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Abstract
We demonstrated a 978 nm laser diode (LD) side-pumped YSGG/Er:YSGG/YSGG composite crystal with a size of Ф 3 mm × 65 mm and continuous-wave (CW) mode. By optimizing resonator length and output mirror transmittance, a maximum output power of 28.02 W is generated, corresponding to slope efficiency of 17.55% and optical-optical efficiency of 12.29%, respectively. The thermal focal lengths are obtained by resonator stability condition. The laser wavelength is centered near 2.8 µm. Moreover, the beam quality factors $M^2_x$/$M^2_y$ are fitted to be 8.14 and 7.35, respectively. The above results indicate that a high-performance 2.8 µm CW laser can be achieved by LD side-pumped YSGG/Er:YSGG/YSGG composite crystal with excellent heat dissipation ability, which promotes effectively the development and applications of the mid-infrared solid-state lasers.
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1. Introduction
In recent years, the development of 2.7-3 µm mid-infrared laser has attracted wide attention since it falls within the water absorption band, leading to important and extensive applications in medical surgery, remote atmospheric sensing, and pollution monitoring [1–4]. Additionally, the 2.7-3 µm laser can also be used for nonlinear optics, generating 3-5 and 8-14 µm lasers by nonlinear frequency conversion [5,6].
Generally, the transition between 4I11/2 and 4I13/2 of Er3+ can realize ∼2.8 µm laser [7,8]. However, since the lifetime of the laser lower level is much longer than that of the laser upper level, it is prone to a self-terminating effect, which makes the population inversion difficult and is not conducive to the ∼2.8 µm laser oscillation. Some technologies have been proposed and developed to shorten the lifetime of the laser lower level 4I13/2 [9–12], including the cascade laser technology, co-doping deactivating ions, and increasing doping concentration up to over 30 at.%. Among them, the high-concentration doping of Er3+ is a simple method for shortening the lifetime of the 4I13/2 level and inhibiting the self-terminating effect [7].
Up to now, the 2.7-3 µm laser has been realized in many Er3+-doped crystals and fibers, such as Er:YAG [13], Er:YSGG [14], Er:GGG [15], Er:YAP [16], Er:Lu2O3 [17], Er:SrF2 [18], Er:ZBLAN [19], Er:ZrF4 [20], etc. However, Er3+-doped perovskite YAP crystals are easy to crack during growth due to the anisotropy. The melting point of Er3+-doped sesquioxide crystals is as high as about 2400 °C [21], which brings great difficulty to the crystal growth. The growth conditions of the Er3+-doped fluorides are harsh and have adverse influence on the equipment and environments [22]. In contrast, the Er3+-doped oxide crystals with garnet structure have excellent optical and mechanical properties, making them the mainstream laser gain media for high-power mid-infrared solid-state lasers [23]. Compared with the Er:YAG and Er:GGG crystals, the Er:YSGG crystal has a lower phonon energy of about 728 cm-1 [24], which is beneficial for decreasing the laser threshold and improving the laser efficiency.
In 2015, a maximum CW laser output of 1.84 W was obtained via a 970 nm LD side-pumped Er:YSGG slab with a slope efficiency of 11.2% [25]. In 2019, a pulse laser output with a maximum average output power of 34.9 W and a slope efficiency of 13.7% was yielded by a 970 nm LD side-pumped Er:YSGG crystal rod [26]. In 2021, a maximum output power of 1.37 W with a slope efficiency of 23.62% was realized by an LD end-pumped Er:YSGG crystal cuboid [14]. In 2022, a maximum average power of 61.02 W was demonstrated with the optical-optical efficiency of 12.6% by LD side-pumped two-rod Er:YSGG crystals [27]. In the same year, a 2.8 µm CW laser output with a maximum power of 2.01 W and a slope efficiency of 28.89% was achieved through LD end-pumped Er:YSGG crystal [28]. These studies show that the output power of the Er:YSGG laser under the pulsed mode has been as high as tens of watts, while the laser output power under CW mode was only a few watts.
