Open Access
Add to BibSonomy
Abstract
In this work, we have demonstrated tunable 1.8-µm laser operation based on a Tm:YVO4 cladding waveguide fabricated by means of femtosecond laser direct writing. Benefiting from the good optical confinement of the fabricated waveguide, efficient thulium laser operation, with a maximum slope efficiency of 36%, a minimum lasing threshold of 176.8 mW, and a tunable output wavelength from 1804 to 1830nm, has been achieved in a compact package via adjusting and optimizing the pump and resonant conditions of the waveguide laser design. The lasing performance using output couplers with different reflectivity has been well studied in detail. In particular, due to the good optical confinement and relatively high optical gain of the waveguide design, efficient lasing can be obtained even without using any cavity mirrors, thereby opening up new possibilities for compact and integrated mid-infrared laser sources.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Due to the pronounced absorption of H2O and CH2 [1], laser sources emitting at ∼1.8 µm have triggered much research interest in mid-infrared gas spectroscopy [2,3] and tomography [4]. Moreover, because of the strong absorption for polymers and the resonating features in C-H bonds, 1.8-µm laser sources also find important applications in polymer processing [5] as well as in the selective treatment of hydrocarbon-rich sites (such as collagen-based tissues) [6–8]. In this respect, trivalent thulium (Tm3+) doped laser gain media are identified as one of the most attractive candidates for mid-infrared, including 1.8-µm, laser sources as a result of the wideband emission characteristics of Tm3+ at the wavelength range of 1.7-2.1 µm [9]. Compared with Ho3+-doped lasers, which generally operate at the wavelength of around 2.1 µm, Tm3+-doped lasers operate at wavelengths close to or less than 2 µm [10–13]. However, the closely overlapped absorption bands of Tm3+ at the 3F4 energy manifold leads to a prominent three-level behavior and reabsorption at the end of its emission band, making it a challenge to demonstrate Tm lasers operating at ∼1.8 µm [14]. In recent years, mid-infrared lasers that operate at 1.8 µm and below have been fabricated with thulium-doped fibers [14–17], microcavities [18], and crystals [19]. Nevertheless, thulium-doped fiber lasers generally need the help of optical filtering such as acousto-optic- [14], tunable bandpass- [15] and hybrid interference filters [17] to access the shorter end of the laser emission, leading to a complicated installation and a wide-band output with significant background noises [15,17]. The thulium-doped microcavities are suffering from the complicated fabrication processes, relatively low lasing power, and multi-mode operation. In case of solid-state Tm lasers, in 2009, M.J.D. Esser et al. has achieved a diode-end-pumped Tm:GdVO4 laser operating at 1818nm [20]. Therein, the reflectivity of the laser output coupler (OC) is adjusted from 95% (at 1.9 µm) to 28% (at 1.82 µm), but the intracavity power intensity is largely reduced, making the lasing threshold increases from 5.6 W (at 1.9 µm) to 21.9 W (at 1.82 µm).
With the capability of limiting light field within a µm-scale area along a relatively long interaction length, optical waveguides offer a much higher light density in the guiding channel, as such the light-matter interaction and thus the localized optical properties can be largely enhanced [21–24]. In particular, if the waveguide gain medium exhibits both high optical gain and low propagation loss, the tiny Fresnel reflection at the in-/out-coupling end-faces can provide sufficient optical feedback for laser oscillation [21]. Up to now, several Tm3+-based waveguides have been fabricated by different methods, such as ion beam etching [25], ion diffusion [26], reactive co-sputtering [27], diamond saw dicing [28] and femtosecond-laser direct writing (FsLDW) [29–32]. With the capability of fabricating flexible three-dimensional waveguide structures in varieties of optical materials, FsLDW has become a prominent fabrication method for optical waveguides since its first demonstration at 1996 [32–34]. In 2019, Esrom Kifle et al. has achieved a Q-switched laser operation with thulium-doped FsLDW waveguides [29], exhibiting the great potential of FsLDW for the fabrication of Tm3+-doped waveguides. Therein, the so-far shortest wavelength of ∼1850nm for Tm3+-doped FsLDW waveguide laser output has been demonstrated. However, only high-reflectivity OCs (with transmittance of T = 20% and 30%) were used while no boundary value of the cavity conditions has been discussed. Moreover, the output wavelengths of around 1850nm are still subject to the strong re-absorption of Tm3+ ions under high-power operation. Up to date, the experimental demonstration of Tm waveguide lasers without using any cavity couplers for providing optical feedback is still missing.
