5 kHz. As far as we know, this is the highest output power reported for the CW and pulsed 1.83 μm laser. In addition, the relationship between the output wavelength and crystal length is theoretically analyzed, which shows that increasing the loss of 1880 nm is the key to high-power 1.83 μm laser output.

1. Introduction

A laser at an eye-safe wavelength of ~1.8 µm has attracted much attention in the fields of polymer laser-welding, mid-infrared gas spectroscopy and nonlinear frequency conversion [1,2,3,4]. In particular, a 1.8 µm laser is the ideal source to pump Cr:ZnS/ZnSe crystals to generate a 2.5 µm mid-infrared laser [5]. However, in the eye-safe band of 1.4~2.1 µm, a high-power laser output around 1.7~1.8 µm remains a challenge, although Er³⁺ (1.5~1.6 µm), Tm³⁺ and Ho³⁺ (1.9~2.1 µm) ion-doped crystals have been developed and commercialized.

At present, radiation around 1.7~1.8 µm can be generated by using Nd³⁺ or Tm³⁺-doped laser crystals [6,7] or nonlinear crystals [8,9,10]. In 2021, P. Lei et al. reported a 1855.5 nm MgO:PPLN optical parametric oscillator, and the maximum average output power was only 1.57 W at a pulse repetition frequency (PRF) of 40 kHz [11]. However, the output power for the frequency conversion method is usually constrained by the damage threshold of nonlinear crystals. In contrast, rare earth ion-doped laser crystals are a direct and effective method for obtaining high-power laser output. For example, the Nd³⁺ ion has attracted special attention due to its ~1.83 µm emission for the energy transition from ⁴F_3/2 to ⁴I_15/2. In fact, the first Nd³⁺ laser operating at 1830 nm can be dated back to 1971 based on a Nd:YAG rod [12]. Recently, the output power of the 1.83 µm Nd:YAG laser has been improved to 3.28 W [13]. Unfortunately, the rollover phenomenon is inescapable due to the serious thermal effects caused by the quantum defect [14]. In addition to Nd³⁺-based laser materials, the Tm³⁺-doped gain materials have been tested for 1.8 µm laser generation. In fact, Tm³⁺-doped bulk crystals are broadly used to generate a 1.9~2 µm laser by the transitions from ³F₄ to ³H₆ with the advantages of broad emission spectrum and high Stokes efficiency [15,16,17]. As we all know, the gain spectrum is located at 1.9~2 µm for the free-running Tm³⁺-doped lasers due to the re-absorption effect [18]. A possible solution is to use a wavelength-tuning element to obtain the 1.8 µm emission of Tm³⁺. In 2008, a 1.835 µm Tm:YLF laser was realized with an output power below 1 W using a birefringent filter [19]. In 2019, a tunable Tm:CaYAlO₄ laser was reported based on a YAG etalon, but the output power was under 2 W over the entire wavelength range between 1848 nm and 1876 nm [20]. In addition, it has been proven that controlling the transmittance of the output coupler (OC) can efficiently generate a 1.8 µm laser, but the output power scaling is still constrained below 10 W. For example, in 2019, Na et al. demonstrated a high-efficiency 1.83 µm Tm:YLF laser with a continuous-wave (CW) output power of 8.5 W by utilizing handpicked output couplers [21]. Later, Na et al. demonstrated a continuous-wave Tm:YAP laser, and the maximum output powers are 7.04 W and 7.1 W, corresponding to 1825 nm and 1844 nm, respectively [22]. Evidently, the potential of Tm³⁺-doped crystals to produce a high-power output at 1.8 µm needs to be further investigated. And there is no report on the powerful Q-switched Tm³⁺-doped bulk laser at ~1.8 µm with an output power exceeding 10 W.

In this study, we report an acoustic-optical (AO) Q-switched 1.83 µm Tm:YLF laser based on the approach of controlling transmittance of OC. The relationship between the output wavelength and the length of the Tm:YLF crystal is analyzed theoretically. A maximum output power of 13 W is generated in the CW operation, and the slope efficiency is up to 32.7%. In order to stabilize and compress the linewidth of 1830 nm, an undoped YAG etalon with a thickness of 125 µm is inserted into the cavity under the CW operation. The maximum output power of 12.2 W is demonstrated with a linewidth below 0.5 nm. When the AO Q-switch is turned on, the maximum pulsed output power is 10.32 W at 15 kHz with a pulse width of 150 ns. And the maximum pulse energy is 1.13 mJ at 5 kHz with a pulse width of 131 ns.

