Open Access
Add to BibSonomy
Abstract
We demonstrated a novel cascaded nonlinear frequency conversion that integrated OPO, SFG and SRS in two KTA crystals, realizing an efficient wavelength conversion from 1064 nm to multi-wavelength around 630 nm. The OPO and SRS were both performed in an x-cut KTA crystal to realize a noncritically phase-matched OPO and X(ZZ)X Raman conversion. The SFG was achieved in a (θ = 90◦, φ = 24.3◦)-cut KTA crystal with a type-II phase-matching configuration. Benefitting from the strong pulse-narrowing induced by the combined effect of OPO and SRS, a minimum pulse width of 13.5 ns was obtained, corresponding to a pulse energy of 0.3 mJ and a pulse peak power of 22.2 kW. The multiple operation parameters of wavelength, average output power, pulse duration and repetition rate can be coordinated to explore new treatment plan in photodynamic therapy. Furthermore, the designability of cascaded nonlinear optical frequency conversion could make the nonlinear optical technology accessible to a much wider range of potential users.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Nonlinear optics has been generally believed to have its origins in a seminal experiment of frequency doubling a ruby laser beam focused into a quartz crystal, demonstrated by Peter Franken and associates in 1961 [1]. Then the necessity to achieve new wavelengths that are not directly accessible with lasers and to develop tunable coherent light sources from ultraviolet to visible, mid-infrared, and terahertz band has led to a continuous exploration of nonlinear optical frequency conversions (NOFCs) [2–5]. Phase matching is one of the paramount requirements for achieving efficient NOFCs. However limited by the chromatic dispersion and birefringence properties, a nonlinear bulk crystal usually corresponds to a single phase-matched parametric nonlinear frequency conversion, such as frequency doubling, sum frequency generation (SFG), and optical parametric oscillation (OPO). Therefore, wavelengths that can be produced by NOFCs are limited by the available fundamental laser wavelengths and the used nonlinear interaction processes.
Cascaded NOFC is accordingly developed to satisfy the practical need for coherent light sources with a desired wavelength or range of wavelengths that cannot be produced by a single NOFC. Cascaded NOFC is usually cooperatively accomplished by different χ(2)-nonlinearity-based nonlinear processes in several nonlinear optical crystals. Quasi-phase matching (QPM) technique can integrate several separate frequency conversion steps in a single periodically poled crystal [6,7]. However, the fabrication of micro-structured QPM-crystal with high and reliable quality is a challenge, and is possible only with certain ferroelectric crystals with fairly limited thickness. This limits the effective aperture of the QPM-crystal for high power levels. The cascaded NOFC employing those technologically important and industrially mature bulk crystals should be more cost-effective and more user-friendly. Frequency tripling of 1064 nm can be conveniently realized as a cascaded scheme, beginning with frequency doubling of the input 1064 nm beam in one crystal and subsequent sum frequency mixing of 1064 and 532 nm in another crystal [8,9]. The χ(3) nonlinearities in nonlinear optical crystal (such as stimulated Raman scattering, SRS) can also be utilized in a cascaded NOFC, offering new functionalities to the available device. A self-frequency-doubled KTiOAsO4 (KTA) Raman laser at 573 nm intracavity pumped by an AO Q-switched 1064 nm laser has been realized by Z. J. Liu and associates [10]. F. Bai et al. have demonstrated a 1810 nm KTiOPO4 (KTP) OPO intracavity driven by a 1180 nm SrWO4 Raman laser with a 1064 nm pumping [11]. Our group had demonstrated a compact noncritically phase-matched (NCPM) KTP OPO intracavity driven by a 1092 nm KTA Raman laser, generating 1625 nm OPO signal with high beam quality [12]. The advantage of cascaded NOFC lies in the efficient wavelength conversion of well-developed fundamental lasers to desired wavelengths beyond the reach of single NOFC by using commercially available nonlinear optical crystals. This cost-effective method could make the nonlinear optical technology accessible to a much wider range of potential users.