The ∼2.8 µm laser generated by the radiative transition of Er3+ belongs to a four-level system [7,28], which involves excited state absorption, up-conversion, cross-relaxation, and non-radiative transition, etc. Usually, a significant amount of waste heat is generated in the gain medium during the laser operation. Simultaneously, the cooling of the outer surface of the gain medium by circulating water inevitably brings temperature gradients, resulting in the thermal effect. In addition, the high-concentration doping of Er3+ (≥30 at.%) leads to a decrease in the thermal conductivity of the Er:YSGG crystal (3.27 W m−1 K−1) [29]. As the pump power increases, the heat accumulation inside the crystal becomes more severe, thus limiting the improvement of laser performance. To weaken the influence of the thermal effect, the thermal bonding technology is adopted in this study, in which the YSGG crystal with high conductivity (6.83 W m−1 K−1) [30] is bonded to both end-faces of the Er:YSGG crystal as a heat sink to improve the temperature distribution inside this crystal and enhance the laser output power and efficiency.
In this paper, a 978 nm LD side-pumped YSGG/Er:YSGG/YSGG laser under CW mode was demonstrated. By optimizing the length of the resonator cavity and the transmittance of the output mirror, a maximum average output power of 28.02 W was obtained, corresponding to the slope efficiency of 17.55%. Meanwhile, the thermal focal lengths were determined, and the laser spectrum and beam quality were also studied.
2. Experiment setup
Selecting high-optical quality parts from the YSGG and Er:YSGG crystals grown by the Czochralski (Cz) method, directional cutting was carried out along the <111 > direction. Then, a pre-bonding between the YSGG and Er:YSGG crystal surfaces with high processing accuracy and low roughness was realized at room temperature based on the intermolecular attraction force. Currently, the bonding energy was weak and the bonding interface was prone to break, requiring further high-temperature heat treatment. The pre-bonded crystal was placed in a muffle furnace and heated at 1200 °C for 12 h. During the heating process, the diffusion of ions and holes at the bonding interface gradually intensified, forming stable and strong covalent bonds at the bonding interface, thereby achieving permanent bonding. Combined with the effective pumping length of the LD side-pumping source and the thermal properties of YSGG, the YSGG crystal rods with sizes of Ф 3 mm × 6.5 mm were bonded to the two end-faces of the 35 at.% Er:YSGG crystal with a size of Ф 3 mm × 52 mm. Thereby, the YSGG/Er:YSGG/YSGG composite crystal with a size of Ф 3 mm × 65 mm was fabricated, and the high quality of thermal bonding was characterized by transmission spectra in our previous work [31]. Subsequently, both end-faces of the composite crystal were coated with antireflection film in the range of 2.65-3 µm. The crystal rod and LD array heat exchanger were installed in the water flow tube, and the circulating water flowed through the laser module from the water chiller to cool the crystal rod directly. To make the laser system work steadily, the temperature and flow rate of the circulating water were controlled at 288 K and 15 L/min, respectively. The side-pumping source is a 978 nm LD (HTOE DPL-0980-45) with a full width at half maximum (FWHM) of 3.09 nm and a maximum power of 300 W. The laser experimental setup is exhibited in Fig. 1. The M1 was a plane mirror coated with the high reflection (HR) film in the range of 2.7-3 µm, and the M2 used as the output coupling mirror (OC) was also plane with the transmissions of 0.5%, 2%, 5%, and 10% at 2.79 µm, respectively. A power meter (Ophir 30A-BB-18) was used to record the laser output power, and a pyro-electric array camera (Ophir-Spiricon PY-III-HR) was employed to measure the laser beam profile. Moreover, the laser spectrum was determined via a Fourier Transform Infrared Spectrometer (Nicolet iS50R AM FTIR).
3. Results and discussion
To optimize the resonator parameters, the influence of different cavity lengths on the laser performance of 978 nm LD side-pumped YSGG/Er:YSGG/YSGG composite crystal is investigated, as shown in Fig. 2. It can be observed that the length of the resonator cavity has little effect on the laser output power at low pump power. When the pump power exceeds 124 W, the shorter the resonator cavity length, the higher the maximum output power. This may be because the diffraction loss at high pump power becomes obvious with long cavity length, leading to the decreases in laser output power and slope efficiency. Additionally, due to the volume limitation of the pump source, the shortest length of the resonator cavity can only be compressed to 80 mm.
