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Abstract
We evaluate the thermal effects of a c-cut Tm:YAP slab laser by considering the anisotropic properties of the biaxial YAP crystal. Significant improvements in thermal stress were demonstrated by selecting the crystallographic a-axis, which possesses higher thermal conductivity and thermal expansion, as the direction of the slab thickness. A maximum laser power of 30 W (E//a) at 2 µm was obtained under an incident LD power of 55 W with an optical conversion efficiency of 55.4% and slope efficiency of 61.8% using the a-slab. The slab laser was then used for realizing compact Ho lasers via intra-cavity pumping, resulting in a 0.8 W Ho:YAG laser and a 5.5 W Ho:YAP laser (E//b) at 2.12 µm and 2.13 µm, respectively.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Wavelengths around 2 µm, which match well with several absorption lines of the atmosphere, and moderate absorption bands of water and macromolecules, make Tm and Ho lasers suitable for several applications including atmospheric monitoring, medical treatment, optical communications, chemical detection, and material processing [1–4]. Moreover, the 2 µm lasers, especially the Ho lasers whose wavelengths exceed 2.1 µm, are the preferred driving sources for extending coherent wavelengths towards the mid-infrared (3-14 µm) via optical parametric oscillation, optical parametric amplification, or differential frequency generation [5–7].
Tm lasers, especially the Tm:YAG, Tm:YLF and Tm fiber lasers, have been demonstrated to be efficient for high power operations [8–10], owing to their long fluorescence lifetime, approximately 200% quantum yield via cross relaxation [8], and the existing diode lasers (LDs) at 800 nm. However, the lack of an absorption band around 800 nm diversified the mechanisms of the Ho lasers, which were achieved mainly by co-doping Tm and Ho ions inside the same gain medium [11], in-band pumping the Ho laser by 1.9 µm lasers [12], and intra-cavity pumping the Ho laser with Tm lasers [13]. Using a highly efficient Tm laser, intra-cavity pumped Ho lasers are more suitable for room temperature operations, which facilitate the direct use of common LDs in a compact structure with high total efficiency from LD to Ho laser [14].
YAP crystals doped with Tm3+ ions which have thermal and mechanical properties similar to isotropic YAG crystals, have received growing attention recently owing to their twice higher emission cross sections for energy storage [15] and their natural birefringence for linearly polarized radiation without the use of extra polarizers [16]. Since the early studies reported by Elder and Payne in 1997, where output power of 730 mW was achieved [17], the output power and efficiency of Tm:YAP lasers have increased rapidly. A 50 W Tm:YAP laser at 1.94 µm with a slope efficiency of 56% was achieved under a cooling temperature of 10 °C by Sullivan et al. in 2004 [18]. In 2018, 11.8 W continuous-wave Tm:YAP laser with a slope efficiency of 62% and optical conversion efficiency of 56% was reported by Cole et. al. [16]. Compared to rod lasers, slab Tm:YAP lasers are more competent at power scaling owing to the rapid heat transfer along the slab thickness [19–21]. A 72 W Tm:YAP laser at 1993 nm, with a slope efficiency of 37.9%, was reported by Cheng et. al. in a dual end-pumped slab structure with incident LD power of 220 W [20]. Further research on such lasers focused mainly on the lasing wavelength around 1.94 µm. Using an incident pump power of 140 W, a 48 W Tm:YAP laser at 1938 nm with slope efficiency of 43.6%, where two YAG etalons were inserted intra-cavity for wavelength control was obtained by Han et. al.[22]. Recently, an electro-optically Q-switched Tm:YAP laser with a free running power of 26.57 W at 1937.87 nm and slope efficiency of 32.56% was reported by Wen et al. [15].
The temperature field and stress distribution inside the slab were usually simulated analytically or numerically assuming a unique thermal conductivity and thermal expansion for evaluating the thermal effects of the Tm:YAP slab under high power pumping [21,23–25]. This is, however, unsuitable for the biaxial YAP crystal with orthorhombic D162h symmetry, owing to its varied thermal conductivities, thermal expansions, and thermal optical coefficients along the different crystal axes [17].
In this paper, the anisotropic properties of the YAP host were first considered for evaluating the temperature distribution and thermal stress of the Tm:YAP slab, which helped in reducing the thermal effects by selecting a suitable crystal axis along the slab thickness. A maximum output power of 30.5 W at 2000.6 nm was obtained using a single end-pumped c-cut Tm:YAP slab under an incident LD power of 55 W, corresponding to an optical efficiency of 55.4% and a slope efficiency of 61.8%. Meanwhile, 2.1 µm Ho lasers pumped intra-cavity by the slab laser were explored considering the overlaps between the absorption spectra of the Ho-doped YAG and YAP crystals and the polarized emission spectra of the Tm:YAP crystal.
