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Abstract
We demonstrate an efficient wavelength-selectable output in the attractive deep-red spectral region from an intracavity frequency converted Nd:YLF/KGW Raman laser. Driven by an acousto-optic Q-switched 1314 nm Nd:YLF laser, two first-Stokes waves at 1461 and 1490 nm were generated owing to the bi-axial properties of KGW crystal. By incorporating intracavity sum-frequency generation and second-harmonic generation with an angle-tuned bismuth borate (BIBO) crystal, four discrete deep-red laser emission lines were yielded at the wavelengths of 692, 698, 731, and 745 nm. Under the incident pump power of 50 W and the repetition rate of 4 kHz, the maximum average output powers of 2.4, 2.7, 3.3, and 3.6 W were attained with the pulse durations of 3.4, 3.2, 4.3, and 3.7 ns, respectively, corresponding to the peak powers up to 177, 209, 190, and 245 kW. The results indicate that the Nd:YLF/KGW Raman laser combined with an angle-adjusted BIBO crystal provides a reliable and convenient approach to achieve the selectable multi-wavelength deep-red laser with short pulse duration and high peak power.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In recent years, deep-red radiation sources with wavelengths around 700 nm have been of increasing interest for a crowd of realistic applications, such as polymer fiber transmission, optically pumped alkali-metal vapor laser and remote sensing [1–3]. Additionally, benefiting from the weak absorption in both water and hemoglobin, these deep-red lasers can penetrate centimeters of biological tissues so as to be attracted to dermatology and stimulated emission depletion microscopy [4,5]. Moreover, the several-nanosecond (ns) deep-red lasers could satisfy the stress confinement criteria and produce strong photoacoustic transients, which are perfectly desirable for photoacoustic imaging [6]. Especially, the few-ns-class and multi-kilohertz pulsed deep-red laser can be utilized to enhance the spatial height resolution and high sampling rate for applications with complicated environments such as altimetry and vegetation monitoring [7].
Assorted methods have been adopted with the intention of producing the deep-red radiation sources. The vertical-external-cavity surface-emitting laser (VECSEL) are capable of actualizing the high-power continuous-wave (CW) deep-red laser with the average output power exceeding 8 W but rarely work in the ns pulsed mode [8,9]. Pumped by a green or blue laser source, Eu3+ or Pr3+-doped lasers were able to directly generate the deep-red lasers [10,11], but the average output power for Q-switched operation was limited to hundreds of milliwatts due to its insufficient emission cross-section [12,13]. Optical parametric oscillators and Ti:sapphire lasers pumped by the solid-state green lasers provide a viable tactic for the generation of the ns pulsed deep-red lasers with broad tunable range [14,15]. However, the costly and cumbersome solid-state green lasers partly hampered the further development of these kinds of deep-red lasers. Red-diode-pumped Alexandrite lasers have been approved to be a feasible approach to realize the high-power broadly tunable deep-red laser output [3,16], but the intricate cavity-dumped Q-switching structure is commonly needed for achieving the several-ns deep-red laser radiation [7].
