1. Introduction

Powerful laser sources within the spectral region between 1.4 and 1.8 µm, which have the attractive features of eye-safety and enhanced atmospheric transmission, are of intense interest for numerous applications in range finding, scanning lidar, remote sensing, and laser countermeasures [1–3]. In addition, this waveband coincides precisely with the lowest loss transmission window of silica-based optical glasses, making it have important applications in optical fiber communications [4]. Furthermore, nanosecond (ns) pulsed eye-safe lasers with high average power and short pulse duration are highly desirable for low altitude navigation, targeting infrared for night, and mid-infrared laser generation [5]. So far, several strategies have been proposed to fulfill the eye-safe laser radiation. The optically pumped vertical-external-cavity surface-emitting lasers (VECSELs) are capable of producing average power of several watts in the continuous wave (CW) regime [6] and peak power of kilowatt level in the pulse regime [7], but the complexities of growth and fabrication hinder its further development. Er/Yb co-doped fibers can deliver the high-power eye-safe lasers with the CW output power exceeding 300 W [8,9], whereas it is a great challenge to achieve the high energy and high peak power eye-safe laser output owing to the onset of parasitic nonlinear effects [10]. Er-doped crystalline lasers are constantly recognized as an efficient approach for the realization of the high power and high energy eye-safe lasers [11,12]. Nonetheless, further power or energy scalability faces enormous difficulties due to the problems associated with upconversion effect, dopant ion clustering, and expensive pump sources [13]. Optical parametric oscillators have produced tens-of-watts of average power and hundreds-of-microjoules of pulse energy in eye-safe wavelength region [14], but the serious thermal effect deposited on the nonlinear crystal will inevitably lead to the deterioration of beam quality and the damage of nonlinear crystal [15].

During the past two decades, stimulated Raman scattering (SRS) has been emerged as an interesting alternative for generating the high power and high beam quality eye-safe lasers as its automatic phase matching, pulse compression and beam purification. Cascaded Raman fiber lasers have yielded about 300 W of CW output power at 1480 nm with good beam quality, while it is quite difficult to produce the high peak power and narrow linewidth ns pulsed eye-safe laser radiation stemming from some other nonlinear effects [16]. Alternatively, Raman-active crystalline media provide an efficient and practical method for accessing high average power, high peak power and high beam quality ns pulsed eye-safe laser. To date, considerable advances on the eye-safe crystalline Raman lasers have been demonstrated in the ns pulsed regime [1,17–21]. For instance, a high brightness extracavity cascaded diamond Raman laser in the eye-safe spectral region was achieved with an average output power of 16.2 W and a near diffraction limited beam quality of M² ≈ 1.17 [1]. Nevertheless, further enhancement in output power was limited by the immature growth and coating processes of mono-crystalline diamond crystal. Remarkably, in contrast to the extracavity crystalline Raman lasers, the intracavity crystalline Raman lasers have the advantages of compactness and low Raman threshold. In 2014, an ns pulsed eye-safe intracavity Nd:YVO₄/YVO₄ Raman laser at 1525 nm was demonstrated with an average output power of 5.2 W and a peak power of 3.4 kW, while the beam quality was quite poor (M² ≈ 3.6) due to the low mode-to-pump overlap efficiency and the heavy thermal load arising in Nd:YVO₄ crystal [18]. Subsequently, by virtue of the V-shaped resonator and the composite Nd:YVO₄ crystal, a high beam quality ns pulsed intracavity KGW Raman laser operating at 1526 nm was obtained with an average output power of 4.9 W and a peak power of 13 kW [21]. However, the sign of power saturation was still manifested that could be inherently caused by the poor thermal and mechanical characteristics of Nd:YVO₄ crystal. Intriguingly, Nd:YLF crystal is an attractive candidate for generating the high peak power and high beam quality 1314 nm ns pulsed lasers owing to its long upper-laser-level lifetime and weak thermal lens effect [22,23]. Recently, our group reported a high peak power and high beam quality narrowband eye-safe intracavity Nd:YLF/KGW Raman laser by using a simple linear cavity that can provide the maximum average output powers of 3.6 W at 1461 nm and 4.0 W at 1490 nm with the respective peak powers of approximately 330 and 480 kW [24]. Nevertheless, further improvements in terms of average power and pulse energy were severely hampered by the low fracture stress limit of Nd:YLF crystal and the low laser damage threshold of optical coating deposited on KGW crystal. To meet a plethora of practical applications, it is imperative to surmount the above difficulties for exploring the ns pulsed eye-safe intracavity Raman laser sources with upgraded power and enhanced efficiency, which are beneficial for enabling the long-distance transmission and improving the signal-to-noise ratio.

