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Abstract
A type of intra-cavity polarizer based on a selective absorption mechanism was reported. For Sm:GdCa4O(BO3)3 (Sm:GdCOB) crystal, its polarized absorption property takes on significant anisotropy, and at 1 µm waveband its Y- polarized absorption is close to zero. Utilizing such special property, Sm:GdCOB intra-cavity polarizer are developed for 1 µm solid-state lasers, to generate Y-polarized laser output. This method has been successfully applied to different laser crystals, including cubic Nd:Y3Al5O12 (Nd:YAG), uniaxial Nd:LiGd(MoO4)2 (Nd:LGMO), and biaxial Nd:Lu2Y2SiO5 (Nd:LYSO). In summary, this research supplies a novel, effective, convenient, and cost-saving route to control the polarization and wavelength of solid-state lasers.
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1. Introduction
Lasers with wavelength around 1 µm play an important role in many fields, such as laser processing, ranging, medical treatment, etc [1–4]. At present, Nd3+-doped gain medium are popularly used to generate such laser [5–7]. It is well known that the effect of laser on materials is closely related to the wavelength and polarization state. For example, the nonlinear frequency conversion has strict requirement for the polarization of fundamental frequency light, to satisfy the phase matching condition. Therefore, it is a very practical subject to study how to realize the polarized operation of lasers.
The polarization generation methods can be summarized into two categories: extra-cavity and intra-cavity. The extra-cavity approach is to turn the laser into desired linearly polarized light by placing an optical device outside the laser resonator, such as a wave plate [8–11]. It uses the phase delay caused by the birefringence feature to control the polarization state of light. Usually, the extra-cavity polarization device is simple and efficient, but the optical loss is large. Another method is to place the optical element inside the laser resonator, such as polarizing prisms or glass [12–18]. They can achieve the polarized laser output through selective transmission, and their sizes are relatively large which affect the miniaturization of lasers in some extent. In 2009, Shen et al. realized dual-wavelength Nd:GdVO4 laser operation at 1063 and 1065 nm with different polarization [19]. The light selection effect of polarization beam splitter (PBS) is used to make the laser oscillate at a certain polarization direction inside the laser resonator, leading to adjustable laser output with different polarizations and wavelengths. Generally, a stretched polymer film can be used for the visible light of 400-700 nm, and a polarizing prism is available for wider waveband. The organic components have the disadvantage of low damage threshold. When a glass sheet is introduced into a laser resonator, the intra-cavity loss can be adjusted by its rotating, and the pump threshold and gain intensity will change accordingly. At Brewster angle, linear polarized laser output can be obtained. Its sensitivity to the rotation angle is not conducive to acquire high-power laser. By time now, people are still actively exploring new ways that can conveniently control laser polarization.
In this paper, Sm:GdCa4O(BO3)3 (Sm:GdCOB) crystal was used as a polarization element. It has a group of absorbance peaks from 1000 to 1140 nm, and the polarized absorption properties have great difference. At a familiar Nd3+ laser wavelength, 1064 nm, the polarized absorption coefficients of Sm:GdCOB crystal are αX = 1.58 cm-1, αY = 0.12 cm-1, and αZ = 0.52 cm-1, respectively. Such difference can be utilized to select polarization in a laser resonator. The laser oscillating along NX or NZ direction will be suppressed due to the large absorption loss, and a NY polarized laser output can be obtained. In many mixed crystals such as Nd:LGMO, Nd:LYSO, the inherent energy level of Nd3+ ions will split into many sub-energy levels because of the crystal field effect. Correspondingly, there will be rich transition spectral lines with different gain coefficients. As a result, for such lasers without polarization optics, the outputs were usually multi-wavelengths with different polarizations. By utilizing Sm:GdCOB crystal, the output polarization as well as wavelength can be adjusted flexibly for these lasers. In this paper, the polarization selecting function of Sm:GdCOB crystal was proved for Nd3+ laser crystals with different symmetries, including Nd:YAG, Nd:LGMO, and Nd:LYSO. Under unified experimental conditions, the laser output power and spectrum with different linear polarizations were measured and discussed.
