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A compact, actively Q-switched intracavity simultaneous orthogonally polarized multi-wavelength KGW Raman laser is realized with a b-cut Yb3+:GdAl3(BO3)4 (Yb:GAB) crystal for the first time. The gain balanced dual-polarization of the Yb:GAB fundamental laser can be adjusted by varying the angular tilt of the output mirror. Benefitting from the two strong orthogonally polarized Raman lines at 768 and 901 cm−1 in KGW crystal, two sets of orthogonally polarized dual-wavelength Raman pulse lasers at 1133.1, 1156.6 nm and 1137.8, 1151.9 nm, respectively are achieved by rotating a 90 degrees. Under an absorbed pump power of 6.6 W, the corresponding peak powers are 10.33 and 10.26 kW, respectively. Along with the power considerable fundamental lasers at 1042.8, 1047.5 nm and 1043.4, 1046.9 nm, a total of eight wavelengths lasers are achieved in the above operation. The simple and reliable intracavity coupled orthogonally polarized multi-wavelength Raman laser shows great potential for the generations of broadband visible and THz radiation.
© 2016 Optical Society of America
1. Introduction
Stimulated Raman scattering (SRS) is an efficient method to produce new laser lines including visible, near- and mid-infrared waveband. In recent years, due to the important applications in spectroscopy, lidar, holography and terahertz-wave (THz) difference-frequency generation [1–6], researchers show great interests in dual- or multi-wavelength lasers, especially, for orthogonal polarized dual- or multi-wavelength laser [7]. Combining with the SRS technique, a common way to obtain dual-wavelength Raman lasers is using a dual-wavelength fundamental laser or a Raman media with two Raman modes of similar gain coefficient [8–17]. In 2013, Shayeganrad reported a 1178.9 and 1199.9 nm c-cut Nd:YVO4/YVO4 Raman laser with a dual-wavelength fundamental laser at 1066.7 and 1083.9 nm [8]. Shen et al. achieved second-stokes dual-wavelengths operation at 1321 and 1325 nm BaWO4 Raman laser from the 1061 and 1064 nm fundamental wavelength [9]. Using Nd:YVO4 as the self-Raman crystal and undoped GdVO4 as the Raman crystal, they also realized several dual-wavelength Raman lasers emitting at 1174, 1175 nm, and 1522, 1524 nm [10,11]. In 2014, Ignatyev et al. realized simultaneous dual-wavelength Raman conversion of 532 nm laser radiation with 889 and 925 cm−1 Stokes shifts in Ba(MoO4)0.45(WO4)0.55 solid solution single crystals [12]. In 2015, Huang et al. obtained two eye-safe wavelengths by using different Raman gain peaks occurring at 267 and 694 cm−1, respectively, in KTP, and 233 and 671 cm−1 in KTA [13]. However, those Raman lines have the same polarizations, which will degenerate the performance of dual-wavelength laser for laser interferometry and precision metrology. In Zhao’s report, using two complex laser cavities to form σ- and π- polarized fundamental radiations, two lines at 1159.4 nm and 1166.8 nm were obtained in BaWO4 crystal [14]. By balancing the Raman gain of KTP and KTA with their Y axes perpendicular to each other, Huang et al. achieved orthogonally polarized dual-wavelength at 1091 and 1095 nm [15]. The design of orthogonally polarized Raman modes intrinsically avoid the gain competition from the multi-line laser conversions, which provides a good idea to obtain expected dual-wavelength emissions.
KGd(WO4)2 (KGW) is of an efficient Raman medium that has large Raman gain, good thermal conductivity and high damage threshold. Compared to some other popular Raman materials, a unique advantage of KGW is that it has two strong orthogonally polarized Raman lines at 768 cm−1 and at 901 cm−1, with similar Raman gain [18]. The corresponding polarization is oriented along the Ng and Nm crystal axes, respectively. In the recent years, by rotating the crystal about the laser axis or inserting a λ/2 electro-optical switch, a greater diversity of output wavelengths have been realized using either the Raman shift lines of 768 or the 901cm−1 [16,17,19–26]. Many UV-visible lines have also been achieved by second harmonic generation (SHG) or sum frequency generation (SFG) [27–30]. However, simultaneous orthogonally polarized dual-wavelength KGW Raman laser has seldom been reported as far as we know.