In this work, we have fabricated FsLDW channel waveguides based on Tm:YVO4 crystals. Tm:YVO4 is widely used in mid-infrared laser operations, but the performance of Tm:YVO4 waveguide lasers has not been studied before. By means of adjusting the reflectivity of OCs (with a reflectivity of R = 20%, 40%, and ∼5%), a low-threshold laser tunable from 1804nm to 1830nm with a slope efficiency of ∼36% at maximum has been achieved. Our work has finely achieved a narrow-band short-end emission of thulium lasers with barely affecting the laser properties, exhibiting the great potential of Tm:YVO4, as well as the fascinating cooperation of low-reflectivity coupler system together with FsLDW waveguides for thulium lasers operating at the short end of the emission band of Tm3+. Besides, we have achieved a slope efficiency of ∼17% without using any cavity mirrors, providing favorable conditions for compact and integrated mid-infrared laser sources.
2. Experimental
The z-cut Tm:YVO4 crystal (fabricated by the Czochralski method with Tm3+ concentration of 3 at.%) was purchased from Atom Optics Co., Ltd., with a dimension of 10(x) × 3(y) × 2(z) mm3 and optically polished before the FsLDW fabrication. The x-, y-, and z-axes are parallel to the a-, b-, and c-axes of the crystal, respectively. The waveguide was fabricated by utilizing a fiber-coupled fs-laser system (FemtoYL-25, YSL Photonics). With an optical microscope objective (Sigma, 50×, N.A. = 0.45), the linearly polarized fs laser (with a central wavelength of 1030 nm, a pulse width of 400 fs, a repetition rate of 25 kHz, and a pulse energy of 2.21 µJ) was focused ∼ 100 µm beneath the crystal surface. The sample was placed on a PC-controlled translation stage with a moving speed of 3 mm/s. The polarization of the fs laser was parallel with the direction of stage translation, which was parallel to the y-axis of the crystal. We note here that the scale of the Tm:YVO4 crystal was 10(x) × 10(y) × 2(z) mm3 when fabricated by fs-laser writing, and it was cut into two parts (10(x) × 3(y) × 2(z) mm3 and 10(x) × 6(y) × 2(z) mm3, corresponding to waveguide lengths of 3 and 6 mm, respectively) for better lasing properties. As such, a 6-mm-long waveguide with identical parameters in a separate piece of Tm:YVO4 crystal wafer was also obtained for further comparison. The microscope image of the waveguide end-face was obtained with a metalloscope (Axio Imager, Carl Zeiss), while an end-face coupling system is set to investigate the lasing performance (mode profiles, slope efficiency, polarization properties, and so on) of the fabricated Tm:YVO4 waveguides (see Fig. 1). Three types of waveguide cavities, namely G-G (glass wafers at the both sides of waveguides), IC-G (an input coupler and a glass wafer as OC), and IC-OC (with the OC reflectivity of R = 60% or 80%) cavities. The glass wafer (with a refractive index of about 1.44 at lasing wavelength) can be regarded approximately as a low-reflectance coupler with a reflectivity of ∼5%. Here, the Fresnel reflectivity is calculated by $\frac{{{{({{n_i} - {n_t}} )}^2}}}{{{{({n_i} + {n_t})}^2}}}$, where the ni and nt represent the refractive indices for the crystal-glass (R = ∼2.3%) and glass-air (R = ∼3.3%) interfaces, respectively. Besides, the Fresnel reflectivity is calculated to be around 10% without using any couplers, i.e., the glass plays a role for sample fixing and theoretically reduction of the Fresnel reflection compared with the crystal facet only. Generally, ceteris paribus, the intracavity light intensity will increase by the increase of OC reflectivity, except for extreme high-loss conditions. Different OCs will also influence the quality factor (Q-factor) of cavities.
Fig. 1. The end-face coupling system of Tm:YVO4 waveguide laser. L1 & L2, Optical Lens; IC, input coupler; WG, Tm:YVO4 waveguides; OC, output coupler. The inset exhibits the sketch of the chip and fs laser writing.