In this paper, we implemented a ten-watt-level eye-safe intracavity crystalline Raman laser based on the sophisticated L-type resonator embedded with two laser crystals for the first time. The efficient powerful ns pulsed eye-safe Raman laser output was enabled by improving the launched pump power with the Nd:YLF two-crystal configuration, enhancing the pump beam absorption efficiency with the linearly polarized pump beam, and slightly enlarging the fundamental beam size in the Raman crystal with the comprehensively optimized resonator. By exploiting two different Raman vibrational modes of KGW crystal at 768 (N_g) and 901 cm⁻¹ (N_m) in orthogonal orientations, two groups of eye-safe dual-wavelength Raman lasers at 1461, 1645 nm and 1490, 1721nm were achieved with the total average output powers of 9.3 and 9.7 W as well as the near-diffraction limited beam quality of M² < 1.6 under the optimal repetition rate of 4 kHz. Moreover, the pulse durations at 1461, 1490, 1645, 1721nm were measured to be 4.8, 5.5, 4.3, and 3.6 ns, respectively, leading to the peak powers of approximately 410, 370, 80, 100 kW. To the best of our knowledge, the highest average output power obtained here is about two times higher than previously reported eye-safe intracavity Raman lasers.

2. Experimental setup

The schematic diagram is depicted in Fig. 1. The pump source was a fiber Bragg grating (FBG) stabilized laser diode (HAN’S TCS M880-150-DK) with a numerical aperture of 0.22 and a core diameter of 200 µm. Over the entire pump power range, the center wavelength was stabilized at ∼880 nm with the spectral bandwidth of ∼0.2 nm (FWHM). After being collimated by lens F1 with a focal length of 50 mm, the unpolarized pump beam was split by a polarizing beam splitter (PBS) into two branches with nearly equalized power. Afterwards, the collimated pump beams were re-imaged by lenses F2 and F3 with the focal lengths of 200 and 250 mm, respectively, providing the focus spot diameters of ∼ 0.8 and 1 mm. Two a-cut Nd:YLF crystals (1 at.%, 3 × 3 × 30 mm³) were selected as the laser gain media, which were anti-reflection (AR) coated at 880, 1047–1321 nm on the front facet, and partial-reflection (PR) coated at 880 nm (R > 70%) and AR coated at 1047–1321 nm on the rear facet. Moreover, the c-axis of two Nd:YLF crystals was laid along the horizontal direction. The half-wave plate (HWP) was utilized to readjust the s-polarized pump beam to the p-polarized pump beam for aiming the c-axis of Nd:YLF crystal [25], thereby the pump beam absorption efficiencies for both branches can be up to about 93% under non-lasing circumstances. Compared to the naturally polarized light, the pump beam absorption efficiency of the linearly polarized light has been improved by about 9 percentage points [26]. The Q-switched device was an acousto-optic modulator (Gooch & Housego), which was AR coated at 1314 nm on both ends and driven by a 27.12 MHz ultrasonic frequency generator powered by a 100 W radio frequency source. A 30 mm long N_p-cut KGW crystal (EKSMA Optics) was chosen as the Raman gain medium owing to its attractive thermal properties, comparatively large Raman gain coefficient, high damage threshold, and unique bi-axial properties [27], and it was AR coated at 1314–1721nm on both sides. All the crystals were wrapped with indium foil and mounted in the water-cooled copper heat sinks, and the temperature was maintained at 18°C.

 

Fig. 1. Experimental configuration of the high-power eye-safe intracavity Raman laser.

