An actively Q-switched eye-safe orthogonally-polarized dual-wavelength intracavity Raman laser was demonstrated for the first time, to the best of our knowledge. The gain balanced dual-wavelength operation at 1314 and 1321 nm within an in-band pumped Nd:YLF laser was realized by slightly titling the cavity mirrors. Owing to the KGW bi-axial properties, two sets of simultaneous orthogonally-polarized dual-wavelength Raman lasers at 1470, 1490 nm and 1461, 1499 nm were achieved by simply rotating the KGW crystal for 90°, respectively. With an incident pump power of 30 W and an optimized pulse repetition frequency of 5 kHz, the maximum dual-wavelength Raman output powers of 2.6 and 2.4 W were obtained with the pulse widths of 5.8 and 6.3 ns, respectively, corresponding to the peak powers up to 89.7 and 76.5 kW.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Lasers at wavelengths within the spectral region between 1.4 and 1.8 µm are retina safe and exhibit excellent transmission performance in the atmosphere and optical glasses, which make these laser sources widely used for applications including optical communications, laser range finding, laser countermeasures and remote sensing [1,2]. Particularly, simultaneous eye-safe dual-wavelength lasers with small wavelength interval have great potential applications in the fields of laser interferometry , precision metrology , coherent THz radiation [5,6] and eye-safe differential absorption lidar [7,8]. The eye-safe dual-wavelength laser can be generated directly through the Er3+/Yb3+ co-doped materials [9,10]. However, due to the large quantum defect (about 37%) and low fluorescence quantum efficiency (less than 10%) of the upper laser level, the output performances of the eye-safe dual-wavelength laser are seriously affected. There are two other methods to produce eye-safe dual-wavelength laser sources: optical parametric oscillators (OPOs) and stimulated Raman scattering (SRS). To date, there are several works reported on the eye-safe dual-wavelength OPOs [10–13]. For instance, the synchronized eye-safe dual-wavelength laser at 1562.1 and 1567.4 nm was realized by pumping the non-critical KTP OPO with a Nd:GYSGG laser, and the maximum average output power was 750 mW with a peak power of 22.7 kW . Based on a diffusion-bonded Nd:YVO4/Nd:GdVO4 crystal, an eye-safe dual-wavelength KTP OPO at 1570.4 and 1572.6 nm was demonstrated with an average output power of 2.3 W and a peak power of approximately 13 kW . Unfortunately, OPOs need phase matching and working near the damage thresholds for many nonlinear crystals, resulting in problems for creating stable and robust systems .
Compared with OPOs, Raman lasers have many advantages, such as free from phase-matching constraints, pulse duration shortening and beam quality enhancement through Raman beam cleanup [15,16]. Over the past decade, serials of eye-safe dual-wavelength Raman lasers have been demonstrated [17–21]. In 2012, by employing two different Raman crystals, a 1.62 W eye-safe dual-wavelength Nd:YVO4/GdVO4 Raman laser operating at 1522 and 1524 nm was obtained with a peak power of 14.5 kW . By pumping a single Raman crystal with a dual-wavelength fundamental laser, an eye-safe dual-wavelength ceramic Nd:YAG/SrWO4 Raman laser at 1501 and 1526 nm was achieved with an average output power of 3.36 W and a peak power of 3.8 kW in 2014 . In the following year, based on a single-wavelength Nd:YAP fundamental laser and different Raman gain peaks and Stokes order within a single KTP/KTA Raman crystal, two sets of eye-safe dual-wavelength Raman lasers working at 1478, 1503 nm and 1474, 1480 nm were delivered with the average output powers of 506 and 220 mW, respectively, and the corresponding peak powers were 21.1 and 7.3 kW . However, those dual-wavelength Raman laser lines were either the natural polarization or having the same polarization, which will degrade the performance of the application in precision metrology and laser interferometry  and make it difficult to separate the beams at the two wavelengths. Recently, an orthogonally-polarized multi-wavelength laser at 1.6-1.7 µm by using an OPO to pump the Raman conversion was demonstrated with an average output power of 1.2 W and a peak power of 28.5 kW . However, the complex nonlinear optical frequency conversion progress makes the laser system inconvenient and unstable, and exhibits a very low optical-to-optical conversion efficiency of 0.5%.