2. Experimental Setup

The experimental configuration of the LD-pumped 1.83 µm Tm:YLF laser is shown in Figure 1. A 50 W fiber-coupled diode laser emitting at 790 nm is utilized as the pump source. The core diameter is 105 µm, and the numerical aperture is 0.22. Based on a 1:4 coupling lens, the pump spot is expanded to 530 µm. A c-cut 3.0 at.% Tm³⁺-doped YLF crystal is used as the gain medium with dimensions of 3 mm × 3 mm × 10 mm. The ends of Tm:YLF have antireflection (AR) coatings at 793 nm and 1908 nm. The acoustic aperture of the AO Q-switch is 2 mm, and the physical length is 50 mm. The AO Q-switch is AR-coated at 2 µm. In order to dissipate the waste heat, a laser chiller is used with a cooling temperature of 20 ℃. The laser resonator is composed of a plane input mirror (M1) and a concave output mirror (M2). The physical length of the cavity is 85 mm. The optical transmission curves of the input (red solid line) and output (black solid line) mirrors are shown in Figure 2. M1 is high reflectivity (HR) coated at 1750–2200 nm (reflectivity > 99%) and antireflection (AR) coated at 792 nm (reflectivity < 2%). M2 has a transmission of 10% at 1830 nm and a transmission of more than 45% at 1880–2100 nm. In our experiment, three output mirrors with a radius of curvature (ROC) of 100 mm (OC1), 200 mm (OC2) and 500 mm (OC3) are used for comparison. In addition, a concave output mirror OC4 (ROC = 300 mm) coated at 1850–2150 nm with a 20% transmission is also used in the following spectrum research. The output power is measured by a laser power meter (PM150 Coherent, Santa Clara, CA, USA). The temporal pulse profiles are recorded by a fast InGaAs photodetector (EOT, ET-5000F) with a rise time of 28 ps and monitored by a digital oscilloscope (InfiniiVision DSOX6002A, KEYSIGHT).

3. Results and Discussions

First, the spectral output characteristics of Tm:YLF crystal are studied based on OC1. We test five different lengths of Tm:YLF crystals, and find an interesting phenomenon that the output wavelength is related to the Tm:YLF crystal length. As shown in

Table 1. Oscillating wavelengths in theory and experiment for different lengths of Tm:YLF crystal.

Length of Tm:YLF/mm	Output Wavelength/nm
	Theory	Experiment
10	1830	1830
12	1830	1828
15	1830	1828
22	1880	1887
25	1880	1886

, when the crystal lengths are 10 mm, 12 mm and 15 mm, the Tm:YLF laser emits at ~1.83 µm. However, when the crystal lengths are 22 mm and 25 mm, the output wavelengths move to ~1.88 µm. As is well known, the laser radiation wavelength depends on the laser threshold. Therefore, it is necessary to keep the 1830 nm laser at a lower pump threshold power. The threshold pump power equation is given as [21]

$$P_{th}=\pi h{\nu }_{\mathrm{p}}({\omega }_o^2+{\omega }_p^2)(2{\sigma }_{\alpha }{N}_{t}l+L-\ln (1-T_{OC}))/4{\eta }_p\tau ({\sigma }_{\alpha }+{\sigma }_e)$$

(1)

where hν_p is pump photon energy, ω_o is the radius of laser in the crystal, ω_p is the radius of pump laser in the crystal, η_p is the pumping efficiency, τ is the upper-level lifetime, σ_α and σ_e are absorption cross-section and emission cross-section at laser wavelength, N_t is the number of active particles, l is the crystal length, L is intrinsic double-pass cavity loss and T_OC is the transmittance of the output coupler. The parameters for the calculation of Equation (1) are listed in

Table 2. Parameters used in theory calculation.