2. Experimental design and setup
2.1 Thermal effects of the anisotropic Tm:YAP slab
A c-cut Tm:YAP slab (Pbnm notation, one of the coordinate systems for the YAP crystal [26]) with doping concentration of 3 at.% and dimensions of 1.5 mm×6 mm×17 mm, is wrapped with indium foil and mounted onto a cooper heat sink to facilitate water cooling. Considering the anisotropic absorption, the heat source inside the slab is expressed as:
The above calculations were performed using COMSOL Multiphysics, where the thermal stress interface consisted of a heat transfer interface with an applied solid mechanics interface. The temperature gradient ΔTi(r, z) = Ti(r, z) - Tbi obtained was then used to calculate the optical path difference OPDi(r), and thus the focal lengths fi, of the thermal lenses, along the direction i [29]:
Table 1. Material and experimental parameters for simulating thermal effects. (Pbmn notation)
The anisotropic K, αT, and ∂n/∂T were taken following the Pbnm notation as the definition of the Tm:YAP crystal used, and were commonly assumed to be unique in previous reports on the thermal effects of Tm:YAP lasers [21,23,34]. Although these crystal parameters varied in different references owing to the differences in the doping ion and doping concentrations [32], the relatively small K, αT,, and ∂n/∂T along the b axis remained the same [30–32,37,38]. This makes it feasible for evaluating the thermal effects of the YAP slab along varied crystallographic orientations by assuming that the thermal parameters along different axes have similar relative magnitudes inside a uniformly doped crystal with an acceptable doping density for laser operation. For simplicity, we use x-slab (x = a, b) for indicating a c-cut Tm:YAP slab with the crystallographic x axis as the direction of the thickness. Considering the fact that K, αT, and ∂n/∂T were not measured directly, the above model could provide a strategy for optimizing the YAP slab and subsequently be adopted in other anisotropic hosts such as YLF, YVO4, or KGW for high power operations.
An elliptic temperature distribution is shown at the pump surface owing to the anisotropic thermal conductivity and the large aspect ratio of the slab structure (Figs. 1(c)–1(d)). Under the same incident pump power of 60 W, the maximum temperatures have slightly different values of 59 °C and 62.1 °C for the a- and b-slabs, respectively. This is attributed to the approximate thermal conductivities of 11.6 W/m/K and 9.9 W/m/K along the a- and b-axes respectively [30]. Hence, the assumption of isotropic conductivity is sufficient for simulating the temperature distribution. However, since the thermal expansion along the a-axis is about twice that along the b-axis [31], greater deformation in the b-slab (Figs. 1(a) and 1(b)) leads to higher thermal stress inside it (Fig. 2). Unlike the maximum temperatures located in mid-plain parallel to the cooling surface and across the symmetry axis of the slab (Figs. 1(a) and 1(b)), high thermal stresses were observed at positions away from the pump surface in the mid-plain region (Figs. 2(a) and 2(c)) and close to the pump surface in the main cooling surfaces (Figs. 2(b) and 2(d)), respectively.
Fig. 1. Temperature and stress distributions inside the Tm:YAP slab under a LD power of 60 W. (a, b) 3D temperature fields with thermal deformation, (c, d) temperature profiles on the pump surface, (e, f) thermal stress on the pump surface of the a-slab and b-slab, respectively.
Fig. 2. Stress contours on the lateral plain across the symmetry axis of the slab (a, c), and on the cooling surfaces (b, d) for the a- and b-slabs, respectively.
Figure 3 compares the maximum temperatures, maximum stresses and thermal lenses of the two slab types under different LD powers. Here, ∂n/∂T = 8.3×10−6 K−1 (E//a) was considered by assuming the laser is polarized parallel to the a-axis. The focal lengths are calculated from temperature distribution parallel a- and b-axis. Besides the slightly higher thermal gradient, the higher maximum stress of 147.5 MPa calculated for the b-slab (compared to the value of 126.2 MPa for the a-slab), approaches the estimated fracture limit around 160 MPa [34]. Owing to the lower αT along the b-axis, thermal focal lengths along the b-axis of the a- slab are longer under different incident powers, and the curves of the thermal lens along the a-axis fa and b-axis fb nearly merge if the b-axis is selected along the slab thickness with high thermal gradient (Fig. 3(c)). Under maximum incident power, fa and fb are 26.78 mm and 59.8 mm respectively in the a-slab and 41.2 mm and 32.7 mm respectively in the b-slab. Using a compactly designed cavity to tolerate the short fa, the a-slab was selected for subsequent experiments in order to minimize the risk of thermally-induced fractures under high power pumping, since the stress resulting from the high thermal expansion along the a-axis can be suppressed by the limited slab thickness.