Yb3+:GdAl3(BO3)4 (Yb:GAB) is a good host material for solid-state lasers because of its good chemical and physical properties. It belongs to the trigonal space group R32 with an asymmetric crystal structure, resulting in the optical anisotropy and birefringence, and thus outputting the naturally e- and o-polarized emissions around 1045 nm [31–33]. On the other hand, the output wavelengths of Yb3+ doped lasers are different from the traditional 1064 nm, thus may providing an unusual range of Stokes wavelengths and corresponding visible wavelengths.
In this letter, we report on a diode-pumped, actively Q-switched intracavity SRS in KGW with a Yb-doped GdAl3(BO3)4 laser. Yb:GAB crystal emits a gain balance orthogonally polarized dual-wavelength fundamental laser via adjusting the loss resulting from output coupler. Simultaneous orthogonally polarized multi-wavelength Raman lasers at 1133.1, 1156.6 nm and 1137.8, 1151.9 nm are achieved by rotating the KGW crystal.
2. Experimental details
The experimental setup for the simultaneous orthogonally polarized multi-wavelength laser was shown in Fig. 1. The pump source used was a 976 nm fiber-coupled laser diode with a fiber core diameter of 200 μm and a numerical aperture of 0.22. The output laser of the diode was unpolarized. The pump light was focused into the laser crystal with a beam spot radius (1/e2) of ~200 μm by a 1:1 collimating focusing system. A plano-concave cavity with a cavity length of 80 mm was used. The input mirror IM was coated with highly reflection (> 99.9%) at 1030-1200 nm and high transmission (> 97%) at 940–990 nm. The output coupler OC was a plano-concave mirror with 200 mm radius of curvature, which was highly reflective at the fundamental wavelength and had a transmission of T = 2% at the 1st Stokes wavelength.
Fig. 1 Schematic of the simultaneous orthogonally polarized multi-wavelength diode-pumped actively Q-switched Yb:GAB/KGW Raman laser.
The Yb:GAB crystal and KGW crystal were grown by the top seeded solution growth (TSSG) method in a vertically cylindrical resistance-heated furnace in our group. In this experiment, a 10 at.% b-cut 3 × 3 × 2 mm3 Yb:GAB crystal was used. With the b-cut crystal, each o- and e-wave could be successively fully excited by adjusting the output coupler [33]. The KGW crystal was cut along the Np axis with a size of 5 × 5 × 24 mm3 (Nm × Ng × Np). Consequently, the orthogonally polarized SRS at 768 cm−1 (E//Ng) and 901 cm−1(E//Nm) could be achieved with the orthogonally polarized o- and e-wave from Yb:GAB crystal. A 30-mm-long acousto-optic Q-switcher (from Gooch & Housego Co.) was placed between the Yb:GAB crystal and the KGW crystal, and was driven at a 40 MHz center frequency with 20 W of radio-frequency power. All the crystals were mounted in a copper block and water-cooled with temperature keeping at 293 K.
3. Results and discussion
At First, we study the laser characteristics of the Yb:GAB crystal. As reported in [34], the laser oscillation, for anisotropic laser crystals and cavities free of any polarizing elements or surfaces, will be naturally polarized depending on the inversion ratio necessary to compensate to the cavity losses (in particular the loss induced by the output transmission) in the stationary state. Figure 2(a) reveals that the emission cross sections in each polarization have a big difference. If the cavity losses of the two competing wavelengths at different sides of the gain intersection can be moderately adjusted to realize gain-to-loss balance in both the polarization and wavelength, an orthogonally polarized dual-wavelength laser will be done. This balance can be achieved via adjusting the loss resulting from the position of output coupler. A similar method has been reported in [35]. Figure 2(b) shows some experiment results in our previous work. The laser emission spectra were recorded by using a spectrometer (HR4000, Ocean Optics). It can be seen that the laser wavelength, polarization, and intensity of a b-cut Yb:GAB can be effectively adjusted through regulating the angular tilt of the output mirror. The lasing wavelengths near 1041 nm and 1052 nm belong to the e-emission and near 1048nm belong to the o-emission, respectively.