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As illustrated in Fig. 1, the L-shaped fundamental cavity was adopted for inserting two Nd:YLF crystals, and it was constituted by two plano–concave mirrors M1 and M4 with the same radius-of-curvature of 300 mm and a flat dichroic mirror M2. The plano–concave mirror M1 was coated to be high-transmission (HT) at 880 and 1047–1053 nm as well as high-reflection (HR) at 1314–1321 nm. The flat dichroic mirror M2 was AR coated at 880 nm and HR coated at 1314–1321 nm. The output coupler M4 was coated for HR at the fundamental field and partial-transmission (PT) at the first-Stoke and second-Stokes fields, and the transmissions at the wavelengths of 1461, 1490, 1645, and 1721nm were 2.0%, 2.3%, 35%, and 66%, respectively. The separations between M1 and M2 as well as M2 and M4 were set to be 60 and 200 mm, respectively, leading to a large fundamental resonator length of 260 mm, which can enforce the fundamental laser to operate on the σ-polarization at 1314 nm [19]. Based on the ABCD transfer matrix theory, the beam radii of TEM₀₀ fundamental mode at Nd:YLF1, Nd:YLF2, and KGW crystals were roughly estimated to be about 360, 290, 220 µm under the maximum launched pump power of about 85 W, respectively. The Raman resonator was composed of the plano–concave mirror M4 and a flat dichroic mirror M3 that had a HT coating at 1314–1321 nm and a HR coating at 1461–1721nm. The physical length of Raman resonator was designed to be approximately 60 mm. Another flat dichroic mirror with HR coating at 1461–1721nm and HT coating at 1314–1321 nm was utilized to separate the fundamental radiation and the Stokes radiations. In order to further characterize the output performance of second-Stokes laser, a long-pass filter (Thorlabs, FELH1500) was exploited to filter out the residual first-Stokes lasers. The output power was gauged with an optical power meter (Physcience Opto-Electronics, LP-3C). The laser spectra were registered by an optical spectrum analyzer (Yokogawa, AQ6374) with a resolution of 0.05 nm. The pulse temporal characteristics were monitored by a fast photodiode linked to a digital oscilloscope (Agilent, DSO90604A).

3. Experimental results and discussions

The output characteristics of the actively Q-switched two-crystal Nd:YLF laser were firstly evaluated by a plane mirror with ∼10% output coupling at 1314–1321 nm after removing the KGW crystal and the flat dichroic mirror M3. For ease of comparison, the repetition rate was elected to be 4 kHz as its highest optical power conversion efficiency in the following intracavity Raman laser experiments. The pump power for the lasing threshold was approximately 9 W. The maximum average output power was up to 16.7 W under the incident pump power of 84.9 W, resulting in the optical-to-optical conversion efficiency of 19.7% and the slope efficiency of 22%. Compared to the previous result [24], the average output power has been enhanced by more than two-fold accompanied with a much higher optical power conversion efficiency. It could be introduced by the upgraded incident pump power and the improved pump beam absorption efficiency. Under the maximum incident pump power, the pulse duration was determined to be ∼70 ns, and the central wavelength was measured to be 1313.5 nm with the full-width at half-maximum (FWHM) of about 0.15 nm. Meanwhile, by using the scanning-knife-edge method, the beam quality factors along the x and y directions were measured to be $M_x^2 = 2.4$ and $M_y^2 = 2.0$, respectively. During the experiments, crystal fracture or deleterious thermal effects were not observed, and there is substantial promise for further improving the fundamental laser power by use of the multi-segmented Nd:YLF crystal [23].