In this paper, we demonstrated a novel cascaded nonlinear frequency convertor that integrated OPO, SFG and SRS in two KTA crystals. This cascaded process began with a 1064 nm pumped x-cut KTA OPO to generate 1535 nm signal radiation. Then it was followed by sum-frequency mixing the resonated 1535 nm with 1064 nm light in another critically phase-matched KTA crystal to produce 628 nm light. A subsequent multi-order SRS conversion in that x-cut KTA pumped by the 628 nm finished up the cascaded process, generating Stokes-shifted red radiation at 637 and 647 nm. The OPO and SRS conversion were both performed in an x-cut KTA crystal to realize a NCPM-OPO and X(ZZ)X Raman conversion. The SFG was achieved in a (θ = 90◦, φ = 24.3◦)-cut KTA crystal with a type-II phase-matching configuration. A maximum average output power of 2.1 W, a minimum pulse width of 13.5 ns and a maximum pulse peak power of 22.2 kW were obtained from this cascaded frequency convertor, enabling this coherent red light source to have practical applicability. Red laser around 630 nm have been proved to be an efficient tool for photodynamic therapy (PDT) due to the low dose absorption in epidermis layer and non-ionizing interaction [13].
2. Design issues and experimental setup
In comparison with KTP, KTA has the advantages of slightly higher values of second-order nonlinear coefficients, a longer infrared cutoff wavelength, a significantly reduced absorption at 3.5 μm, and the absence of gray tracking damage [14–16]. Furthermore, KTA also exhibits a large χ(3)-nonlinearity to realize efficient SRS frequency conversion [17–21]. The most prominent Raman shifts of KTA is 233 cm−1, corresponding to X(ZZ)X Raman configuration which are associated with the TiO6 octahedron torsional modes. So KTA is chosen as the nonlinear optical crystal used in this cascaded NOFC. The starting wavelength for cascaded NOFC is set at 1064 nm, the most common emission wavelength from the well-developed neodymium-doped (Nd-doped) solid-state lasers. The signal output of a NCPM x-cut KTA OPO will be at 1535 nm with a 1064 nm pumping [22]. NCPM maximizes the effective nonlinear-optical coefficient and essentially eliminates spatial walk-off, which makes it interesting for OPO application. Therefore, fundamental waves at 1064 and 1535 nm can be conveniently achieved in an intracavity KTA OPO. A second conversion process can be performed by sum-frequency mixing the 1535 nm with 1064 nm light inside the OPO cavity to generate a red light at 628 nm. By further integrating the promising Raman conversion in the x-cut KTA crystal, new Stokes-shifted red wavelength can be obtained from this visible intracavity Raman laser.
For the SFG at 628 nm, there may be multiple phase-matching configurations in these often-used nonlinear crystals, as shown in Table 1. It should be noted that the other possible phase-matching configurations for a given crystal have not been listed in Table 1 either due to the low effective nonlinearity or strong spatial walk-off. The type-II SFG in a (θ = 90◦, φ = 24.3◦)-cut KTA offers the best performance balance between a high effective nonlinearity and weak spatial walk-off. While the OPO and SRS conversion can be both performed in an x-cut KTA crystal to efficiently realize a NCPM-OPO and X(ZZ)X Raman conversion. Therefore, the cascaded conversion can be designed by using two KTA crystals to successively convert 1064 nm to 1535 nm by KTA-OPO, 628 nm by KTA-SFG, and 637 and 647 nm by KTA-SRS, producing new wavelengths that cannot be reached by the usual nonlinear conversion processes for 1064 nm laser light. Taking a polarization requirement for the two SFG input waves into consideration, Nd:YAG is further chosen as the gain medium for generating fundamental wave at 1064 nm due to its characteristic of un-polarized lasing.
Table 1. Phase-matching configurations for 1535 nm + 1064 nm → 628 nm in the often-used nonlinear crystals.