Stimulated Raman scattering (SRS) has been recognized as a classic method for producing the short pulse duration and high beam quality ns pulsed laser on account of its profound characteristics such as pulse width shorting and Raman beam cleanup. So far, watt-level ns pulsed deep-red laser source has been yielded through frequency-doubled a cascaded phosphosilicate Raman fiber amplifier [17], but they suffer from limited pulse energies of hundreds of nanojoules arising from other nonlinear effects, including stimulated Brillouin scattering and four-wave mixing. Ulteriorly, a combination of crystalline Raman laser followed by second harmonic generation (SHG) or sum-frequency generation (SFG) represents an important frontier for accessing the wavelength-switchable visible light output from the green to red spectral region based on the well-established 1 µm Nd-doped lasers [18–21]. Nevertheless, tremendous challenge needs to be confronted in acquiring the efficient deep-red laser output due to the relatively low stimulated-emission cross-section of the traditional Nd-doped crystals at 1.3 µm [22]. Lately, we presented a ns pulsed deep-red laser source operating at 730 and 745 nm based on the SHG of the Nd:YLF/KGW Raman laser with a bismuth borate (BIBO) crystal [23]. Aside from the large round-trip losses at the fundamental and first-Stokes fields, the simultaneous unsought second-Stokes field also impeded the SHG process; hence, the maximum average output powers at 730 and 745 nm were restricted to 1.7 and 2.0 W with the low conversion efficiencies of 3.5% and 4.1%, respectively. Subsequently, with a view to improving the output power and conversion efficiency, we replaced the BIBO crystal with a critically phase-matched lithium triborate (LBO) crystal to enable efficient SHG and to suppress undesired cascaded Stokes fields. Benefiting from the negligible walk-off angle of LBO crystal and the reduced round-trip loss at first-Stokes wavelength, an efficient ns pulsed Raman deep-red laser at 745 nm was achieved with the average output power of 4.1 W and the conversion efficiency of 8.2% [24]. Nonetheless, it is difficult to offer the ability for wavelength flexibility and switching because there is a big difference in phase matching (PM) angle between the SHG and SFG procedures. Albeit the temperature tuning LBO crystal can offer great potential for unprecedented flexibility in wavelength control, the switching speed is relatively slow [20]. To further meet the requirements of some specific applications such as fibroblasts proliferation, fluorescence spectroscopy of small bismuth clusters (692 nm) [25,26], and strontium atoms clock (698 nm) [27], it is imperative to explore the efficient high-power wavelength-switchable deep-red lasers. Furthermore, the wavelength-versatile laser source realized by frequency mixing of crystalline Raman laser can conquer the dependence of multiple single-wavelength laser system in the field of multi-wavelength laser applications.
In this work, we report, to the best of our knowledge, the first demonstration of a wavelength-versatile intracavity frequency converted crystalline Raman laser that can emit four laser lines in the deep-red spectral range, which was achieved by the selective frequency mixing in a Q-switched Nd:YLF/KGW Raman laser with the aid of an angle-tuned BIBO crystal. By rotating the KGW crystal along the propagation axis and slightly tuning the PM angle of BIBO crystal, four different deep-red wavelengths of 692, 698, 731, and 745 nm were separately obtained via the SFG/SHG of the fundamental and/or first-Stokes waves. Under an optimal pulse repetition frequency (PRF) of 4 kHz, the maximum average output powers of these four wavelengths amounted to 2.4, 2.7, 3.3, and 3.6 W with the pulse durations of 3.4, 3.2, 4.3, and 3.7 ns, respectively, which gave rise to the peak powers as high as 177, 209, 190, and 245 kW. The beam qualities for SHG and SFG were found to be near diffraction limited with M2 < 1.7.
2. Experimental setup
The layout of the wavelength-versatile Raman deep-red laser source is depicted in Fig. 1. We elected a 1.0 at.% single-doped a-cut Nd:YLF crystal with a transverse of 3 × 3 mm2 and a length of 40 mm as the laser crystal. The entrance side of Nd:YLF crystal was coated to be high transmission (HT) at 880 nm (T > 99.8%) and 1047–1314 nm (T > 99.5%), and the other side was coated to be HT at 1047–1314 nm (T > 99%). The laser crystal was pumped by a fiber Bragg grating (FBG) locked laser diode (200 µm core, 0.22 NA), whose central wavelength was stabilized at 880 nm to match its weak absorption peak. The lens F1 with a focal length of 50 mm was used for collimation and the lens F2 with a focal length of 200 mm was adopted for focusing, thus delivering a pump beam spot diameter of 0.8 mm into the Nd:YLF crystal. Under non-lasing situations, the pump absorption efficiency was measured to be approaching 93%. An acousto-optic Q-switching device (Gooch & Housego, I-QS027-4S4H-B5) was coated with anti-reflection (AR) at 1314 nm (R < 0.2%) on both surfaces, and was driven by a 27.12 MHz ultrasonic frequency as well as a 100 W radio-frequency power. The Raman crystal was a 30 mm long Np-cut KGW crystal coated for AR at 1314–1490 nm (R < 0.1%) on both sides. Apart from the profound properties such as high damage threshold, appealing thermal characteristic, and comparatively high Raman gain coefficient, the KGW crystal has the unique bi-axial properties that could provide two Raman shifts of 768 cm-1 (Ng) and 901 cm-1 (Nm), which are beneficial for implementing the wavelength-switchable deep-red laser output. The SHG of the first-Stokes wave and the SFG of the fundamental and first-Stokes waves were fulfilled by a 10 mm long angle-tuned BIBO crystal, which was cut along θ = 10.5°, φ = 0°, and AR coated at 690–745 nm (R ≈ 4%) and 1314–1490 nm (R < 0.05%) on both ends. All the crystals were wrapped with indium foil and mounted in the water-cooled copper holders at a constant temperature of 19°C.