The output performances of the high-power eye-safe intracavity crystalline Raman laser were explored at the optimal repetition rate of 4 kHz. Remarkably, different from our prior report [24], the second-Stokes laser was also generated along with the first-Stokes laser under high power pumping, which could be mainly attributed to the sufficient intracavity first-Stoke laser intensity induced by the relatively low output coupling at first-Stokes field (≤ 2.3%). Initially, the N_g axis of KGW crystal was aligned with the c-axis of Nd:YLF crystal for accessing the Raman shift of 768 cm^-1, and the laser emission spectrum under the maximum incident pump power is displayed in Fig. 2(a). One can see that the central wavelengths of the first-Stokes and second-Stokes lasers were determined to be 1460.9 and 1645.3 nm with the FWHMs of 0.12 and 0.11 nm, respectively. Figure 3(a) illustrates the power transfers of the first-Stokes laser at 1461 nm and the second-Stokes laser at 1645 nm with respect to the incident pump power. Thresholds for the first-Stokes laser at 1461 nm and the second-Stokes laser at 1645 nm occurred for the incident pump powers of 12 and 25 W, respectively. Correspondingly, the maximum average output powers of 7.9 and 1.4 W were attained under the incident pump power of 84.9 W, resulting in the eye-safe dual-wavelength Raman output power of 9.3 W with the overall optical power conversion efficiency of 11%. By rotating the KGW crystal by 90° for addressing the Raman shift of 901 cm^-1, we achieved another set of eye-safe dual-wavelength Raman laser at 1489.9 and 1720.5 nm with the respective FWHMs of 0.14 and 0.11 nm, as visualized in Fig. 2(b). Similarly, the first-Stokes laser at 1490 nm and the second-Stoke laser at 1721nm reached thresholds at the incident pump powers of 10 and 24 W, respectively, and the maximum average output powers approached 8.2 and 1.5 W under the full incident pump power of 84.9 W [see Fig. 3(b)]. As a consequence, the highest eye-safe dual-wavelength Raman output power at 1490 and 1721nm amounted to 9.7 W with the total optical power conversion efficiency of 11.4%. Meanwhile, the power fluctuations at the first-Stokes and second-Stokes fields were found to be less than 2.0% and 2.8% (RMS) over two hours, respectively. Noting that although the Raman gain coefficient at 768 cm^-1 mode is slightly higher than that at 901 cm^-1 mode, the lower output power obtained at 1461 nm could be mainly introduced by the onset of competition between the 768 cm^-1 and 901 cm^-1 modes [28]. In contrast to the previously reported eye-safe intracavity crystalline Raman lasers [18–21,24], the highest average output power has been increased by about two times. To the best of our knowledge, the eye-safe laser power reported here can also be favorably compared against the highest value achieved by the cumbersome extracavity diamond Raman laser [1,17]. Notably, taking the advantages of reduced optical scattering loss, decreased water absorption, and diminished phototoxicity, the second-Stokes laser operating near 1.73 µm is permanently considered as the best choice for the imaging of brain tissue [29,30].

 

Fig. 2. Stokes spectra of the eye-safe intracavity Raman laser under the incident pump power of 84.9 W.

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Fig. 3. The dependence of the first-Stokes and second-Stokes radiations on the incident pump power.

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During laser characterizations, thermal roll-off of output power did not occur owing to the weak thermal lens of Nd:YLF crystal associated with σ-polarization. Meantime, measurements of beam quality factors were implemented by using the scanning-knife-edge method. By leveraging the beam cleanup effect of SRS process, the improved beam qualities of the first-Stokes radiations were observed compared with those of the fundamental beam, and they were measured to be M² < 1.6 under the maximum output power. Moreover, the spatial beam qualities of the second-Stokes radiations have been further improved over those of the first-Stokes radiations, and their M² factors were determined to be less than 1.5 across the full output power range. Better beam quality can be further achieved by reducing the pump beam size or enlarging the TEM₀₀ fundamental mode size. According to the modeling of actively Q-switched intracavity Raman lasers [31], the second-Stokes field might be eliminated with a higher output coupling (for example T > 10%) at the first-Stokes wavelength. Numerical simulations further indicated that substantial improvement in output power and conversion efficiency might be anticipated by optimizing the output coupling as well as reducing the round-trip losses of the fundamental and first-Stoke lasers. The experiments will be carried out in the near future. It is worth noting that the high-order Hermite-Gaussian transverse modes were not occurred over the whole pump power range, indicating a weak astigmatic thermal lens arising in the KGW crystal, which could be mainly attributed to the alleviated heat load that arose from the low duty cycle [32,33]. Nevertheless, the astigmatic thermal lensing effect and laser-induced damage of KGW crystal still be the huge obstacles for power scalability in this kind of eye-safe intracavity Raman laser.