Fig. 3. (a) Maximum temperatures, (b) maximum thermal stresses, and (c) thermal focal lengths under different incident LD powers of the a-slab and b-slab respectively, where the stress is expressed by the Van der Waals stress.
2.2 Spectral analyses for intra-cavity pumping
Figure 4 depicts the absorption spectra of the 0.8 at% HoYAG and the 0.5 at% Ho:YAP crystals, where emission spectra of the 3 at.% Tm:YAP crystal is inserted (Fig. 4(a)). The absorption spectra were measured using a Perkin Elmer UV-VIS-NIR spectrometer (Lambda-950). Samples of the three crystals were cut to the same dimensions of 4 mm×4 mm×1 mm, and the YAP crystals were cut along c axis. Polarized emission spectra of the Tm:YAP crystal were calculated based on the polarized absorption along the a- and b- axes according to the Macumber relationship [39], where noise at longer wavelength is attributed to the divergence from this model at wavelengths away from the emission peaks. As shown in Fig. 4, the polarized emission bands of Tm:YAP around 1.94 µm were consistent with the sub-absorption peak of Ho:YAG and the polarized absorption bands of Ho:YAP. At the emission peak of 1936 nm, the absorption coefficients were 0.31 cm−1 for Ho:YAG, 0.49 cm−1 and 0.36 cm−1 for Ho:YAP crystal along the a-axis and b-axis respectively. Such small absorption coefficients make it unsuitable for in-band pumping both Ho:YAG and Ho:YAP lasers in a compact structure, using a separated Tm:YAP laser. However, potential intra-cavity pumped Ho lasers were of interest since the measured absorption coefficients are higher than the maximum effective absorption coefficient of 0.17 cm−1 from the Ho:YAG crystal to the intra-cavity Tm:YAG laser in Ref. [14].
Fig. 4. (a) Polarized emission spectra of the c-cut Tm:YAP crystal and the absorption spectrum of the Ho:YAG crystal; (b) Polarized absorption spectra of the c-cut Ho:YAP crystal..
2.3 Experimental setup
The experimental setup of the Tm:YAP laser and its pumping for the Ho lasers is depicted in Fig. 5. A fiber-coupled 792 nm LD with core diameter of 200 µm and NA of 0.22 served as the pump source, which was imaged with a pump waist radius of 230 µm onto the surface of the a-slab by two plano-convex lenses with focal lengths of 30 mm and 60 mm respectively. The slab was anti-reflection (AR) coated from 760 nm to 820 nm at both end surfaces and wrapped with indium foil before being mounted into a cooper heat sink with micro channels for water-cooling at 16 °C. A plano-concave cavity with a length of 40 mm was applied for the TmYAP slab laser. M1 was AR coated at the pump wavelength and high reflection (HR) coated at 1800∼2200 nm. The output coupler (OC) M2 whose radius of curvature was 400 mm, has transmittances of 5% and 40% respectively at 1.9-2.2 µm. In order to explore the intra-cavity pumped Ho laser, 0.8 at.% Ho YAG crystal with 4 mm in diameter of 4 mm and 6 mm long, and 0.5 at.% c-cut Ho:YAP crystal with dimensions of 3×3×10 mm3 were considered. Each crystal was cooled at 16 °C and AR coated for 1.8-2.2 µm at either end. The plano-concave OC was then replaced by flat mirrors with transmittances of 2%, 10%, and 20% for the Ho laser (2.09∼2.2 µm) and HR coated for the Tm laser (1.8∼2.02 µm). The wavelengths of the Tm and Ho lasers were measured using a mid-infrared spectral analyzer (771 Bristol Inc.).
3. Results and discussions
3.1 Highly efficient Tm:YAP slab lasers
Using the 5% OC, a maximum output power of 30.5 W was obtained at 2000.6 nm under incident LD power of 55 W corresponding to a slope efficiency of 61.8% and optical conversion efficiency of 55.4% (Fig. 6(a)), which are higher than those reported by Tm:YAP lasers with similar power levels [19–22]. As shown in Fig. 6(a), as the pump power increases, the wavelength of the Tm:YAP laser red-shifts from 1993 nm to 2001 nm owing to the increased re-absorption loss of the lower Stark levels of the ground state manifolds 3H6 [14]. The beam quality at the output power of 30 W was measured with Mx2 of 3.19 and My2 of 2.68 in the transverse and vertical directions respectively by a Nanomodescan device (Ophir Optronics Ltd.)