2. Polarization selection principle of Sm:GdCOB
The polarized transmission spectra around 1 µm of Sm:GdCOB crystal are shown in Fig. 1. The position and intensity of absorption peaks are greatly influenced by the polarization direction, which takes on obvious anisotropy. NX polarization has the largest absorption intensity at 1025-1067 nm and 1073-1085 nm. At other wavebands (1067-1073 nm, 1085-1130 nm), NZ polarization presents the largest absorption. For the whole test band, NY polarization gives the maximum transmittance, which can reach 97% above. So, for familiar solid-state laser wavelength of 1064 nm, we processed a NZ-cut Sm:GdCOB sample as intra-cavity selective absorption polarizer, because in its cross-section the polarized absorption difference of NX and NY is the largest, which was the most favorable configuration for high purity linear polarization laser generation.
Sm:GdCOB crystal has large specific heat (0.605 to 0.881 J·g-1·K-1) and small thermal expansion coefficients (6.73-13.76 × 10−6 K-1) [20]. With a 1064 nm, 20 ps, 10 Hz laser as the light source, the laser damage threshold of Sm:GdCOB crystal was measured to be as high as 155 GW/cm2. All of these properties are favorable for the intracavity polarizer application of solid-state lasers.
3. Polarized laser experiments
3.1 Experimental set-up
The schematic of experimental setup is shown in Fig. 2. Nd-doped laser crystal was end-pumped by a fiber-coupled laser diode whose central wavelength was 808 nm. The laser cavity was optimized before polarized laser experiments, and the result showed that a concave-flat resonator presented the largest output power. So, we unified used a concave-flat resonator with a length of 30 mm.
The input mirror is concave with a radius of -100 mm, which is high reflective (HR) coated at 1050-1100 nm and anti-reflective (AR) coated at 808 nm. A flat mirror is used as the output coupler with a transmittance of 5% around 1064 nm. A Nz-cut Sm:GdCOB crystal plate with sizes of 4 mm × 4 mm × 1 mm was inserted between the laser crystal and the output coupler. Three kinds of laser crystals were used alternately in this experiment, including Nd:YAG, Nd:LGMO, and Nd:LYSO, which were ready-made samples in our laboratory. The sizes of Nd:YAG, Nd:LGMO, and Nd:LYSO crystals are 3 mm × 3 mm × 10 mm, 3 mm × 3 mm × 8 mm and 3 mm × 3 mm × 10 mm, respectively. They were wrapped with indium foil and placed in a hot sink copper block, whose temperature was controlled at 15 °C by flowing water.
3.2 Polarized laser output of Nd:YAG crystal
Nd:YAG belongs to the cubic crystal system, which is optical isotropic. According to its room temperature emission properties, the maximum emission is at 1064 nm, and the next is at 1062 nm [21]. The polarization laser experiment was carried out with a < 111>-cut, 1.1 at.% Nd:YAG crystal, whose end faces were AR coated around 1064 nm. The laser output properties without polarizer were measured first. The 1064 nm laser appears in either polarization direction. The maximum output power is 2.5 W at an absorbed pump power of 4.8 W, corresponding to an optical conversion efficiency of 52.1%.
Then the experimental results with polarizer were tested and shown in Fig. 3. The output laser is always linear polarized, with polarization direction along the NY axis of Sm:GdCOB crystal. When NY axis rotated from vertical direction to horizonal direction, the output polarization also changed accordingly. It is worth mentioning that the vertical and horizontal directions of Nd:YAG crystal behave differently in output power and wavelength, as seen in Fig. 3. The phenomenon can attribute to the nonuniformity of Nd3+ ions. The difference of ions radius between Nd3+ and Y3+ will cause lattice deform. The fractional condensation during growth will intensify the inhomogeneous distribution of Nd3+. As a result, the emission characteristic presents some discrepancy for different polarizations. When the NY axis of Sm:GdCOB was along vertical direction (Fig. 3(a)), a maximum output power of 0.95 W was obtained at vertical polarization direction, at the same time 11 mW laser was detected at horizontal polarization direction, which gave a polarization degree of 86:1. Correspondingly, when the NY axis of Sm:GdCOB was along horizontal direction (Fig. 3(b)), a maximum output power of 1.14 W was obtained at horizontal polarization direction, at the same time 10 mW laser was detected at vertical polarization direction, which gave a polarization degree of 114:1.