The pulse durations of the first-Stokes and second-Stokes lasers with respect to the incident pump power were registered at the repetition rate of 4 kHz. As visualized in Fig. 4(a), the pulse durations of the first-Stokes radiations monotonically declined with the incident pump power, and the SRS process represented a prominent pulse compression in contrast to the fundamental pulse [34]. Under the maximum incident pump power, the pulse durations at 1461 and 1490 nm was measured to be 4.8 and 5.5 ns, respectively, corresponding to the peak powers of about 410 and 370 kW. Although the pulse durations are considerably longer than the values reported in the prior literature [24], which can be ascribed to the much longer fundamental resonator, the peak powers are essentially comparable. Moreover, it is evident from Fig. 4(b) that the pulse durations of the second-Stokes radiations were further shortened compared to the first-Stokes pulses, and they fluctuated between 4.3 and 5.3 ns as well as 3.4 and 4.0 ns, respectively. Whereas, the peak powers of the second-Stokes lasers at 1645 and 1721nm continued to ascend with the incident pump power, and the highest peak powers were up to about 80 and 100 kW, respectively. Furthermore, the pulse trains and temporal pulse profiles of the first-Stokes and second-Stokes radiations were monitored under the full incident pump power, as depicted in Fig. 5. Clearly, it can be seen from Figs. 5(a) and 5(b) that the pulse trains of the first-Stokes lasers were very stable, and the peak-to-peak intensity fluctuations at 1461 and 1490 nm were found to be less than ± 5% and ± 3%, respectively. Meanwhile, for cascaded-Stokes radiations, the peak-to-peak instabilities of pulse amplitudes became even worse, which could be introduced by the secondary SRS process and the gain competitions associated with the first-Stokes radiations. As a result, the peak-to-peak intensity fluctuations at 1645 and 1721nm were determined to be about ± 9% and ± 10% under the maximum output power, respectively. As illustrated in Fig. 5, both the first-Stokes and second-Stoke lasers were operated stably in a single pulse without any evidence of satellite pulses. However, the falling edge of the first-Stokes pulse was somewhat smooth, which would decrease the effective peak power densities of the first-Stoke laser on the KGW crystal and the output coupler, thereby reducing the risk of damage to these optical components to some extent.

 

Fig. 4. Pulse duration and peak power versus the incident pump power for the high-power eye-safe intracavity Raman laser at the repetition rate of 4 kHz.

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Fig. 5. Pulse trains and temporal pulse profiles of the first-Stoke and second-Stokes radiations under the repetition rate of 4 kHz and the incident pump power of 84.9 W.

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4. Conclusion

In summary, a ten-watt-level eye-safe intracavity crystalline Raman laser has been successfully developed for the first time, to our knowledge. The significantly upgraded eye-safe laser power has been confirmed by the special two-crystal configuration as well as the comprehensively optimized designs of resonator structure and pump system. Two pairs of eye-safe dual-wavelength Raman lasers at 1461, 1645 nm and 1490, 1721nm were produced within the acousto-optically Q-switched Nd:YLF/KGW intracavity Raman laser. Under the optimal repetition rate of 4 kHz, the maximum average output powers of 9.3 and 9.7 W were acquired with near-diffraction limited beam quality of M² < 1.6. The pulse durations at 1461, 1490, 1645, 1721nm were determined to be 4.8, 5.5, 4.3, and 3.6 ns with the peak powers of up to about 410, 370, 80, 100 kW, respectively. As far as we know, this is the highest average power of the eye-safe intracavity Raman lasers to date. Furthermore, we envision the possibility of scaling up the system to unprecedented average eye-safe laser powers of tens-of-watts by employing the multi-segmented Nd:YLF crystal, reducing the thermal lensing effect of KGW crystal and improving its coating quality, optimizing the coating parameters of optical elements, etc.