The pump laser source used for waveguide laser characterization is a continuous-wave Ti:sapphire laser (Coherent MBR-110) with output laser tunable at 760-820 nm. The linearly-polarized pump laser was focused into waveguides by an optical lens (Daheng Optics, 20×, N.A. = 0.40), and the light output from waveguides was collected onto a detector by an another objective lens (Daheng Optics, 20×, N.A. = 0.40). Different waveguide cavity conditions were applied by using input coupler/OCs with different reflectivity at pump/lasing wavelength or even low-reflection design (R = ∼5% at both end-faces) to investigate the lasing performance of the fabricated Tm:YVO4 waveguide. A longpass filter with the cut-off wavelength of 1600 nm was set to eliminate the pump interference, after which we use a laser beam profiler (Scanning Silt Beam Profilers, DataRay Inc.), an infrared spectrometer (WaveScan A.P.E.), and power meters to study the laser performance.
3. Results and discussion
Figure 2(a) shows an optical microscopic image of the fabricated waveguide end-face, and no crystalline damage can be identified in the waveguide area, indicating that the original lattice matrix and optical properties of Tm:YVO4 are well-preserved within the waveguide volume. However, there are semicircle tracks upside of the waveguide area, because the birefringence of Tm:YVO4 crystals can lead to the fs laser focusing into two different depths in a single-scan process (Fig. 2(b)). This phenomenon has been reported with Nd:GdVO4 crystals [35,36], which own the similar birefringence properties to YVO4 crystals. The relatively darker performance inside the waveguide volume is due to the blocking effect of the cladding area, while part of the blocked light is limited near the fs-induced cladding tracks, exhibiting localized lattice expansion caused by FsLDW and the resulting residual stress field. Figure 2(c) exhibits the linear optical absorption spectrum measured at the unmodified bulk area of the Tm:YVO4 crystal we used in this work. The absorption was measured by UV spectrophotometer (UV-1800, Shimadzu), where the unpolarized light has input along z-axis (with a thickness of 2 mm), and the absorption spectrum is well-matched with the 3H6→3H4 absorption bands of Tm:YVO4 as studied before [37,38]. The polarized absorption of Tm:YVO4 crystals have also been studied previously [38]. The absorption reaches the maximum at 797 nm, therefore, the pump laser in our work was operated at 797 nm for the highest gain property and optimized lasing performance.
Fig. 2. (a) The end-face image of the FsLDW waveguide. (b) the art works of twin tracks of single-scan FsLDW. (c) Wavelength-dependent linear optical absorption of Tm:YVO4 at around 800 nm. The insert shows.
The near-field distributions of the output laser along TE (parallel with y axis of the crystal) and TM (parallel with z axis of the crystal) polarizations are shown in Figs. 3(a) and 3(b), respectively. The laser modes are captured under the pump power of 1 W, with the input coupler (high reflectivity of >99% at 1850-1950nm, high transmittance of >95% at 780-980 nm) and glass OC (∼5% reflectivity). We note here that no significant difference can be identified for the laser modes generated by using different pump conditions or OCs, suggesting the good robustness of the used waveguide cavity. The relationship between the ∼1.8 µm output power and launched power under ∼797 nm is shown in Figs. 3(c) and 3(d), respectively. The laser power is measured by using Thorlabs PM100D powermeter and S121C probe. While calculating, the transmittance of the input lens and output objective at pump and lasing wavelengths, respectively, has been considered. In this work, the output laser power along TE polarization is always larger than that along TM polarization under the same pump and cavity conditions. It is most likely because that the cladding geometry that fabricated by FsLDW exhibits a better optical confinement along TE polarization and some residual stress around the waveguide core that mentioned above, as demonstrated previously in many other works that based on FsLDW YVO4 waveguides [39]. The relatively higher absorption for Tm:YVO4 crystals of the 797-nm light along TE polarization (x-axis) could also be another cause [37]. With TE-polarization pumping, the slope efficiency η reaches the highest of ∼35.7% under the waveguide cavity composed by IC and glass (R = ∼5%), where the maximum of output power reaches 274.3 mW. By lowering the OC transmittance, or to say increasing the OC reflectivity, the laser slope efficiency values and the determined lasing thresholds (with a minima of 176.8 mW) decrease due to the stronger optical feedback while weakened output coupling efficiency provided by using high-reflectivity OCs. Besides, benefiting from the high gain and low loss, the Tm waveguide in our work can even operates with G-G cavity, where the minor Fresnel reflection at incoupling and outcoupling end-faces can already provide sufficient optical feedback. The slope efficiency of the low-reflectivity coupler system remains 16.6%, where the threshold power is ∼213.4 mW along TE polarization. As a reference, the 6-mm-long waveguide, which owns the same parameters as the 3-mm-long sample as is mentioned above, exhibits a lower slope efficiency and a much higher lasing threshold compared to that of the one we used in this work (with a length of 3 mm), for the longer interaction length leads to higher losses, declining the power intensity and the gain properties in the cavity. Furthermore, no thermal bleaching effect have been observed during lasing operation.