The diagrammatic sketch and photo picture for the experimental setup of this cascaded NOFC is displayed in Fig. 1. An intracavity frequency conversion scheme was established by introducing two KTA crystals into the 1064 nm fundamental laser cavity that could resonate the interacting light waves. The laser cavity was made up of an input mirror M1, an acousto-optic (AO) Q-switch, a Nd:YAG module, an intermediate mirror M2, a pair of KTA crystals, and an output coupler M3. M1 was a plano-concave mirror with 1000 mm radius of curvature, which was high-reflection coated at 1064 nm (R>99.8%) on the concave surface. The used AO Q-switch had antireflection coatings at 1064 nm on both light-passing surfaces and was driven at a center frequency of 24 MHz with a radio-frequency power of 50 W. The 1064 nm source was a commercial Nd:YAG module, in which a water-cooled Nd:YAG rod 3 mm in diameter and 65 mm in length was side-pumped by radial laser-diode (LD) arrays. The plano-plane mirror M2 was high-reflection coated at 1500-1600 and 600-650 nm (R>99.8%), high-transmission coated at 1064 nm on one surface, and anti-reflection coated at 1064 nm on the other surface. Both the 4 × 4 × 20 mm3, x-cut KTA and 4 × 4 × 8 mm3, (θ = 90◦, φ = 24.3◦)-cut KTA crystal were wrapped with indium foil and mounted in copper block cooled by water at a temperature of 20°C. Both the two KTA crystals were high-transmission coated at 1064 and 1500-1550 nm. A plano-plane output coupler M3 was high-reflection coated at 1500-1600 and 1064 nm (R>99.8%) and high-transmission coated at 600-650 nm.
Fig. 1 The diagrammatic sketch and photo picture for the experimental setup of the cascaded frequency convertor.
Therefore, the 1064 nm resonance could be established between M1 and M3, while KTA-OPO, KTA-SFG and KTA-SRS shared the same resonator comprised of M2 and M3. As for KTA OPO operation, the 1535 nm radiation with a polarization being parallel to the Y-axis of KTA crystal could be generated. So the SFG-KTA should be properly placed to match the polarization state of 1535 nm to realize the o (1535 nm) + e (1064 nm)→ e (628 nm) conversion. Then 1535 nm could be frequency mixed with the orthogonally-polarized 1064 nm that was unconverted in OPO, maximizing the utilization of 1064 nm fundamental wave. The generated e-polarized 628 nm by KTA-SFG would correspond to a X(ZZ)X Raman configuration in the x-cut KTA, obtaining the largest Raman gain.
3. Experimental results and discussions
The output performance of this cascaded frequency convertor was initially evaluated by tuning the repetition rate of AO Q-switch, resulting in an optimized repetition rate of 7 kHz. Dependences of the total average output power and pulse width on the LD pump power are depicted in Fig. 2. Due to the little wavelength separation between multi-wavelengths around 630 nm, it was hard for us to accurately measure the average output power at a single wavelength. So the average output powers shown in Fig. 2 were the total average output power. When the LD pump power for the Nd:YAG module was increased to 42 W, red laser beam began to emit from the resonator. The corresponding spectrum of the red radiation was measured by a spectrometer (HR4000, Ocean Optics), showing single-wavelength emission at 628 nm. When the LD pump power was increased from 50 to 72 W, new peaks at 637 and 647 nm appeared, and 628 nm still dominated the output, as shown in Fig. 3(a). 637 and 647 nm should be attributed to the first-order and second-order Raman Stokes of 628 nm induced by the 233 cm−1 Raman shift of KTA in a X(ZZ)X configuration. The Raman gain coefficient of bulk crystalline media theoretically follows the 1/λ asymptotic wavelength dependence. This contributed to the efficient multi-order SRS conversions at visible wavelengths for KTA. When the LD pump power was further increased, the Raman Stokes gradually dominated the spectrum, indicating the successful realization of a cascaded OPO, SFG and SRS in the two KTA. Figure 3(b) shows the emission spectrum obtained at a maximum average output power of 2.1 W, with a strong second-order Raman Stokes at 647 nm dominating the output.
Fig. 2 The dependences of the total average output power and pulse width on the LD pump power for this cascaded frequency convertor.