Fig. 3. Linear polarization output performance of Nd:YAG crystal. (a) NY of Sm:GdCOB is along vertical direction; (b) NY of Sm:GdCOB is along horizontal direction. Insets: the spectra of polarized lasers measured at the highest output powers.
3.3 Polarized laser output of Nd:LGMO crystal
Nd:LGMO belongs to the tetragonal crystal system, which is optical uniaxial. By selecting crystal orientation, Nd:LGMO could generate laser emissions with different wavelengths [22]. In this experiment, an uncoated, X-cut (i.e. a-cut), 0.5 at.% Nd:LGMO sample was used as the laser medium, which corresponded to the most efficient laser configuration for this crystal. The laser output power and wavelength without polarizer were measured first. 1061 nm and 1068 nm were obtained in the Z and Y polarization directions, respectively. The output power is mainly at 1061 nm, and the polarization degree is 5:1. The maximum output power is 0.57 W at an absorbed pump power of 3.0 W, corresponding to an optical conversion efficiency of 19.0%.
Then the experimental results with polarizer were measured, as shown in Fig. 4. The output laser is always linear polarized, with polarization direction along the NY axis of Sm:GdCOB crystal. When NY axis of Sm:GdCOB was along Z axis (i.e. c axis) of Nd:LGMO (Fig. 4(a)), a maximum output power of 0.27 W was obtained at Z polarization direction of Nd:LGMO with a wavelength of 1068 nm, at the same time 2 mW laser was detected at X polarization direction of Nd:LGMO, which gave a polarization degree of 135:1. Correspondingly, when NY axis of Sm:GdCOB was along Y axis of Nd:LGMO (Fig. 4(b)), a maximum output power of 0.31 W was obtained at Y polarization direction of Nd:LGMO with a wavelength of 1061 nm, at the same time 3 mW laser was detected at Z polarization direction of Nd:LGMO, which gave a polarization degree of 103:1.
Fig. 4. Linear polarization output performance of Nd:LGMO crystal. (a) NY axis of Sm:GdCOB is along Y axis of Nd:LGMO; (b) NY of Sm:GdCOB is along Z axis of Nd:LGMO. Insets: the spectra of polarized lasers measured at the highest output powers.
For anisotropic laser crystals, the laser oscillation will be naturally polarized in accordance with the maximum gain. According to the emission cross-section distribution of Nd:LGMO crystal [22], it can be known that π(Z) polarization direction is easier to output 1061 nm laser, and σ(X/Y) polarization direction is easier to output 1068 nm laser. It explains the experimental phenomenon that 1061 and 1068 nm lasers appeared at different polarization directions. For X-cut Nd:LGMO crystal, the laser emission cross-section of Z polarization is larger than that of X polarization, so the output power and slope efficiency of Z polarization laser are superior to the data of X polarization laser.
3.4 Polarized laser output of Nd:LYSO crystal
Nd:LYSO attributes to the monoclinic crystal system, which is optical biaxial. Its emission spectra exhibit strong optical anisotropy. The emission intensity of X polarization is the largest, with representative emission peaks at 1060, 1076, and 1080 nm [23]. The laser experiment was performed with two 1.0 at.% Nd:LYSO crystals, whose end faces were AR coated around 1064 nm. One Y-cut crystal was used to measure the laser outputs with X and Z polarizations, and the other Z-cut crystal was used to measure the one with Y polarization. Firstly, the output laser power and wavelength were measured without polarizer. Each crystal exhibit laser output in both polarization directions. For Y-cut crystal, the output is mainly X-polarized with a polarization degree of 2:1. The maximum output power is 1.69 W at an absorbed pump power of 5.5 W, corresponding to an optical conversion efficiency of 30.7%. For Z-cut crystal, the output is mainly Y-polarized with a polarization degree of 2:1. The maximum output power is 1.16 W at an absorbed pump power of 5.5 W, corresponding to an optical conversion efficiency of 21.1%.