Fig. 2 (a) Emission cross sections in both polarizations of Yb:GAB (b) Some output laser spectrum through regulating the angular tilt of the output mirror.
In this work, with a 20 mm-long plano-concave cavity (T = 3%, R = 75 mm), orthogonally polarized dual-wavelength laser has been obtained by slightly inclining output coupler. The laser characteristics are shown in Fig. 3. A maximum output power of 2.78 W is achieved under an absorbed pump power of 5.93 W with a slope efficiency of 63.3%. Figure 3(b) presents the corresponding optical spectrum for the dual-polarization, dual-wavelength Yb:GAB continuous-wave laser at 1047.2 nm in σ polarization and 1050 nm in π polarization. The emission wavelength is recorded with a spectrometer (WaveScan, APE, GmBH). The power ratio for each spectral component is always remained around 50% when the absorbed pump power increases from 1.94 to 5.93 W. Such a gain balanced Yb:GAB output will be a ideal fundamental light for the simultaneous orthogonally polarized KGW SRS conversion.
Fig. 3 (a) Output powers, (b) optical spectrum for the dual-polarization, dual-wavelength Yb:GAB continuous-wave laser.
When the Ng crystal axis of KGW parallels to the optical axis c of the Yb:GAB gain medium, the typical laser emission spectrum is shown in Fig. 4(a) under an absorbed pump power of 5.27 W, at a repetition rate of 5 kHz. As can be seen, four laser lines are obtained. The central wavelength of the fundamental and the first-Stokes dual-lines are centred at 1042.8, 1047.5 nm and 1133.1, 1156.6 nm, respectively. The Raman lines have the same polarization direction with theirs corresponding fundamental waves that the 1042.8 nm and 1133.1 nm beams are determined to be π-polarized, corresponding to the frequency shift of 768 cm−1, and the 1047.5 nm and 1156.6 nm beams are σ-polarized with a frequency shift of 901 cm−1. The frequency shifts agree very well with the optical vibration modes in [23]. Note that that the fundamental waves located at 1043.4 (π-polarized) and 1046.9 nm (σ-polarized) are different from the results above due to the broad emission at 1.0 μm of Yb:GAB crystal. As shown in Fig. 2(a), Yb:GAB crystal has a broad gain band. The change of the intracavity loss thus leads to a selected laser emitting at different wavelength.
Fig. 4 (a) Optical spectrum, (b) total and individual output power, (c) pulse train, (d) single pulse profile, and (e) far-field beam profile of the simultaneous orthogonally polarized multi-wavelength laser at 1133.1 and 1156.6 nm at the PRF of 5 kHz.
Figure 4(b) displays the output power with respect to the absorbed pump power. The individual output powers at 1133.1 and 1156.6 nm are separated with a prism and a Glan-Taylor polarizer. Under an absorbed pump power of 6.6 W, the highest output power 155 mW is obtained, comprising of 75 mW at 1133.1 nm and 80 mW at 1156.6 nm, Each Stokes component is increased linearly, indicating a weak competition effect. To avoid the fracture on the GAB crystal induced by thermal stress, a higher pump power is not tried. In our experiment, a power considerable fundamental light is emitted in the whole operating range. The total output power including fundamental laser is measured to be 266 mW at the absorbed pump power of 6.6 W. The corresponding optical-to-optical conversion efficiency is of 4%, comparable with the previously reported dual-wavelength Raman lasers in [8,13,14]. The pulse characteristic of the Raman light is recorded by a digital oscilloscope (500 GHz bandwidth, 4 Gs/s sampling rate) and is shown in Fig. 4(c,d). Under an absorbed pump power of 6.6 W. the fundamental and dual-wavelength Raman pulse widths are measured to be 17 and 3 ns, respectively. The corresponding peak power of the dual-wavelength Raman laser is 10.33 kW. The stable single pulse profile indicates no time delay between the dual-wavelength signals. The profile of the four-wavelength laser beam was measured with a pyroelectric camera (Pyrocam III, Ophir Optronics Ltd.) and is given in Fig. 4(e).