Fig. 2. Laser performance of 978 nm LD side-pumped YSGG/Er:YSGG/YSGG composite crystal rod with different cavity lengths (T: OC transmittance, L: cavity length).
Concurrently, the laser performance of LD side-pumped YSGG/Er:YSGG/YSGG composite crystal under different transmittances of the output mirror is determined with the optimal cavity length of 80 mm, as shown in Fig. 3. The laser thresholds are about 59.76, 60.79, 72.23, and 115.05 W for the OC transmittances of 0.5%, 2%, 5%, and 10%, respectively. With the increase of the OC transmittance, the intracavity transmittance loss increases, resulting in the increase of the laser threshold. Also, due to the relatively low transmittance loss and high power density in the cavity when the OC transmittance is 0.5%, it is easy to cause damage to the OC film layer, so that the maximum pump power is only increased to 168.96 W. When the OC transmittance is 2%, the maximum output power of 28.02 W is obtained at the pump power of 228.06 W, corresponding to the slope efficiency of 17.55% and optical-optical efficiency of 12.29%, respectively. To avoid damage to the laser rod under high pump power, the instability of laser output power within 30 s is determined, which is less than 2%. When the OC transmittance is 5%, the maximum output power of 27.66 W is slightly lower. But the corresponding slope efficiency is 19.03%, which is higher than the slope efficiency when the OC transmittance is 2%. Compared with the maximum output power of 18.95 W and slope efficiency of 13.48% for the un-bonded Er:YSGG crystal side-pumped by 978 nm LD, the laser output power and efficiency are effectively improved after thermal bonding. The 2.7-3 µm mid-infrared laser performance of other Er3+-doped oxide crystals operated at CW mode is listed in Table 1. To the best of our knowledge, the output power of 28.02 W is the highest ever reported 2.7-3 µm CW laser generated in the oxide crystals.
Fig. 3. Laser performance of 978 nm LD side-pumped YSGG/Er:YSGG/YSGG composite crystal rod with different OC transmittances.
Table 1. 2.7-3 µm laser performance of Er3+-doped oxide crystals operated at CW mode.
Usually, the evaluation of the thermal lensing effect is characterized by the thermal focal length of the laser medium, which is related to multiple parameters and some of them cannot be accurately determined [33,34]. Therefore, the thermal focal lengths under different pump powers are usually measured by experimental method. Asymmetric stable resonator is the most commonly used method for measuring the thermal focal length, and the schematic diagram is shown in the illustration of Fig. 4. The stability conditions of the resonator cavity are shown in formula (1) [35].
where L1 and L2 are the distances from the crystal rod center to the input mirror M1 and the output mirror M2, respectively, and f is the thermal focal length of the laser crystal. In theory, the position of the input mirror and crystal rod remains unchanged and L1<L2. When the pump power is maintained at a certain value, the thermal focal length of the crystal can be obtained by moving the output mirror until the output power drops to zero, which is the distance from the output mirror to the center of the crystal rod. Nevertheless, in practical experiments, it is difficult to obtain the critical state of stable operation of the resonator cavity by changing the position of the output mirror, and the measurement error is relatively large. The cavity length is usually fixed, and the resonator cavity transitions from a stable state to a non-steady state with the output power increasing firstly and then decreasing until suddenly dropping to zero as the pump power increases [36].Fig. 4. Thermal focal length of the experimental and theoretical values as a function of pump power on the LD side-pumped Er:YSGG and YSGG/Er:YSGG/YSGG composite crystal rods.