Fig. 3 The emission spectrum of the cascaded frequency convertor at an average output power of 0.6 W (a) and 2.1W (b).
An interesting question may arise. An x-cut KTA can also shift the fundamental at 1064 nm to Stokes at 1091 and 1120 nm. The SFG wavelengths of (1091 nm + 1535 nm) and (1120 nm + 1535 nm) are respectively 637.7 and 647.5 nm, nearly coinciding with the wavelengths of the first and second-order Raman Stokes based on the 233 cm−1 Raman mode of KTA. It was confirmed that the 637 and 647 nm red radiations were generated by the KTA Raman conversion in our experiment. According to the Sellmeier equations of KTA [23], there exist no phase matching configurations for the SFG of 1091 nm + 1535 nm and 1120 nm + 1535 nm in the XY-plane of KTA. So both the (θ = 90◦, φ = 0◦) and (θ = 90◦, φ = 24.3◦)-cut KTA crystal used in our experiment could not support the SFG-based generation of 637 and 647 nm. Furthermore, the optical spectrum of the weakly leaked light from mirror M1 was further measured by using an optical spectrum analyzer (AQ6370, Yokogawa). The spectral components of 1091 and 1120 nm induced by KTA Raman conversion of 1064 nm were not observed. The reason could be understood as follows. The OPO and SFG exploited the χ(2)-nonlinearities of KTA, allowing for a higher parametric gain than the χ(3)-nonlinearity-based 1091 nm Raman conversion in the same crystal. By using a LAMBDA 950 spectrophotometer, the reflectivities of M3 at 1091 and 1120 nm were measured to be 96.1% and 78.4%, respectively. This further increased the Raman conversion threshold. However, both M2 and M3 were high-reflection coated at 1500-1600 nm, enabling an extremely low-threshold OPO conversion. Then the subsequent threshold-less SFG would consume the Z-polarized 1064 nm wave (with respect to the x-cut KTA). Therefore, the greatly depleted Z-polarized component could not drive a X(ZZ)X Raman conversion in KTA, failing to generate the Raman-shifted fundamental waves.
It is worthy to note that a LD side-pumped Nd:YAG module suffers from poor spatial mode-matching between the volume of the 808 nm pump and 1064 nm beams in the Nd:YAG rod. So the optical conversion efficiency from 808 nm pump to red Raman light is relatively low. For a direct reference, the laser performance of the Nd:YAG module at 1064 nm was investigated in the continuous-wave (CW) and Q-switched mode, with the results shown in Fig. 4. The two KTA crystals and mirror M2 remained fixed to keep the same linear losses for 1064 nm with that in the cascaded frequency conversion. The mirror M3 was replaced by an optimized 1064 nm output coupler with 10% transmission. Under the same LD pump power with that for achieving maximum average output power of 2.1 W in Fig. 2, 1064 nm output powers of 12.6 and 9.1 W were obtained in the CW and Q-switching (7 kHz) mode, respectively. An effective optical conversion efficiency of 23% was achieved in this cascaded frequency convertor, with a definition as the ratio between the average output powers achieved in the scheme of cascaded NOFC and 1064 nm lasing. A LD end-pumped 1064 nm laser can be used to increase the conversion efficiency from 808 to 1064 nm. Furthermore, the OPO, SFG and SRS conversions can be finally coupled into the dynamical behavior of fundamental 1064 nm laser. The parameters of nonlinear crystals determine a nonlinear loss being equivalent to the output coupling of 1064 nm wave [24]. So the lengths of used KTA crystals could be further optimized to improve the conversion efficiency from 1064 nm to red emissions. The two KTA crystals can also be diffusion-bonded together to behave as an optical assembly to improve the conversion performance [25].
Fig. 4 The dependences of the output powers on the LD pump power for the 1064 nm Nd:YAG laser in the CW and Q-switched mode.