Overall, the peak powers of these eye-safe dual-wavelength lasers mentioned above are still low. Further improvements in terms of average power and peak power are essential to increase the conversion efficiency of THz radiation and reduce data acquisition time in practical applications. In this work, we developed a high peak power eye-safe orthogonally-polarized dual-wavelength Raman laser by using a 1.3 µm orthogonally-polarized dual-wavelength fundamental laser to pump a single Raman crystal. In order to obtain the 1.3 µm orthogonally-polarized dual-wavelength fundamental laser with high average power and high peak power, an anisotropic Nd:YLF crystal is a preferred candidate due to its relatively weak thermal lens effect , high energy storage capability , and very close stimulated emission cross sections for both polarizations . However, major drawbacks of Nd:YLF are its fragility and high energy-transfer upconversion rate, which will limit its further power enhancement. Apart from the two most commonly used methods, such as reducing the doping concentration and increasing the pump beam volume, the in-band pumping technique is another efficient method to overcome these problems via reducing the quantum defect in turn lowering the heat generation. Compared with traditional 797/806 nm pumping, the quantum defect with the adoption of the 880 nm pumping could be decreased by ∼15% for the 1.3 µm laser emission. In addition, we chose a tungstate crystal KGd(WO4)2 (KGW) to be the Raman shifter due to its high damage threshold, large Raman gain, good thermal conductivity, and negative thermo-optic coefficient. Moreover, KGW has two different strong Raman lines at 768 cm−1 (Ng) and 901 cm−1 (Nm) in an orthogonal orientation, with a similar Raman gain coefficient [27,28]. Consequently, based on the combination of Nd:YLF and KGW, it is possible to obtain two sets of eye-safe orthogonally-polarized dual-wavelength Raman lasers at 1470, 1490 nm and 1461, 1499 nm by the orientation of the KGW.
In this paper, we demonstrate two sets of simultaneous eye-safe orthogonally-polarized dual-wavelength Raman lasers within an 880 nm laser diode (LD) end-pumped actively Q-switched Nd:YLF/KGW intracavity Raman laser. At an optimized pulse repetition frequency (PRF) of 5 kHz, we obtained 2.6 W average output power of 89.7 kW peak power at 1470 and 1490 nm. After rotating the KGW crystal for 90°, another set of orthogonally-polarized dual-wavelength Raman laser at 1461 and 1499 nm was generated with an average output power of 2.4 W and a peak power of 76.5 kW. To our knowledge, this is the first demonstration of an eye-safe orthogonally-polarized dual-wavelength Raman laser, and the maximum peak power reported here is much higher than those of other eye-safe dual-wavelength Raman lasers [20,23]. This high peak power orthogonally-polarized dual-wavelength laser source can be potentially beneficial to future applications in long-distance remote sensing and eye-safe differential absorption lidar.