The experimental results with polarizer in cavity were summarized in Fig. 5. All the polarization directions of output lasers are along the NY axis of Sm:GdCOB crystal. For Y-cut Nd:LYSO laser crystal, when NY axis of Sm:GdCOB was along X axis of Nd:LYSO (Fig. 5(a)), a maximum output power of 0.81 W was obtained at X polarization direction of Nd:LYSO with three wavelengths of 1060, 1076 and 1080 nm, at the same time 5 mW laser was detected at Z polarization direction of Nd:LYSO, which gave a polarization degree of 162:1. When NY axis of Sm:GdCOB was along Z axis of Nd:LYSO (Fig. 5(c)), a maximum output power of 0.70 W was obtained at Z polarization direction of Nd:LYSO with two wavelengths of 1076 and 1080 nm, at the same time 3 mW laser was detected at X polarization direction of Nd:LGMO, which gave a polarization degree of 233:1. For Z-cut Nd:LYSO laser crystal, when NY axis of Sm:GdCOB was along Y axis of Nd:LYSO (Fig. 5(b)), a maximum output power of 0.59 W was obtained at Y polarization direction of Nd:LYSO with single wavelength of 1076 nm, at the same time 3 mW laser was detected at X polarization direction of Nd:LYSO, which gave a polarization degree of 197:1. In short, by intra-cavity polarization selecting of Sm:GdCOB, multiple wavelength combinations with the same linear polarization direction have been obtained from Nd:LYSO crystals, including single wavelength (1076 nm), double wavelengths (1076 nm & 1080 nm), and three wavelengths (1060 nm & 1076 nm & 1080 nm), which will be very favorable for further nonlinear optical frequency conversions.
Fig. 5. Linear polarization output performance of Nd:LYSO crystal. (a) NY axis of Sm:GdCOB is along X axis of Nd:LYSO; (b) NY of Sm:GdCOB is along Y axis of Nd:LYSO. (c) NY of Sm:GdCOB is along Z axis of Nd:LYSO. Insets: the spectra of polarized lasers measured at the highest output powers.
In near infrared waveband, Sm:GdCOB exhibits strong absorption anisotropy, and the NY polarization always has the weakest absorption intensity. Utilizing this property, the Sm:GdCOB polarizer reported here can also be applied to some other wavebands, such as 1.2 µm, 1.45 µm.
4. Conclusion
The large absorption anisotropy of Sm:GdCOB crystal is used for intra-cavity polarization selecting. The main experimental results are summarized in Table 1. With a NZ-cut Sm:GdCOB as polarization selector, linear polarized laser outputs have been successfully obtained from different types of Nd3+ doped crystals, including Nd:YAG, Nd:LGMO, and Nd:LYSO. In most situations, the polarization degree is greater than 100:1, and the best result reached 233:1. Considering the Sm:GdCOB sample was not coated, and Sm3+ concentration as well as crystal thickness were not optimized, there is still much room for improvement in the current results. In summary, the absorption anisotropy of crystal can be utilized to adjust intra-cavity loss, which supplies a simple, effective, convenient, and cost saving method to select laser polarization and wavelength.
Table 1. Polarization selecting output properties of NZ-cut Sm:GdCOB for different Nd3+-doped laser crystals
Funding
National Natural Science Foundation of China (51702185, 61975096); Natural Science Foundation of Shandong Province (ZR2020KA003, ZR2020QA072, ZR2022MA067); China Postdoctoral Science Foundation (2020M672025); Primary Research & Development Plan of Shandong Province (2019JZZY010313).
Disclosures
The authors declare no conflicts of interests.
Data availability
The data that support the findings of this study are available from the corresponding authors upon reasonable request.
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