The temporal shape of the output red laser pulse was recorded using a 1 GHz digital oscilloscope (Tektronix DPO7104) and a photodetector (Thorlabs, Det10A/M). A minimum pulse width of 13.5 ns was obtained, corresponding to a pulse energy of 0.3 mJ and a pulse peak power of 22.2 kW. Compared with the common red lasers by frequency doubling the lasers emitting in 1.2-1.3 μm spectral region, this cascaded-NOFC-based red laser exhibited a strong pulse-narrowing induced by the combined effect of OPO and SRS [26]. Figure 5 displays the wave form with a pulse width of 13.5 ns and corresponding pulse train with a repetition rate of 7 kHz. The pulse-to-pulse intensity instability in the pulse trains was less than 5%. The 628 nm pulses can be generated by temporal overlap of the wave of 1535 and 1064 nm. Actually, OPO is a typical cavity dumping process, which will make the 1535 nm pulse shorter than the 1064 nm pulse. So the 628 nm pulse and subsequent Stokes pulse would be temporally modulated by the short 1535 nm pulse, resulting in an asymmetry pulse shape shown in Fig. 5(a).
Fig. 5 Typical single pulse with the pulse width of 13.5 ns (a); the corresponding pulse train with a repetition rate of 7 kHz (b).
This multi-wavelength NOFC-based solid-state laser source around 630 nm can provide new PDT treatment plan. On the one hand, the single wavelength in 630 nm region matches well with the narrow absorption peak of the photosensitizer of photofrins, and can produce high irradiance to minimize the therapeutic exposure time. On the other hand, the multi wavelengths can be used for the case of multicomponent-photosensitizers with different absorption maxima, offering potential advantages over the usually used lamps with a broad range of wavelengths but at reduced fluence rates [27]. Furthermore, the more working parameters, such as wavelengths, output powers, pulse widths and pulse repetition rates, enabling to determine the optimum light irradiation condition to PDT [28].
Based on the obtained experimental results, the advantages of cascaded NOFC can be listed as follows. Firstly, this method can use the matured Nd-doped lasers. The four-level characteristics and broadband pump absorption enable the 1064 nm Nd-doped lasers to operate with favorable properties, such as the compact configuration with 808 nm LD pumping and ability of high-power Q-switched operation. Secondly, it can use the commercially available nonlinear crystals with high quality, and does not require the high precision engineering of micro-structured samples. An optical frequency convertor employing this technique should be more cost-effective and more user-friendly. Thirdly, designable combinations of χ(2)-nonlinearity-based or χ(3)-nonlinearity-based NOFCs in the big family of nonlinear crystals can provide more possibilities that can never be realized with a single NOFC. For example, the x-cut KTA used in this work can be replaced by an x-cut KTP. New SFG wavelength at 635 nm and subsequent first-order and second-order Raman-converted wavelengths of 646 and 657 nm induced by the 267 cm−1 Raman shift of X(ZZ)X KTP scheme will be produced. The designability of cascaded nonlinear optical frequency conversion could make the nonlinear optical technology accessible to a much wider range of potential users.
4. Conclusions
In conclusions, a new scheme of cascaded NOFC was demonstrated to produce laser beams of various visible wavelengths around 630 nm, starting with a pulsed 1064 nm pump beam. The cascaded conversion were designed by using two KTA crystals to successively convert the 1064 nm to 1535 nm by KTA-OPO, 628 nm by KTA-SFG, and 637 and 647 nm by KTA-SRS. A maximum average output power of 2.1 W, a minimum pulse width of 13.5 ns and a maximum pulse peak power of 22.2 kW are obtained from this cascaded frequency convertor, enabling this coherent red light source to have practical applicability in PDT. By using other combination of commercially available nonlinear optical crystals, the wavelength of well-developed fundamental lasers can be further converted to the wavelengths of interest beyond the reach of single nonlinear frequency convertor.