Fig. 3. The image of laser beam quality analyzer along TE (a) and TM (b) polarizations. Output powers with different launched powers and optical couplers along TE (c) and TM (d) polarizations.
We further investigated the dependence of laser slope efficiency with pump polarization under different pump and OC conditions. The pump wavelength is set as 797 nm with an output power of 1 W. The polarization-dependent graph of lasing efficiency is shown in Fig. 4(a). It is worth mentioning that with the increase of the coupler transmittance (from 20% to 95%), the ratio ηTE/ηTM between the slope efficiency values along TE and TM polarizations has also been decreased from 3.33 to 1.55. It is because the re-absorption effect that related to the 3H6→3F4 transition of Tm3+ ions is stronger along TE polarization. In other words, with the increment of light intensity in the waveguide cavity (i.e., with higher coupler reflectivity), the output power along TE polarization increases slower compared with that of TM polarization, leading to the decrease of ηTE/ηTM. Simultaneously, Tm:YVO4 owns a relatively lower gain property along TM polarization [37,38], which makes it harder to lase with a lower light intensity in the waveguide. It also results in the higher increase of lasing threshold power along TM polarization, as shown in Fig. 3(d). The pump-wavelength-dependent output laser slope efficiency is shown in Fig. 4(b). The data was measured based on the IC-G cavity, which owns the highest efficiency as shown in Fig. 3(d). Besides, these vibration trends are quite identical by using different cavity conditions, with only decreased efficiency values. The result of Fig. 4(b) is well-fitted with the polarized absorption spectra in the former research [38], achieving a broadband stimulation with the pump wavelength from 780 nm to 810 nm. In particular, the waveguide can lase under a slightly shorter pump wavelength along TE polarization as shown in Fig. 3(a), because the cladding waveguide fabricated by FsLDW exhibits a slight asymmetry of the cladding structures and some residual stress around the waveguide core, providing a better optical confinement along TE polarization [39].
Fig. 4. (a) The relationship between polarization and slope efficiency with different waveguide cavities under the pump power of 1 W. (b) The relationship between the slope efficiency and wavelengths under different polarization.
Figure 5(a) exhibits the wavelengths of output laser under different waveguide cavities. The wavelength information is measured by a spectrometer (A•P•E waveScan, Angewandte Physik und Elektronik GmbH). The waveguide tends to lase at longer wavelengths with high-reflection OCs, corresponding to high intra-cavity intensity. As the peak of thulium emission spectrum is near 1.8 µm, while the peak of thulium absorption spectrum exhibits a blue shift compared with it of the emission spectrum, the waveguide tends to exhibit gain-oriented lase at ∼1.8 µm with a lower Q-factor cavity (lower quality factor indicates a higher roundtrip loss, corresponding to lower reflectivity coupler). On the contrary, with a high-Q (high reflectivity coupler) the waveguide is more likely to lase at longer wavelengths, where it exhibits a lower re-absorption of Tm3+ and a more easily-achieved population inversion. Meanwhile, the lasing mode is optimized under the higher light intensity in the cavity. Figure 5(b) exhibits the lasing wavelength under different launched power with IC-G cavity along TE polarization. With the increase of pump power, the laser output has shown a red shift, where the lasing mode that shorter than 1805nm has decreased to zero and a secondary peak at ∼1815nm has appeared. This result has re-confirmed that the thulium waveguide laser tends to operate at a longer wavelength with a higher light intensity in the cavity.