2. Experimental setup
The experimental arrangement was schematically depicted in Fig. 1. A continuous-wave (CW) LD (DILAS), coupled into a delivery fiber having a core diameter of 200 µm and a numerical aperture of 0.22, was used as the pump source. Under an incident pump power of 30 W, the central wavelength of the pump beam was temperature tuned to 880 nm with a spectral linewidth of 1.9 nm, which matches the second absorption peak of Nd:YLF crystal. The output from the fiber was focused to a spot diameter of about 1 mm into the laser crystal by using a pair of plane-convex coupling lenses (1:5 magnification) with anti-reflection (AR)-coated at 880 nm (R < 0.06%). The laser crystal was a 1.0 at. % a-cut Nd:YLF crystal with a dimension of 3 × 3 × 30 mm3. It was coated for high transmission (HT) at 880 nm (T > 99.8%), 1047-1321 nm (T > 99.5%) on the front surface, high reflection (HR) at 880 nm (R > 60%) and HT at 1047-1321 nm (T > 99%) on the other surface. The HR coated at pump beam was utilized to distribute the thermal load longitudinally in the laser crystal so that the local temperature rise in the crystal was minimized. Under non-lasing conditions, the round-trip absorption efficiency was estimated to be > 86%, and this is something that could be increased in the future by improving the characteristics of the crystal coatings, particularly the HR-coating at 880 nm. A 46-mm-long acousto-optic Q-switcher (Gooch & Housego, I-QS027-4S4H-B5) was AR coated around 1.3 µm (R < 0.2%) on both facets, and it was driven at the 27.12 MHz ultrasonic frequency with a radio-frequency power of 100 W. The Raman-active medium was a Np-cut KGW crystal with a transverse cross section of 3 × 3 mm2 and a length of 30 mm, which was AR coated at 1047-1500 nm (R < 0.3%) on both sides. All of the crystals were wrapped with indium foil, and mounted in the micro-channel water-cooled copper holders with a working temperature of 16 °C. The front mirror M1 was a plane-concave mirror with a radius-of-curvature of 150 mm coated for HT at 880 nm (T ≈ 91.4%) and 1047-1053 nm (T > 90.6%), HR at 1314-1321 nm (R > 99.9%). The HT coated at 1047-1053 nm was used to suppress the 1 µm parasitic oscillation. The flat intracavity mirror M2 has HT coating at 1314-1321 nm (T > 98.8%) as well as HR coating at 1460-1500 nm (R > 99.8%), which was employed to reflect and gather the backward Raman emission. The plane mirror M3 coated for HR at 1314-1321 nm (R > 99.9%) and partial reflectivity (PR) at 1460-1500 nm was chosen to be the Raman output coupler. The values of transmission for the mirror M3 at the wavelengths of 1461, 1470, 1490 and 1499 nm were 4.9%, 5.0%, 6.8%, and 8.1%, respectively. The Raman resonator was made up of M2 and M3. The lengths of the fundamental resonator and Raman resonator were set up to be approximately 140 and 35 mm, respectively. An optical spectrum analyzer (Zolix, Omni-λ300) was employed to record the output laser spectra with a resolution of 0.1 nm. The pulse temporal behaviors were recorded by a fast photodiode (DET08CL/M, 5 GHz) connected to an Agilent digital oscilloscope (DSO90604A, 6 GHz).
3. Experimental results and discussions
Initially, the laser characteristics of the actively Q-switched orthogonally-polarized dual-wavelength fundamental laser at 1314 and 1321 nm were evaluated, where the aforementioned Raman output coupler was replaced by a plane output coupler with PR at 1314-1321 nm (TOC = 5%). As well known, the gain balanced dual-wavelength fundamental laser output is very essential for the simultaneous orthogonally-polarized dual-wavelength KGW Raman conversion. Over the full range of pump power, the power ratio of the two wavelengths was maintained to be 1 by slightly regulating the angular tilt of the cavity mirrors. As shown in Fig. 2, the average output power rose monotonously with the incident pump power. Under the incident pump power of 30 W, the maximum average output powers at the PRF of 5 and 10 kHz were measured to be 5.1 and 5.6 W with the slope efficiencies of 20.6% and 22.5%, respectively. We anticipate that substantial improvements in efficiency will be possible by improving the 880 nm coating quality of the input mirror and laser crystal. Moreover, the corresponding dual-wavelength pulse widths were measured to be 73 and 115 ns, respectively. Under the full output power, the emission wavelengths of σ-polarization and π-polarization were centered at 1313.7 and 1320.9 nm with the same full width at half-maximum (FWHM) of approximately 0.9 nm. By using a scanning-knife-edge method, the M2 factor of the 1321 nm laser on π-polarization was measured to be 1.5, while the poor beam quality of M2 = 2.3 was obtained for the 1314 nm laser on σ-polarization. The poor beam quality of σ-polarization could be explained by two reasons. One is the gain competition with π-polarization . The other reason is that the cavity is not optimal for both wavelengths causing that higher modes oscillate in one laser wavelength. By using Eq. (1) presented in , the maximum stress in the Nd:YLF crystal is calculated to be ∼7 MPa, which is about one-fifth of the thermal fracture limit (33–40 MPa) [29,30]. The theoretical result confirms that our current laser system can work without the risk of fracture.