Funding
National Natural Science Foundation of China (NSFC) (61308047 and 61605068); The Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
References and links
1. P. A. Franken, A. E. Hill, C. W. Peters, and G. Weinreich, “Generation of optical harmonics,” Phys. Rev. Lett. 7(4), 118–119 (1961). [CrossRef]
2. C. Chen, J. Lu, T. Togashi, T. Suganuma, T. Sekikawa, S. Watanabe, Z. Xu, and J. Wang, “Second-harmonic generation from a KBe2 BO3F2 crystal in the deep ultraviolet,” Opt. Lett. 27(8), 637–639 (2002). [CrossRef] [PubMed]
3. B. Q. Chen, C. Zhang, C. Y. Hu, R. J. Liu, and Z. Y. Li, “High-Efficiency Broadband High-Harmonic Generation from a Single Quasi-Phase-Matching Nonlinear Crystal,” Phys. Rev. Lett. 115(8), 083902 (2015). [CrossRef] [PubMed]
4. K. Balskus, Z. Zhang, R. A. McCracken, and D. T. Reid, “Mid-infrared 333 MHz frequency comb continuously tunable from 1.95 to 4.0 μm,” Opt. Lett. 40(17), 4178–4181 (2015). [CrossRef] [PubMed]
5. Y. Wang, L. Tang, D. Xu, C. Yan, Y. He, J. Shi, D. Yan, H. Liu, M. Nie, J. Feng, and J. Yao, “Energy scaling and extended tunability of terahertz wave parametric oscillator with MgO-doped near-stoichiometric LiNbO3 crystal,” Opt. Express 25(8), 8926–8936 (2017). [CrossRef] [PubMed]
6. W. R. Bosenberg, J. I. Alexander, L. E. Myers, and R. W. Wallace, “2.5-W, continuous-wave, 629-nm solid-state laser source,” Opt. Lett. 23(3), 207–209 (1998). [CrossRef] [PubMed]
7. Z. D. Gao, S. N. Zhu, S. Tu, and A. H. Kung, “Monolithic red-green-blue laser light source based on cascaded wavelength conversion in periodically poled stoichiometric lithium tantalite,” Appl. Phys. Lett. 89(18), 181101 (2006). [CrossRef]
8. H. T. Huang, B. T. Zhang, J. L. He, J. F. Yang, X. L. Dong, J. L. Xu, C. H. Zuo, and S. Zhao, “An efficient and compact laser-diode end-pumped intracavity frequency-tripled Nd: YVO4 355 nm laser,” Opt. Commun. 282(13), 2586–2589 (2009). [CrossRef]
9. D. Eimerl, J. M. Auerbach, C. E. Barker, D. Milam, and P. W. Milonni, “Multicrystal designs for efficient third-harmonic generation,” Opt. Lett. 22(16), 1208–1210 (1997). [CrossRef] [PubMed]
10. F. Bai, Q. Wang, Z. Liu, X. Zhang, W. Sun, X. Wan, P. Li, G. Jin, and H. Zhang, “Efficient 1.8 μm KTiOPO4 optical parametric oscillator pumped within an Nd:YAG/SrWO4 Raman laser,” Opt. Lett. 36(6), 813–815 (2011). [CrossRef] [PubMed]
11. Z. Liu, Q. Wang, X. Zhang, S. Zhang, J. Chang, S. Fan, W. Sun, G. Jin, X. Tao, Y. Sun, S. Zhang, and Z. Liu, “Self-frequency-doubled KTiOAsO4 Raman laser emitting at 573 nm,” Opt. Lett. 34(14), 2183–2185 (2009). [CrossRef] [PubMed]
12. H. T. Huang, D. Y. Shen, and J. L. He, “Compact 1625-nm noncritically phase-matched KTiOPO4 optical parametric oscillator intracavity driven by the KTiOAsO4 Raman laser,” IEEE Photonics Technol. Lett. 25(4), 359–361 (2013). [CrossRef]
13. Y. Hu and K. Masamune, “Flexible laser endoscope for minimally invasive photodynamic diagnosis (PDD) and therapy (PDT) toward efficient tumor removal,” Opt. Express 25(14), 16795–16812 (2017). [CrossRef] [PubMed]