Fig. 6. (a) Power of the Tm:YAP laser with the 5% and 40% OC, respectively. (b) Beam quality at the maximum output power of 30 W, where the lasing spectra at maximum powers for the 40% and 5% OCs have been inserted.
Except for the intra-cavity etalons [22] or volume Bragg gratings [15], the Tm:YAP laser at 1.94 µm could also be realized by increasing the transmittance of the OC. Following the model of Ref. [40], for the Tm:YAP slab under consideration, with doping concentration and length product of 51 at.%·mm, 40% OC was used for achieving the 1.94 µm laser, leading to a maximum output power of 12.1 W at 1936.8 nm with slope efficiency of 32.1%. Both the 2 µm and 1.94 µm Tm lasers were polarized parallel to the a-axis. The laser wavelength stabilized at 1936.2 nm ± 0.6 nm when the LD power was scaled up (Fig. 6(a)).
3.2 Intra-cavity pumped Ho:YAG laser
The Ho:YAG crystal was placed inside the cavity of the slab laser in order to achieve lasing at 2.1 µm, where OCs HR-coated at the Tm laser wavelength were applied. The maximum Ho laser powers of 729, 793 and 498 mW, with slope efficiencies of 16.6%, 21.2%, and 10.2% (Fig. 7) were obtained using 2%, 10%, and 20% OCs, respectively. The lasing thresholds increased slightly from 5 W, 6.7 W to 8.1 W with the increased transmittance of the OC.
The maximum output powers with different OCs were attained at the lasing wavelengths of 2122.2 nm for the 2% and 10% OCs and 2090 nm for the 20% OC, respectively (Figs. 8(d)-(f)). Except for the 2% OC, single Tm laser oscillation occurred at thresholds of the Ho:YAG laser with the 10% and 20% OCs due to high threshold of the Ho laser with high OC transmittance (Figs. 8(a)–8(c)). Moreover, the reflection for the Tm laser decreased with the increased OC transmittance for Ho laser, as the wavelengths between the Tm laser at 2000 nm and Ho laser at 2122 nm are too close to be separated in coating. Hence, the Tm:YAP laser oscillated outside the cavity before the Ho laser, where the start in the Ho laser signals were detected at output powers around 281 mW and 296 mW respectively with the 10% and 20% OCs.
Fig. 8. Lasing spectra of the intra-cavity pumped Ho:YAG laser with different output couplings. (a-c) At the Ho laser thresholds; (d-f) At the maximum output powers.
Away from the emission peak around 1936 nm (Fig. 4(a)), wavelength of the Tm laser was around 1995 nm as indicated by the leakage Tm laser in Fig. 8. As the wavelength of the intra-cavity Tm laser falls in the absorption valley of the Ho:YAG crystal for lowering the lasing loss, poor spectral overlap between the intra-cavity Tm:YAP laser and absorption spectrum of the Ho:YAG crystal led to saturation of the power of the Ho laser with different OCs and low Ho laser powers below 1 W. Hence, the combination between Tm:YAP and Ho:YAG crystals is not recommended for efficient Ho laser operation owing to the small absorption cross section of Ho:YAG crystal at the intra-cavity Tm laser wavelength of ∼2000 nm.
3.3 Intra-cavity pumped Ho:YAP laser
The c-cut Ho:YAP crystal with higher absorption at 2000 nm was replaced the Ho:YAG crystal, while the orientation of the crystal axes of the Ho:YAP crystal along the Tm:YAP slab was retained. Using the 2%, 10%, and 20% OCs, polarized Ho lasers with maximum output powers of 2.25 W, 5.51 W and 3.36 W were obtained with slope efficiencies of 8.6%, 20.9%, and 15.7% respectively (Fig. 9). The lasing thresholds of 7 W, 5.5 W, and 8 W were obtained as the transmittance of the OC increased from 2% to 20%. The polarization of the Ho:YAP laser was parallel to the b-axis with an extinction ratio of 17.2 dB at the maximum output power, and the corresponding beam quality was measured to be Mx2=2.18 and My2=1.87 in the horizontal and vertical directions, respectively.
Fig. 9. (a) Power curves of the intra-cavity pumped Ho:YAP laser with the 2%, 10%, and 20% OCs; (b) Beam quality of the Ho:YAP laser at the maximum output power. Inset: typical 2D beam profile.