As displayed in Fig. 1, the fundamental cavity was composed of an input mirror M1 and an output mirror M4. The input mirror M1 was a plane-concave mirror with the radius-of-curvature of 300 mm coated for HT at 880 and 1047–1053 nm (T ≈ 91%) and high reflection (HR) at 1314 nm (R > 99.9%), among which HT at 1047–1053 nm was adopted to suppress the 1 µm laser oscillation. By adopting the special double-sided dichroic coating technology, the plane-concave M4 with the radius-of-curvature of 300 mm had the extremely high reflection at 1314, 1461, and 1490 nm (R > 99.99%) and the relatively high transmission at 692 nm (T = 99.3%), 698 nm (T = 98.7%), 731 and 745 nm (T > 99.7%), thereby minimizing the leakage of the fundamental and first-Stokes waves and enhancing the SHG and SFG processes. The Raman resonator was constituted by the mirror M4 and a middle flat mirror M2, which was HT coated at 1314 nm (T = 98.8%) and HR coated at 1461–1490 nm (R > 99.8%). Furthermore, to prevent the deep-red light from traveling along the backward direction, the flat mirror M3 coated for HT at 1314 nm (T > 99.7%) and 1461–1490 nm (T > 99.8%), and HR at 690–745 nm (R > 99.9%) was incorporated between the KGW and BIBO crystals. The physical lengths of the fundamental and Raman resonators were designed to be approximately 210 and 60 mm, respectively. The fundamental cavity with such a length will bring high losses for the π-polarization at 1321 nm, and thereby enforce the fundamental wave to work only at 1314 nm [28]. Under the incident pump power of 50 W, the beam radii of the 1314 nm TEM00 mode in the Nd:YLF, KGW, and BIBO crystals were assessed to be around 285, 220, and 235 µm, respectively, based on the ABCD transfer-matrix theory. Finally, a short-wavelength-pass filter (Thorlabs, FES0800) was utilized to filter other unwanted wavelengths.
3. Experimental results and discussions
Table 1 represents the PM angles and output wavelengths for the frequency mixing of the Nd:YLF/KGW Raman laser. During the experiment, by slightly tuning the PM angle of BIBO crystal from 9.6° to 10.6°, the wavelength-switchable deep-red emissions could be generated through the SFG of the 1314 nm fundamental wave and 1461 nm first-Stokes wave, the SFG of the 1314 nm wave and 1490 nm first-Stoke wave, the SHG of the 1461 nm wave, and the SHG of the 1490 nm wave, respectively. The output performances of the wavelength-switchable Raman deep-red laser were first explored at the optimal PRF of 4 kHz. We employed an optical spectrum analyzer (Yokogawa, AQ6374) to record the output laser spectra, as portrayed in Fig. 2. The central wavelengths of the SFG emissions were determined to be 691.8 and 698.2 nm with the same spectral linewidth of 0.10 nm (FWHM), respectively, while those of the SHG emissions were detected to be 730.5 and 745.0 nm with the identical FWHM of about 0.07 nm. Meanwhile, similar to our previous report [23], the undesired second-Stokes wavelengths at 1645 and 1720nm were also observed during the experiment owing to the large walk-off angle of BIBO crystal and the high transmission at 1645 nm (T ≈ 51%) and 1720nm (T ≈ 85%) of output coupler M4 [29]. Nevertheless, the intensities of the second-Stokes fields were far less than those of the deep-red emissions. Based on the previous researches, the unwanted second-Stokes fields can be completely eliminated by providing a sufficiently strong coupling in SHG interaction [24] or adding a control crystal for overcritical SFG of the first- and second-Stokes fields [32].
Fig. 2. Optical spectra of the wavelength-switchable Raman deep-red laser under the full incident pump power of 50 W.
Table 1. Phase-matching angles and output wavelengths for frequency mixing of the Nd:YLF/KGW Raman laser.