Then, we systematically explored the output performance of the eye-safe orthogonally-polarized dual-wavelength Raman laser at the PRF of 5 kHz. The Nm optical principal axis of KGW was first aligned with the σ-polarization of Nd:YLF, and the optical spectrum under an incident pump power of 30 W is displayed in Fig. 3(a). As we can see, the central wavelengths of the first-Stokes dual-wavelength laser were determined to be 1470.0 and 1490.0 nm with the FWHMs of 2.4 and 1.8 nm, respectively. The observed spectral broadening is mainly attributed to the frequency-dependent loss for the fundamental field induced by SRS . These two Raman lasers had the same polarization direction with their fundamental lines. The 1313.7 and 1490.0 nm lines were s-polarized associated with the Raman shift of 901 cm−1, and the 1320.9 and 1470.0 nm beams were p-polarized by accessing the Raman shift of 768 cm−1. The frequency shifts are in good agreement with the optical vibration modes in KGW . Furthermore, by rotating the KGW crystal for 90°, we were able to obtain another first-Stokes orthogonally-polarized dual-wavelength laser at 1461.1 nm (first-Stokes at 768 cm−1 of 1313.7 nm beam) and 1499.3 nm (first-Stokes at 901 cm−1 of 1320.9 nm beam), as depicted in Fig. 3(b). The corresponding FWHMs of these two Raman lines were recorded to be 1.6 and 2.4 nm, respectively. From Fig. 3, we can see that there were no other Stokes lines occurred.
Integrating the benefits of the close stimulated emission cross sections at two fundamental wavelengths (1314 and 1321 nm)  and the similar Raman gains at two different strong Raman shifts (768 cm−1//Ng and 901 cm−1//Nm) , the gain balanced dual-wavelength Raman emissions can be easily obtained by modulating the cavity mirrors, just similar to the fundamental operation. Figure 4 illustrates the average output powers of the eye-safe dual-wavelength Raman lasers with respect to the incident pump power. It is noteworthy that the average output power emitted from the fundamental laser has been deducted over the whole operating range. The pump thresholds were nearly the same and approximately 5 W for both kinds of dual-wavelength Raman laser. Under the incident pump power of 30 W, the highest average output powers were acquired to be 2.6 and 2.4 W for the eye-safe dual-wavelength laser at 1470, 1490 nm and 1461, 1499 nm, respectively, resulting in the optical power conversion efficiencies of 8.7% and 8.0%. The average power of 2.6 W can be compared against 3.36 W for the eye-safe dual-wavelength Nd:YAG/SrWO4 Raman laser, which is the highest average power among the eye-safe dual-wavelength Raman lasers . The relatively low optical power conversion efficiency could be attributed to the unoptimized reflectivities of Raman output coupler at the required wavelengths, and the poor beam quality of fundamental laser. In addition, the corresponding instabilities were measured to be 5.4% and 7.6% over one hour, respectively. By virtue of a polarizing beam splitter (PBS), the power ratio between the spectral features at 1470 and 1490 nm was found to be approximately 1 over the whole operating range, while the power ratio between the 1461 nm laser and the 1499 nm laser was observed to be 0.9. Based on the rate equations of actively Q-switched intracavity Raman laser , the different power ratios are mainly raised by the large transmission differences of Raman output coupler at each first-Stokes wavelength. During the laser characterization, rollover of the average output power was not observed, so higher output power can be expected with a higher pump power. Owing to the beam cleanup effect of the SRS process, the beam quality of the first-Stokes has been improved compared to those of the fundamental beam, and the beam quality factors (M2) of the first-Stokes at 1470, 1499 nm and 1490, 1461 nm were measured to be 1.4 and 1.7 under the full output power, respectively.