The Ho laser wavelengths were stabilized at 2129.5 nm±0.2 nm for the 2% and 10% OCs and at 2102.2 nm±0.2 nm for the 20% OC (Figs. 10(d)-(f)). The same as that in the Ho:YAG laser, wavelength of the intra-cavity Tm laser was not within the 1.94 µm emission band of the Tm:YAP crystal (Fig. 4(a)) and fell into absorption valley of the Ho:YAP crystal at 2007 nm, which was indicated by the leakage in Tm laser signal at thresholds of the Ho:YAP laser with the 10% and 20% OCs, respectively (Figs. 10(b)–10(c)). However, unlike the Ho:YAG laser, the signal of the Tm laser could only be detected at powers around 50 mW owing to the higher absorption in Ho:YAP crystal at the absorption valley, where the absorption coefficient of 0.15 cm−1 (E//a) at 2007 nm was higher than of the value of 0.04 cm−1 at 1995 nm for the Ho:YAG crystal. (We note that the polarization of the c-cut Tm:YAP laser was measured along the a-axis.) This absorption is still comparable with the absorption coefficient of the Ho:YAG crystal at the Tm:YAG laser wavelength [14] and contributes to an efficient Ho:YAP laser (Fig. 9(a)). The higher power of the Ho:YAP laser pumped by the Tm:YAP laser can be predicted by optimizing the cavity structure, lengths and doping concentrations of the Tm-doped and Ho-doped gain media.
Fig. 10. Lasing spectra of the intra-cavity pumped Ho:YAP laser with different output couplings. (a-c) At the Ho laser thresholds; (d-f) At the maximum output powers.
The same as other intra-cavity pumped Ho lasers [41,42], self-pulsing was observed in the intra-cavity pumped Ho:YAG and Ho:YAP lasers. This can be explained by the saturable effect of the Ho-doped gain medium, which Q-switched the Tm laser and thus subsequently gain switched the Ho laser [41]. According to the statistical results of the oscilloscope, the pulse width and repetition frequency tended to be decreased and increased with the increased pump power, respectively. However, the pulse separation and amplitude were disorder in the pulse train (Fig. 11), which inhibited Q-switching in the intra-cavity pump Ho laser when directly inserting the acoustic optical modulator [41]. Recently, this issue was solved via inserting a spectral filter intra-cavity for separating the Tm laser from modulated by the Q-switching element, where successful Cr:ZnSe passively Q-switched Tm/Ho:YAG laser has been realized with an average output power of 474 mW at 7 kHz [43]. Hence, Q-switching in the intra-cavity pumped Ho:YAP laser with higher average output power and pulse energy is expected.
Fig. 11. Typical pulse train of the Tm:YAP slab laser pumped Ho lasers. (The image was obtained from the Ho:YAP laser with an output power of 2.8 W, where the pulse width varied from 56 ns to 142 ns).
4. Summary
In summary, since the thermal expansion along the crystallographic a-axis of the biaxial YAP crystal is twice that along the b-axis, conventional isotropic assumptions for evaluating the thermal effects in the Tm:YAP laser have been improved here by introducing the anisotropic properties of the YAP host for optimizing heat management of the Tm:YAP slab laser under high power pumping. Along the thickness of the c-cut slab, the a-axis possessing higher thermal expansion and thermal conductivity can significantly reduce the risk of damage to the slab. A maximum temperature and stress of 71.1 °C and 126 MPa were calculated under LD power of 60 W for the a-axis, compared to 73.3 °C and 148 MPa for the b-axis. In the a-slab structure, maximum output powers of 30 W at 2001 nm and 12 W at 1937 nm were obtained with corresponding optical conversion efficiencies of 55.4% and 25.6% respectively using the 5% and 40% OCs. Away from the emission peak around 1937 nm, wavelengths of the Tm:YAP laser falling into absorption valleys of the Ho:YAG crystal at 1995 nm and the Ho:YAP crystal at 2007 nm were discovered in the search for potential Ho laser operations via intra-cavity pumping. Due to the poor absorption of Ho:YAG crystal in the case of the Tm:YAP laser intra-cavity, maximum output power of 793 mW at 2122 nm with the leakage in Tm laser was obtained. In contrast, the efficient intra-cavity pumped Ho:YAP laser polarized along the b-axis reported a maximum output power of 5.5 W at 2129.5 nm, which is expected for higher output powers owing to the optimized cavity design and gain media structures. Although self-pulsing was observed in the intra-cavity pumping manner owning to the saturable effect of the Ho-doped gain medium, Q-switching in the Tm:YAP slab laser pumped Ho laser was achievable via filtering the Tm:YAP laser from the Ho laser intra-cavity with a spectral filter.
Funding
National Key Research and Development Program of China (2018YFB0407400); National Natural Science Foundation of China (61875200, 61905246).
Disclosures
The authors declare no conflicts of interest.
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