Figure 3 displays the average output powers of the wavelength-switchable deep-red laser with respect to the incident pump power. The thresholds of these deep-red lasers at 692, 698, 731, and 745 nm were 14.5, 12.5, 13.6, and 11.2 W, respectively. The average output powers of these four deep-red lasers increased approximately linearly with the incident pump power. For the case of SFG, the highest output powers of 2.4 and 2.7 W for the 692 and 698 nm lasers were obtained along with the 1645 and 1720nm second-Stokes output powers of 380 and 280 mW, respectively. For the case of SHG, we realized the maximum output powers of 3.3 W for the 731 nm laser and 3.6 W for the 745 nm laser, respectively, accompanied by the 1645 and 1720nm second-Stokes output powers of 155 and 270 mW. The corresponding optical power conversion efficiencies of four deep-red lasers were 4.8%, 5.4%, 6.5%, and 7.3%, respectively. The maximum output powers were just pump limited with no signal of saturation. Our capacity to scale to higher powers has been constrained by the need to prevent the laser-induced damage of the optical coatings and KGW/BIBO crystals. Herein, the output powers for the SFG emissions were considerably lower than those of the SHG radiations. This phenomenon can be well explained by the fact that the pulse duration of the first-Stokes laser is much narrower than that of the fundamental laser owing to the pulse compression effect in the SRS process, which results in a large discrepancy in the peak power overlapped in the time domain between the fundamental pulse and the first-Stokes pulse, thereby yielding a relatively low SFG conversion efficiency [20]. It is worth recalling that the maximum output powers of these SHG radiations obtained here are about 2 times higher than previously reported [23], which can be attributed to the following reasons: firstly, thanks to the improved optical coatings deposited on the KGW and BIBO crystals, the round-trip losses were effectively reduced from 8% to 7.5% at the fundamental field and 2% to 1.4% at the first-Stokes field, respectively; secondly, the combination of the FBG narrowband pump source and the longer laser crystal efficiently enhanced the pump absorption efficiency, thus leading to a 20% increment in the output power for the fundamental laser operation. The rationality of the above explanation has been further confirmed by our numerical simulation. Ulteriorly, under the full incident pump power of 50 W, the spatial features of the four-wavelength deep-red laser were recorded with a laser beam analyzer (Spiricon, Inc. M2–200s). The insets in Fig. 3 illustrate that the intensity profiles of the SFG and SHG laser beams have the ellipticities of about 0.6 and 0.7, respectively. The smaller ellipticity of the SFG emission was ascribed to the conspicuous deviation between the PM angle (∼9.6°) of the SFG process and the cutting angle (10.5°) of the BIBO crystal. As a consequence, the beam quality factors of the SFG emissions along the X and Y directions were measured to be $M_x^2 = 1.83$ and $M_y^2 = 1.60$, respectively, and those of the SFG emissions were determined to be $M_x^2 = 1.62$ and $M_y^2 = 1.51$., respectively.
Fig. 3. Average powers of the wavelength-switchable Raman deep-red laser as a function of the incident pump power. The insets show the far-field beam profiles of (a) SFG and (b) SHG.
The pulse temporal characteristics were recorded by a fast photodiode (DET08CL/M, 5 GHz), which was linked to an Agilent digital oscilloscope (DSO90604A, 6 GHz). Figure 4 illustrates the pulse durations of the wavelength-switchable deep-red laser versus the incident pump power at the PRF of 4 kHz. It is apparent that the pulse durations of these deep-red lasers monotonously decline with the incident pump power. Under the full pump power, the pulse durations at 692, 698, 731, and 745 nm were down to about 3.4, 3.2, 4.3, and 3.7 ns, respectively, corresponding to the peak powers as high as approximately 177, 209, 190, and 245 kW. Additionally, the pulse trains and temporal pulse profiles were investigated under the highest output powers, and the pinnacle instabilities of pulse amplitudes were assessed to be better than ±16% and ±12% for the deep-red radiations at the SFG and SHG procedures, separately. The SFG emissions had the stronger instabilities caused by the different timing jitters between the fundamental pulse and the first-Stokes pulse. Besides, the emergence of the stronger second-Stokes field in the SFG process could be another nonnegligible cause. The peak-to-peak stabilities could be further improved by eradicating the undesired second-Stokes lasers. As exhibited in Fig. 5, four stable single pulse profiles were observed on the oscilloscope. Intriguingly, the pulse durations of the SFG emissions were slightly narrower than those of the SHG radiations. It can be ascribed to the fact that the first-Stokes pulse is generated at the falling edge of the fundamental pulse, which leads to the peak separation of these two pulses and thereafter brings out the narrower pulse durations of the SFG emissions [30,31]. Moreover, modulations were perceived on the two edges of the temporal pulse profile, which can be optimized by eliminating the second-Stokes fields.