Fig. 5. (a) The wavelength of output laser under different waveguide cavities. (b) The wavelength of output laser under different launched power with IC-G optical couplers along TE polarization.
In comparison to previously demonstrated works (see Table 1), our work demonstrates the highest output power under the shortest wavelength output. It mainly benefits from the relatively lower power intensity in the cavity, i.e., lower re-absorption and lower heat bleaching, and the well lasing properties of Tm:YVO4 waveguides. The lower light intensity makes it harder to reach the stimulating threshold, which leads to a relatively higher threshold power as 213 mW compared with some other Tm3+-doped waveguide lasers. The slope efficiency of our work reaches 34%, which may suffer from the absorption peak of 3H6→3F4 transition at ∼1800nm. The end-face of the waveguide core is not a perfect circle, as such the coupling efficiency may be lower than the calculated value of 96%, making the calculated efficiency decreased. Besides, a self Q-switched output waveform is observed in our work (see Fig. 6), and the slope efficiency of 36% would reaches the highest level if compared with the other Tm3+-doped passively Q-switched waveguide lasers so far. The relatively higher efficiency and lower need of light intensity in the cavity also make it possible for utilizing varieties of saturable absorbers with a lower transmittance. It exhibits a great potential for FsLDW Tm:YVO4 waveguides in mid-infrared pulsed lasers. In the future work, we could increase the doping concentration of Tm3+ for a higher absorption cross section. The co-doped materials are also in our consideration, for the cross-relaxation of the co-doped ions may further reduce the re-absorption of the waveguides, leading to better lasing properties such as higher output power and slope efficiency, and relatively lower output wavelengths as well. We could also adjust and test different processing parameters to achieve better waveguide cavity properties, such as better optical confinement, lower propagation loss and a better lasing mode. No-coupler mid-infrared laser operation is also a potential alternative in the future work.
Table 1. Comparisons of 0.8-µm Pumped Continuous-wave Laser Properties Based on FsLDW Tm3+-doped Waveguides
4. Conclusion
In this work, we have achieved a thulium waveguide laser by means of femtosecond laser direct writing. With a little-reflectivity OC cavity configuration, the Tm:YVO4 cladding waveguide have achieved a slope efficiency of 36% at the wavelength as short as 1806nm with an output power as high as 274.3 mW. A low-reflectivity coupler (R = ∼5% at both end-faces) Tm waveguide laser with slope efficiency of 17% have also been fabricated. The relationship between lasing performances (threshold power, lasing wavelength, maximum of output power, and so on) and waveguide cavity properties have also been illustrated in detail. “Through using different optical couplers for waveguide cavity, this work has broadened the spectral range of thulium-doped waveguide lasers. It has also proposed a possibility for the cooperation of low-reflectivity cavity with high-gain thulium waveguides. By combining a number of advantages such as low re-absorption, low heat effect, broadened laser spectrum and easy integration, Tm:YVO4 as well as FsLDW waveguides exhibit a great potential in the research of compact and integrated mid-infrared lasers.”
Funding
National Natural Science Foundation of China (12074223, 12204274); Natural Science Foundation of Shandong Province (2022HWYQ-047, ZR202110280017, ZR2021ZD02); Taishan Scholar Foundation of Shandong Province (tspd20210303, tsqn201909041); Shandong University.
Acknowledgements
The authors gratefully acknowledge Mr. Q. Lu from Shandong University for his kind help on crystal processing.
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 maybe obtained from the authors upon reasonable request.
References
1. A. Stark, L. Correia, M. Teichmann, S. Salewski, C. Larsen, V. Baev, and P. Toschek, “Intracavity absorption spectroscopy with thulium-doped fibre laser,” Opt. Commun. 215(1-3), 113–123 (2003). [CrossRef]
2. P. Fjodorow, O. Hellmig, and V. M. Baev, “A broadband Tm/Ho-doped fiber laser tunable from 1.8 to 2.09 µm for intracavity absorption spectroscopy,” Appl. Phys. B 124(4), 62 (2018). [CrossRef]
3. O. A. Romanovskii, Ya. O. Romanovskii, S. A. Sadovnikov, O. V. Kharchenko, and S. V. Yakovlev, “Simulation of the remote atmospheric sounding by OPO lidar system in the near- and mid-IR,” Proc. SPIE 10791, Lidar Technologies, Techniques, and Measurements for Atmospheric Remote Sensing XIV107910I (2018).