The thermal focal lengths of the Er:YSGG and YSGG/Er:YSGG/YSGG composite crystals under different pump powers are measured with the OC transmittance of 2%, as shown in Fig. 4. With the increase of pump power, the thermal focal length of the two crystal rods gradually decreases, suggesting that the thermal effect inside the two crystal rods are both intensified. When the thermal focal length is 56 mm, the pump power up to 224.43 W for the YSGG/Er:YSGG/YSGG composite crystal is higher than that of 194.59 W for the Er:YSGG crystal. Compared with the un-bonded Er:YSGG crystal, the YSGG/Er:YSGG/YSGG composite crystal has the same thermal focal length for a higher pump power. This is mainly due to the fact that the YSGG crystals are bonded at both ends of Er:YSGG as heat sinks, which can disperse heat effectively and reduce thermal effects. It indicates that the YSGG/Er:YSGG/YSGG composite crystal is more suitable for stable operation at high power. Furthermore, based on the equation modified by Z. Q. Fang [37], the theoretical thermal focal lengths of the Er:YSGG and YSGG/Er:YSGG/YSGG composite crystals are fitted and shown in Fig. 4. The result reveals that the YSGG/Er:YSGG/YSGG composite crystal can decrease the thermal effect and is beneficial to the stability of the laser output power, which is consistent with the experimental data. However, there are differences between the theoretical and experimental curves of the thermal focal length for the YSGG/Er:YSGG/YSGG composite crystal, mainly since the temperature difference between the center and surface of the composite crystal rod is not a constant, and an approximate value is used for calculation.
With the OC transmittance of 2% and the cavity length of 80 mm, the laser wavelength of LD side-pumped YSGG/Er:YSGG/YSGG composite crystal measured by the Fourier spectrometer is presented in Fig. 5. Only a single laser wavelength located near 2.8 µm is observed, and the laser wavelength redshifts from 2796.2 nm around the threshold power to 2796.7 nm when the pump power is up to 228.06 W. Moreover, the beam quality factor (M2) is an important parameter to evaluate the laser performance. Thereby, the laser beam profile and M2 factor of the YSGG/Er:YSGG/YSGG composite crystal are investigated under the pump power of 146.16 W with the OC transmittance of 2% and the cavity length of 80 mm. A CaF2 convex lens with a focal length of 300 mm is placed behind the output mirror and the laser beam is focused on the detector of the spot camera, and the camera is constantly moved before and after the beam waist position, and the spot diameter is recorded versus the distance between the focusing lens and the spot camera. The two-dimensional beam profile of the LD side-pumped YSGG/Er:YSGG/YSGG CW laser is captured near the beam waist, as shown in Fig. 6. By hyperbolic fitting, the beam quality factors in horizontal and vertical directions are obtained to be 8.14 and 7.35, respectively.
Fig. 6. Beam diameter as a function of the propagation distance for LD side-pumped YSGG/Er:YSGG/YSGG composite crystal rod and the laser spot image near the waist.
4. Conclusions
In this paper, the laser performance of a 978 nm LD side-pumped YSGG/Er:YSGG/YSGG composite crystal rod was demonstrated. A maximum output power of 28.02 W, corresponding to the slope efficiency of 17.55% and optical-optical efficiency of 12.29%, is achieved with the resonator cavity length of 80 mm and OC transmittance of 2%. The thermal focal length is about 56 mm when the pump power is up to 224.43 W for the YSGG/Er:YSGG/YSGG composite crystal. Compared with the un-bonded Er:YSGG crystal, the composite crystal has the same thermal focal length for a higher pump power. The laser spectrum shows a single wavelength laser output and the central wavelength is near 2.8 µm. Moreover, the laser beam quality factors M2 are fitted to be 8.14 and 7.35 in the x and y directions, respectively. These results reveal that the YSGG/Er:YSGG/YSGG composite crystal with excellent heat dissipation ability is suitable as the gain medium of high-power lasers, which is conducive to promoting the development of mid-infrared solid-state lasers.
Funding
National Natural Science Foundation of China (52102012); Natural Science Foundation of Anhui Province (2208085QF217); Hefei Institutes of Physical Science (HFIPS) Director’s Fund (YZJJ2022QN08).
Acknowledgments
Thanks to Z. Q. Fang from Yancheng Institute of Technology for his help in fitting the theoretical thermal focal length.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data 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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