The pulse characteristics of the eye-safe orthogonally-polarized dual-wavelength Raman laser with respect to the incident pump power were recorded, where a dichroic mirror with an HR coating at 1460-1500 nm and HT coating at 1314-1321 nm was exploited. Figure 5 shows that the pulse duration of the dual-wavelength Raman laser decreased with the incident pump power, and the SRS process exhibited a significant pulse shortening compared to the fundamental beam . Under the incident pump power of 30 W, the pulse durations of the dual-wavelength Raman laser at 1470, 1490 nm and 1461, 1499 nm were 5.8 and 6.3 ns, respectively, resulting in the peak powers as high as 89.7 and 76.5 kW. As far as we know, the highest peak power reported here is about three times higher than previously reported for other eye-safe dual-wavelength Raman lasers [20,23]. Furthermore, the pulse trains and temporal profiles of the dual-wavelength Raman lasers were monitored under the full output power, as depicted in Fig. 6. The pulse-to-pulse amplitude instabilities shown in Figs. 6(a) and 6(b) were measured to be less than ±8.5% and ±11.8% for both kinds of dual-wavelength Raman laser, respectively. A stable single pulse was observed on the oscilloscope as shown in Figs. 6(c) and 6(d), indicating that the two polarizations are synchronous, without time delay. In addition, a satellite pulse was observed in the experiment, and the intensity of the satellite pulse was much smaller than that of the main pulse. The generation of the satellite Stokes pulse could be attributed to the large Raman gain of the 30 mm long KGW crystal .
Finally, we investigated the output performance of the eye-safe orthogonally-polarized dual-wavelength Raman laser by tuning the PRF from 1 to 10 kHz under the incident pump power of 30 W. As illustrated in Fig. 7, the measured PRF tuning curves have the similar variation trend for different dual-wavelength Raman lasers. It can be seen from Fig. 7(a) that the average output power reaches its maximum value with increasing the PRF to 5 kHz, and then starts to decrease for larger PRF. As described in Fig. 2, the higher PRF is beneficial for the improvement of fundamental laser power. However, the higher PRF will reduce the peak power of fundamental laser, thereby hindering the high efficiency Raman conversion. Therefore, there is an optimized PRF to maximize the Raman output power. The pulse energies of the eye-safe dual-wavelength Raman lasers with respect to the PRF were calculated as exhibited in Fig. 7(b). With the PRF decreasing from 10 to 1 kHz, the pulse energies increased from 0.14 and 0.13 mJ to 1.38 and 1.08 mJ for the dual-wavelength Raman lasers at 1470, 1490 nm and 1461, 1499 nm, respectively. Higher pulse energy can be expected with a lower PRF, but we did not attempt for protecting the optical coatings.
In summary, high-peak-power eye-safe orthogonally-polarized dual-wavelength Raman lasers were obtained by pumping the KGW crystal with an 880 nm LD pumped orthogonally-polarized dual-wavelength Nd:YLF laser. The gain balanced dual-wavelength Raman laser was realized by appropriately adjusting the loss resulting from the angular tilt of the cavity mirrors. With an incident pump power of 30 W and an optimized PRF of 5 kHz, two sets of orthogonally-polarized dual-wavelength Raman lasers operating at 1470, 1490 nm and 1461, 1499 nm were achieved with the maximum average output powers of 2.6 and 2.4 W, respectively, and the corresponding peak powers were up to 89.7 and 76.5 kW. To our knowledge, this is the first implementation of an eye-safe orthogonally-polarized dual-wavelength Raman laser. Future works can be dedicated to improving the average output power and peak power, which would allow greater on-target powers and scan speeds for some practical applications.
National Natural Science Foundation of China (11704155, 51702124, 61605062, 61735005, 61935010); National Key Research and Development Program of China (2017YFB1104500); Guangdong Project of Science and Technology Grants (2018B010114002, 2018B030323017); Guangzhou Science and Technology Program key projects (201903010042).
The authors declare that there are no conflicts of interest related to this article.
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