4. M. Yamanaka, N. Hayakawa, and N. Nishizawa, “High-spatial-resolution deep tissue imaging with spectral-domain optical coherence microscopy in the 1700-nm spectral band,” J. Biomed. Opt. 24(07), 1–070502 (2019). [CrossRef]
5. A. Wittmann, O. Hentschel, A. Sommereyns, and M. Schmidt, “Generation of Polyamide 12 Coatings on Stainless Steel Substrates by Directed Energy Deposition with a Thulium-Doped Fiber Laser (DED-LB/P),” Polymers 14(18), 3729 (2022). [CrossRef]
6. R. H. Wilson, K. P. Nadeau, F. B. Jaworski, B. J. Tromberg, and A. J. Durkin, “Review of short-wave infrared spectroscopy and imaging methods for biological tissue characterization,” J. Biomed. Opt. 20(3), 030901 (2015). [CrossRef]
7. E. Park, Y. J. Lee, C. Lee, and T. J. Eom, “Effective photoacoustic absorption spectrum for collagen-based tissue imaging,” J. Biomed. Opt. 25(05), 1 (2020). [CrossRef]
8. V. V. Alexander, K. Ke, Z. Xu, M. N. Islam, M. J. Freeman, B. Pitt, M. J. Welsh, and J. S. Orringer, “Photothermolysis of sebaceous glands in human skin ex vivo with a 1,708 nm Raman fiber laser and contact cooling,” Lasers Surg. Med. 43(6), 470–480 (2011). [CrossRef]
9. S. D. Jackson, “The spectroscopic and energy transfer characteristics of the rare earth ions used for silicate glass fibre lasers operating in the shortwave infrared,” Laser Photonics Rev. 3(5), 466–482 (2009). [CrossRef]
10. L. Zhang, J. Zhang, Q. Sheng, C. Shi, W. Shi, and J. Yao, “Watt-level 1.7-mu m single-frequency thulium-doped fiber oscillator,” Opt. Express 29(17), 27048–27056 (2021). [CrossRef]
11. T. Li, F. Yan, X. Du, X. Wang, P. Wang, Y. Suo, H. Zhou, and K. Kumamoto, “Wavelength-switchable dual-wavelength thulium-doped fiber laser utilizing photonic crystal fiber,” Opt. Commun. 528(1), 129033 (2023). [CrossRef]
12. A. S. Sharbirin, H. Ahmad, and M. F. Ismail, “Q-switched Thulium-doped fiber laser at 1860nm and 1930nm using a Holmium-doped fiber as an amplified spontaneous emission filter,” Opt. Laser Technol. 123, 105908 (2020). [CrossRef]
13. C. Zhang, J. Liu, X. W. Fan, Q. Q. Peng, X. Guo, D. P. Jiang, X. Qian, and L. B. Su, “Compact passive Q-switching of a diode-pumped Tm, Y:CaF2 laser near 2 µm,” Opt. Laser Technol. 103, 89–92 (2018). [CrossRef]
14. T. Noronen, O. Okhotnikov, and R. Gumenyuk, “Electronically tunable thulium-holmium mode-locked fiber laser for the 1700-1800nm wavelength band,” Opt. Express 24(13), 14703–14708 (2016). [CrossRef]
15. H. Ahmad, A. S. Sharbirin, and M. F. Ismail, “1.8 µm passively Q-switched thulium-doped fiber laser,” Opt. Laser Technol. 120, 105757 (2019). [CrossRef]
16. S. Chen, Y. Chen, K. Liu, R. Sidharthan, H. Li, C. J. Chang, Q. J. Wang, D. Tang, and S. Yoo, “All-fiber short-wavelength tunable mode-locked fiber laser using normal dispersion thulium-doped fiber,” Opt. Express 28(12), 17570–17580 (2020). [CrossRef]
17. H. Wei, Z. Lianqing, D. Mingli, and L. Fei, “A 1.8-µm multiwavelength thulium-doped fiber laser based on a hybrid interference filter,” Int. J. Optomechatronics 10(3-4), 154–161 (2016). [CrossRef]
18. Z. Su, N. Li, E. Salih Magden, M. Byrd, P. Purnawirman, T. N. Adam, G. Leake, D. Coolbaugh, J. D. Bradley, and M. R. Watts, “Ultra-compact and low-threshold thulium microcavity laser monolithically integrated on silicon,” Opt. Lett. 41(24), 5708–5711 (2016). [CrossRef]
19. H. Hu, H. Huang, J. Huang, J. Deng, W. Weng, J. Li, and W. Lin, “Watt-level passively Q-switched Tm: YVO4 laser with few-layer WSe2 saturable absorber,” Infrared Phys. Tech. 113, 103554 (2021). [CrossRef]