Symbol	Value
σα [23]	0.11 × 10−20 cm2	@1830 nm
	0.06 × 10−20 cm2	@1880 nm
σe [23]	0.34 × 10−20 cm2	@1830 nm
	0.39 × 10−20 cm2	@1880 nm
Nt	3.45 × 1020 cm−3
L	0.02
TOC	0.1 @1830 nm
	0.45 @1880 nm
ωp	530 µm
ωo	460 µm

. The relationship between the threshold pump power and the Tm:YLF crystal length for 1830 nm and 1880 nm is shown in Figure 3. And the experimental results agree well with the theoretical predictions. It can be seen that the intersection of the two curves corresponds to a crystal length of approximately 16 mm. For crystal lengths below 16 mm, the threshold pump power for 1830 nm radiation is lower. On the contrary, the 1880 nm laser would oscillate. This phenomenon is due to the reversal of the intracavity losses of these two wavelengths caused by reabsorption losses. Herein, the 10 mm long Tm:YLF crystal is used in the following study.

Second, the characteristics of the 1.83 µm Tm:YLF laser under CW operation are investigated, as shown in Figure 4. To optimize the output power, three output mirrors with different radii of curvature of 100 mm (OC1), 200 mm (OC2) and 500 mm (OC3) are used. When the ROC of the OC is 100 mm, a maximum CW output power of 13 W is obtained, and the slope efficiency is up to 32.7%. When the ROC increases, the maximum output powers decrease to 8.02 W (OC2) and 5.63 W (OC3), and the corresponding slope efficiencies decrease to 21.6% and 16.2%, respectively. The polarization of the output light measured by a linear polarizer is horizontal. Compared with our previous studies [24], it can be found that the laser output performance is not inferior to the 1.9 µm wavelength, although the 1.83 µm emission band is seriously affected by the reabsorption of the Tm:YLF crystal. Considering the difficulty in manufacturing the laser mirrors for the 1.83 µm Tm:YLF laser, the output power will be further improved if volume Bragg grating (VBG) is used to select wavelengths.

Here, we attribute the optimal ROC of 100 mm to the better mode matching when the thermal effect of the gain medium is considered. The thermal focal length for a diode end-pumped bulk laser can be written as [25]

$$f_T=\pi K_c{\omega }_p^2/(\xi P_{in}{\eta }_{abs}(dn/dT))/(1-\exp (-{\alpha }_{p}l))$$

(2)

where P_in is the incident pump power, α_p is the absorption coefficient of the pump laser, ω_p is the pump waist, ξ is the fraction of pump power that results in heating, K_c is the heat conductivity, η_abs is the absorption efficiency, dn/dT is the temperature-dependent coefficient of the index of refraction and l is the length of the crystal. The relationship between the thermal focal length of the Tm:YLF crystal and the incident pump power is shown in Figure 5. When the input pump power is 45 W, the thermal focal length is approximately 80 mm. The laser mode radius in the resonant cavity is simulated using the ABCD matrix theory. The results show that, when the radii of the curvature are 500 mm, 200 mm and 100 mm, the spot diameters at the center of the crystal are theoretically calculated to be 0.72 mm, 0.66 mm and 0.46 mm, respectively. According to the empirical value, the optimal ratio of oscillating laser spot to pump spot at the crystal is about 0.8 [26]. Therefore, a good mode match between the oscillating laser and the pump laser is achieved when the ROC is 100 mm.