14. K. Kato, “Second-harmonic and sum-frequency generation in KTiOAsO4,” IEEE J. Quantum Electron. 30(4), 881–883 (1994). [CrossRef]
15. G. Hansson, H. Karlsson, S. Wang, and F. Laurell, “Transmission measurements in KTP and isomorphic compounds,” Appl. Opt. 39(27), 5058–5069 (2000). [CrossRef] [PubMed]
16. Y. Duan, H. Zhu, C. Xu, X. Ruan, G. Cui, Y. Zhang, D. Tang, and D. Fan, “Compact self-cascaded KTA-OPO for 2.6 μm laser generation,” Opt. Express 24(23), 26529–26535 (2016). [CrossRef] [PubMed]
17. H. Huang, D. Shen, and J. He, “Simultaneous pulse generation of orthogonally polarized dual-wavelength at 1091 and 1095 nm by coupled stimulated Raman scattering,” Opt. Express 20(25), 27838–27846 (2012). [CrossRef] [PubMed]
18. Z. Liu, Q. Wang, X. Zhang, S. Zhang, J. Chang, Z. Cong, W. Sun, G. Jin, X. Tao, Y. Sun, and S. Zhang, “A diode side-pumped KTiOAsO4 Raman laser,” Opt. Express 17(9), 6968–6974 (2009). [CrossRef] [PubMed]
19. H. Zhu, Z. Shao, H. Wang, Y. Duan, J. Zhang, D. Tang, and A. A. Kaminskii, “Multi-order Stokes output based on intra-cavity KTiOAsO4 Raman crystal,” Opt. Express 22(16), 19662–19667 (2014). [CrossRef] [PubMed]
20. Y. Duan, H. Zhu, H. Wang, Y. Zhang, and Z. Chen, “Comparison of 1.15 µm Nd:YAGRaman lasers with 234 and 671 cm-1 shifts,” Opt. Express 24(5), 5565–5571 (2016). [CrossRef] [PubMed]
21. H. Y. Zhu, Y. L. Ye, Y. M. Duan, Y. J. Zhang, D. Zhang, B. Zhao, and A. A. Kaminskii, “Third-Stokes light operation in KTA crystal derived by Nd:Lu0.5Y0.5VO4 crystal laser,” J. Opt. 17(3), 035503 (2015). [CrossRef]
22. H. T. Huang, J. L. He, X. L. Dong, C. H. Zuo, B. T. Zhang, G. Qiu, and Z. K. Liu, “High-repetition-rate eye-safe intracavity KTA OPO driven by a diode-end-pumped Q-switched Nd:YVO4 laser,” Appl. Phys. B 90(1), 43–45 (2008). [CrossRef]
23. D. L. Fenimore, K. L. Schepler, U. B. Ramabadran, and S. R. McPherson, “Infrared corrected Sellmeier coefficients for potassium titanyl arsenate,” J. Opt. Soc. Am. B 12(5), 794–796 (1995). [CrossRef]
24. K. J. Yang, S. Z. Zhao, G. Q. Li, and H. M. Zhao, “A new model of laser-diode end-pumped actively Q-switched intracavity frequency doubling laser,” IEEE J. Quantum Electron. 40(9), 1252–1257 (2004). [CrossRef]
25. B. J. Perrett, P. D. Mason, and D. A. Orchard, “Assessment of diffusion-bonded KTP crystals for efficient, low pulse energy conversion from 1 to 2 µm,” Appl. Opt. 45(18), 4424–4427 (2006). [CrossRef] [PubMed]
26. C. Du, S. Ruan, Y. Yu, and F. Zeng, “6-W diode-end-pumped Nd:GdVO4/LBO quasi-continuous-wave red laser at 671 nm,” Opt. Express 13(6), 2013–2018 (2005). [CrossRef] [PubMed]
27. H. S. Lim, S. C. Lee, and J. O. Kim, “The optimization of laser systems for photodynamic therapy of malignancies,” Proc. SPIE 5689, 588863 (2005).
28. L. Brancaleon and H. Moseley, “Laser and non-laser light sources for photodynamic therapy,” Lasers Med. Sci. 17(3), 173–186 (2002). [CrossRef] [PubMed]