20. M. J. D. Esser, D. Preussler, E. H. Bernhardi, C. Bollig, and M. Posewang, “Diode-end-pumped Tm:GdVO4 laser operating at 1818 and 1915nm,” Appl. Phys. B 97(2), 351–356 (2009). [CrossRef]
21. F. Chen, “Micro- and submicrometric waveguiding structures in optical crystals produced by ion beams for photonic applications,” Laser Photonics Rev. 6(5), 622–640 (2012). [CrossRef]
22. S. Gross and M. J. Withford, “Ultrafast-laser-inscribed 3D integrated photonics: challenges and emerging applications,” Nanophotonics 4(3), 332–352 (2015). [CrossRef]
23. Y. Jia, J. Wu, X. Sun, X. Yan, R. Xie, L. Wang, Y. Chen, and F. Chen, “Integrated photonics based on rare-earth ion-doped thin-film lithium niobate,” Laser Photonics Rev. 16(9), 2200059 (2022). [CrossRef]
24. E. Kifle, P. Loiko, C. Romero, J. R. V. de Aldana, A. Ródenas, V. Jambunathan, V. Zakharov, A. Veniaminov, A. Lucianetti, T. Mocek, M. Aguiló, F. Díaz, U. Griebner, V. Petrov, and X. Mateos, “Fs-laser-written erbium-doped double tungstate waveguide laser,” Opt. Express 26(23), 30826–30836 (2018). [CrossRef]
25. K. van Dalfsen, S. Aravazhi, C. Grivas, S. M. García-Blanco, and M. Pollnau, “Thulium channel waveguide laser with 1.6 W of output power and ∼80% slope efficiency,” Opt. Lett. 39(15), 4380–4383 (2014). [CrossRef]
26. D. P. Shepherd, D. J. B. Brinck, J. Wang, A. C. Tropper, D. C. Hanna, G. Kakarantzas, and P. D. Townsend, “1.9-µm operation of a Tm:lead germanate glass waveguide laser,” Opt. Lett. 19(13), 954–956 (1994). [CrossRef]
27. N. Li, P. Purnawirman, Z. Su, E. Salih Magden, P. T. Callahan, K. Shtyrkova, M. Xin, A. Ruocco, C. Baiocco, E. P. Ippen, F. X. Kärtner, J. D. B. Bradley, D. Vermeulen, and M. R. Watts, “High-power thulium lasers on a silicon photonics platform,” Opt. Lett. 42(6), 1181–1184 (2017). [CrossRef]
28. P. Loiko, R. Soulard, G. Brasse, J. L. Doualan, B. Guichardaz, A. Braud, A. Tyazhev, A. Hideur, and P. Camy, “Watt-level Tm:LiYF4 channel waveguide laser produced by diamond saw dicing,” Opt. Express 26(19), 24653–24662 (2018). [CrossRef]
29. E. Kifle, P. Loiko, J. R. V. de Aldana, C. Romero, A. Ródenas, V. Zakharov, A. Veniaminov, H. Yu, H. Zhang, Y. Chen, M. Aguiló, F. Díaz, U. Griebner, V. Petrov, and X. Mateos, “Fs-laser-written thulium waveguide lasers Q-switched by graphene and MoS2,” Opt. Express 27(6), 8745–8755 (2019). [CrossRef]
30. M. Ams, G. D. Marshall, P. Dekker, J. A. Piper, and M. J. Withford, “Ultrafast laser written active devices,” Laser Photonics Rev. 3(6), 535–544 (2009). [CrossRef]
31. E. Kifle, P. Loiko, C. Romero, J. R. V. de Aldana, M. Aguiló, F. Díaz, P. Camy, U. Griebner, V. Petrov, and X. Mateos, “Watt-level ultrafast laser inscribed thulium waveguide lasers,” Prog. Quantum Electron. 72, 100266 (2020). [CrossRef]
32. F. Chen and J. R. V. de Aldana, “Optical waveguides in crystalline dielectric materials produced by femtosecond-laser micromachining,” Laser Photonics Rev. 8(2), 251–275 (2014). [CrossRef]
33. K. M. Davis, K. Miura, N. Sugimoto, and K. Hirao, “Writing waveguides in glass with a femtosecond laser,” Opt. Lett. 21(21), 1729–1731 (1996). [CrossRef]
34. K. Sugioka and Y. Cheng, “Femtosecond laser three-dimensional micro-and nanofabrication,” Appl. Phys. Rev. 1(4), 041303 (2014). [CrossRef]
35. C. Cheng, J. R. Vázquez de Aldana, and F. Chen, “A novel microprocessing of waveguide coupler in birefringent crystal by twin tracks of single-scan femtosecond laser writing,” Proc. SPIE 9532, Pacific Rim Laser Damage 2015: Optical Materials for High-Power Lasers, 95321 K (2015).