Subsequently, the characteristics of the AO Q-switched Tm:YLF laser are investigated with OC1. When the static AO Q-switch is placed behind the Tm:YLF crystal, the maximum CW output power declines from 13 W to 10.53 W, and the slope efficiency is reduced from 32.7% to 27.5%, as shown in Figure 6. Turn on the AO Q-switch, and the stable pulsed operation is achieved. When the PRFs are set to 5 kHz, 10 kHz and 15 kHz, the maximum average output powers of 5.65 W, 9.22 W and 10.32 W are obtained, corresponding to slope efficiencies of 23.6%, 24.2% and 26.9%, respectively. And the polarization of the pulsed laser remains a horizontal direction. In the inset of Figure 6, the peak-to-peak power fluctuation of 1.58% over 0.5 h at 5 kHz exhibits the high stability of the output power. Figure 7 shows the variation in the pulse duration, the pulse energy and the peak power with the incident pump powers. The pulse energy and the peak power increase with the incident pump power, but the pulse duration is inversely proportional to the input pump power. As shown in Figure 7a, the shortest pulse durations of 131 ns, 138 ns and 150 ns are realized for 5 kHz, 10 kHz and 15 kHz, respectively. In Figure 7b, the maximum single pulse energies of 0.68 mJ, 0.92 mJ and 1.13 mJ are obtained. To avoid damaging the crystal, the pulse energy is limited to about 1 mJ. And the maximum peak powers are 8.63 kW, 6.67 kW and 4.53 kW, as shown in Figure 7c. The temporal pulse train and the single pulse envelope at 5 kHz are shown in Figure 8.

Figure 9 shows the laser spectra of Tm:YLF lasers under different operation modes, which are measured by an APE laser spectrometer (the spectral resolution of 0.5 nm). Considering that the actual spectrum jumps within a certain wavelength range, we use the “Max. Hold” function of the spectrometer to collect and display the light intensities that occur in the measurement process. In the CW regime, the center wavelength is located at 1830 nm when the OC1 is used. However, the laser wavelength slightly blueshifts from 1830 nm to 1824 nm in the Q-switching regime due to increased intracavity loss [27,28,29]. The full width at half-maximum (FWHM) is measured to be approximately 6.5 nm for CW and pulsed states. In contrast, the Tm:YLF laser emits at 1906 nm with a FWHM of ~7 nm when the OC4 is employed as the output mirror. In order to stabilize and compress the linewidth of 1830 nm, an undoped YAG etalon with a thickness of 125 μm is inserted into the cavity under CW operation. The inset of Figure 9 shows the output spectrum when inserting the etalon. The linewidth cannot be measured accurately due to the limitation of spectrometer resolution, and the real linewidth is less than 0.5 nm. As shown in Figure 4 (Square data), the maximum output power decreases to 12.2 W. The slope efficiency of 32.2% changes very slightly, indicating that the etalon does not affect the laser performance. As shown in Figure 10, the output beam quality of the ~1.83 µm laser is measured based on the 90.0/10.0 scanning-knife-edge method. And the M² factors are calculated to be 1.5 and 1.4 in the x and y directions, respectively.

4. Conclusions

In conclusion, a high-power 1.83 µm Tm:YLF laser has been successfully realized with an output power exceeding 10 W for the first time, as far as we know. Under the incident pump power of 45 W, the maximum CW output power of 13 W is achieved, and the slope efficiency is up to 32.7%. When the PRF is 15 kHz, the maximum pulsed output power of 10.32 W is achieved, corresponding to a pulse duration of 150 ns. Under the PRF of 5 kHz, the maximum pulse energy of 1.13 mJ and the narrowest pulse width of 131 ns are realized with a peak power of 8.6 kW. The center emission wavelength of the CW Tm:YLF laser is located at 1.83 µm with a FWHM of 6.5 nm and is shifted to 1824 nm in the Q-switched regime. With a YAG etalon inserted into the cavity, the linewidth is compressed below 0.5 nm with the maximum output power of 12.2 W. In fact, a high-power laser output at 1.83 μm has always been ignored, because the traditional Tm³⁺-doped crystals emit at 1.88–2.05 μm (Tm:YLF: 1.88 μm and 1.908 μm, Tm:YAP: 1.94 μm and 1.99 μm, Tm:YAG: 2.05 μm). Our experiment results demonstrate that the 1.83 µm Tm:YLF crystal is capable of producing powerful laser output, and has the potential to be applied in fields such as material processing and nonlinear optics. In addition, a theory model is established to predict the relationship between output wavelength and crystal length, which point out an effective technical approach for the generation of a 1.83 μm high-power laser. Theoretical analysis results show that increasing the loss of 1880 nm is the key to the high-power 1.83 μm laser. However, considering the difficulty in manufacturing the laser mirrors for 1.83 µm Tm:YLF lasers, volume Bragg grating (VBG) will be used in the future due to its strong wavelength selection ability.