Fig. 4. Pulse durations and peak powers of the wavelength-switchable Raman deep-red laser versus the incident pump power.
Fig. 5. The temporal pulse profiles of the wavelength-switchable deep-red laser under the PRF of 4 kHz and the incident pump power of 50 W.
The output characteristics of the wavelength-switchable deep-red laser were also examined by tuning the PRF from 1 to 10 kHz for a constant pump power of 50 W. It can be found from Figs. 6 and 7 that the PRF varying curves have analogous trends for the SFG and SHG emissions. As depicted in Fig. 6, the average powers reach the peak value with the increase of PRF from 1 to 4 kHz, and then gradually decrease for the higher PRF. On the other hand, as the PRF was rose from 1 to 10 kHz, the pulse energies reduced from 1.3, 1.7, 2.6, and 2.5 mJ to 0.06, 0.1, 0.12, and 0.13 mJ for these four deep-red lasers at 692, 698, 731, and 745 nm, respectively. The achievable supreme pulse energy can be favorably compared against that obtained from the Raman deep-red laser using a 25 mm long LBO crystal [24]. Figure 7 presents the dependence of pulse durations and peak powers on the PRF for the wavelength-switchable deep-red laser. One can observe that the pulse durations of these four deep-red laser emissions climbed from 2.9, 2.8, 3.3, and 3 ns at the PRF of 1 kHz to 18.2, 15.2, 16.6, and 17 ns at the PRF of 10 kHz, respectively, which gave rise to the peak powers decreased from 459, 614, 794, and 857 kW to 3.2, 6.8, 6.9, and 7.4 kW. In contrast to the state-of-the-art cavity-dumped Q-switched Alexandrite lasers [7], the utmost pulse energy and peak power at any one of four deep-red wavelengths have been enhanced by more than 2 times. Most notably, the undesirable second-Stokes fields disappeared when the PRF exceeded 5 kHz. It is very likely that the peak power density of the intracavity first-Stokes field decreases with the PRF so that the threshold power of the second-Stokes field cannot be reached for PRF beyond 5 kHz.
Fig. 6. Average powers and pulse energies with respect to the PRF for the wavelength-switchable deep-red laser under the full incident power of 50 W.
Fig. 7. The dependence of pulse durations and peak powers on the PRF for the wavelength-switchable deep-red laser under the maximum incident power of 50 W.
4. Conclusion
In summary, we have demonstrated an efficient high-power wavelength-switchable deep-red radiation source on the basis of an intracavity frequency converted Nd:YLF/KGW Raman laser, for the first time. The intracavity SFG/SHG of the fundamental and first-Stokes fields were accomplished by means of an angle-tuned BIBO crystal. Substantial improvements in output power and conversion efficiency were enabled by improving the pump absorption efficiency and minimizing the cumulative losses at fundamental and first-Stokes fields. Under the optimized PRF of 4 kHz, the maximum average output powers up to 2.4, 2.7, 3.3, and 3.6 W were produced at the wavelengths of 692, 698, 731, and 745 nm with short pulse durations of 3.4, 3.2, 4.3, and 3.7 ns, respectively. Subsequently, the highest pulse energies were scaled to 1.3, 1.7, 2.6, and 2.5 mJ at 1 kHz with the peak power approaching 459, 614, 794, and 857 kW, respectively. Such an efficient, simplicity and robustness Raman deep-red laser source generating multi-Watt, multiple wavelength output with short pulse duration and high peak power is in high demand for a multiplicity of applications, such as remote sensing, fibroblasts proliferation, fluorescence spectroscopy, altimetry and vegetation monitoring, etc. Further efforts are required to determine whether it is feasible to extract more discrete deep-red wavelengths from this laser system using the 1321 nm fundamental emission.
Funding
National Natural Science Foundation of China (62175093, 61935010, 62175091); Guangdong Province Basic and Applied Basic Research Fund (Joint Fund) (2020A1515110001); Research and Development Program in Key Areas of Guangdong Province (2020B090922006); Guangzhou Science and Technology Project (202102020949).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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