36. S. Li, F. Deng, and Z. Huang, “Femtosecond laser inscription waveguides in Nd:GdVO4 crystal,” Opt. Eng. 55(10), 107104 (2016). [CrossRef]
37. H. Saito, S. Chaddha, R. S. E. Chang, and N. Djeu, “Efficient 1.94-µm Tm3+ laser in YVO4 host,” Opt. Lett. 17(3), 189–191 (1992). [CrossRef]
38. R. Lisiecki, P. Solarz, G. Dominiak-Dzik, W. Ryba-Romanowski, and T. Łukasiewicz, “Effect of temperature on spectroscopic features relevant to laser performance of YVO4:Tm3+, GdVO4:Tm3+, and LuVO4:Tm3+ crystals,” Opt. Lett. 35(23), 3940–3942 (2010). [CrossRef]
39. Y. Jia, R. He, J. R. V. de Aldana, H. Liu, and F. Chen, “Femtosecond laser direct writing of few-mode depressed-cladding waveguide lasers,” Opt. Express 27(21), 30941–30951 (2019). [CrossRef]
40. D. G. Lancaster, S. Gross, H. Ebendorff-Heidepriem, K. Kuan, T. M. Monro, M. Ams, A. Fuerbach, and M. J. Withford, “Fifty percent internal slope efficiency femtosecond direct-written Tm3+:ZBLAN waveguide laser,” Opt. Lett. 36(9), 1587–1589 (2011). [CrossRef]
41. D. G. Lancaster, S. Gross, A. Fuerbach, H. E. Heidepriem, T. M. Monro, and M. J. Withford, “Versatile large-mode-area femtosecond laser-written Tm: ZBLAN glass chip lasers,” Opt. Express 20(25), 27503–27509 (2012). [CrossRef]
42. F. Fusari, R. R. Thomson, G. Jose, F. M. Bain, A. A. Lagatsky, N. D. Psaila, A. K. Kar, A. Jha, W. Sibbett, and C. T. A. Brown, “Lasing action at around 1.9 µm from an ultrafast laser inscribed Tm-doped glass waveguide,” Opt. Lett. 36(9), 1566–1568 (2011). [CrossRef]
43. Y. Ren, G. Brown, A. Ródenas, S. Beecher, F. Chen, and A. K. Kar, “Mid-infrared waveguide lasers in rare-earth-doped YAG,” Opt. Lett. 37(16), 3339–3341 (2012). [CrossRef]
44. J. Morris, N. K. Stevenson, H. T. Bookey, A. K. Kar, C. T. A. Brown, J.-M. Hopkins, M. D. Dawson, and A. A. Lagatsky, “1.9 µm waveguide laser fabricated by ultrafast laser inscription in Tm:Lu2O3 ceramic,” Opt. Express 25(13), 14910–14917 (2017). [CrossRef]
45. Y. Morova, M. Tonelli, and A. Sennaroglu, “Fabrication of femtosecond laser written depressed-cladding waveguides in Tm3+: BaY2F8 crystal and laser operation near 2 µm,” Opt. Mater. (Amsterdam, Neth.) 126, 112121 (2022). [CrossRef]