When rotating the KGW crystal on 90 degrees, we achieve another set of orthogonally polarized dual-wavelength Raman laser as shown in Fig. 5(a). Because of the optical anisotropy of KGW crystal, the index of refraction on each polarization is different thus the intracavity loss for each polarization would be changed when the KGW crystal is rotated. Due to the broad gain band of Yb:GAB crystal, it’s easy to realize a selected laser emitting when the loss at different wavelength varied. Accessing the Stokes shift 901 cm−1 and 768 cm−1, the dual-wavelength Raman lines at 1151.9 nm (π-polarized) and 1137.8 nm (σ-polarized) are obtained, respectively. The total output power at the 1st Stokes waves versus the absorbed pump power is presented in Fig. 5(b), at a pulse repetition rate of 5 kHz. Under an absorbed pump power of 6.6 W, the maximum average output powers for Stokes 1151.9 and 1137.8 nm waves are measured to be 79 and 75 mW, respectively. Combined with the dual-wavelength fundamental laser, a 277 mW output power is achieved, corresponding to an optical-to-optical conversion efficiency of 4.2%. Typical pulse trains and single pulse profiles of the fundamental and dual-crystal Raman lasers are shown Fig. 5(c,d) with the pulse width of 14 and 3 ns, respectively. The corresponding pulse energy and peak power of the dual-wavelength Raman laser were 0.03 mJ and 10.26 kW, respectively. Figure 5(e) presents the profile of the four-wavelength laser beam.
Fig. 5 (a) Optical spectrum, (b) total and individual output power, (c) pulse train, (d) single pulse profile, and (e) far-field beam profile of the simultaneous orthogonally polarized multi-wavelength laser at 1137.8 and 1151.9 nm at the PRF of 5 kHz.
Table 1 displays a comparation of laser performances of simultaneous orthogonally polarized multi-wavelength Raman lasers. As we can see, the output powers are relatively lower. Several weaknesses are existed in this work. Due to the available conditions, we do not try other Raman output couplers. Moreover, all the crystals in this work are only fine polished without any broadband anti-reflection (AR) coatings on the end faces, which would cause a larger reflection loss. The unoptimized Raman cavity and the additional losses would greatly affect the output characteristics of the Stokes laser and account for the lower Stokes conversion efficiency. However, to the best of our knowledge, it is for the first time that simultaneous orthogonally polarized eight-wavelength Raman lasers are obtained with KGW crystal in a simple and reliable cavity. Such simultaneous orthogonally polarized multi-wavelength laser source shows great interest in the generations of broadband visible and THz radiation through second harmonic generation (SHG), sum frequency generation (SFG) and difference frequency generation in a suitable nonlinear crystal, as shown in [15,16,27].
Table 1. Comparation of laser performances of simultaneous orthogonally polarized multi-wavelength Raman lasers.
In addition, it is worth mentioning that green and yellow lights are observed during the experiment, which is attributed to the self-frequency doubling effect in GAB crystal. Thus it will be an interesting design that a self-frequency doubling, broadband green-yellow Raman laser is to be expected in a rational cut Yb:GAB/KGW crystal with an optimized Raman output coupler.
4. Conclusions
In this paper, simultaneous orthogonally polarized multi-wavelength Raman lasers is reported in a KGW Raman medium for the first time. Yb3+:GdAl3(BO3)4 (Yb:GAB) is used as the gain crystal that emits a gain balanced dual-wavelength, dual-polarized fundamental light with a slope efficiency of 63.3%. The laser wavelength, polarization, and intensity of a b-cut Yb:GAB can be effectively balanced by adjusting the loss resulting from the angular tilt of the output mirror. By rotating the KGW crystal, two sets orthogonally polarized Raman pulse lasers located at 1133.1, 1156.6 nm and 1137.8, 1151.9 nm are achieved with the maximum output powers of 155 and 154 mW, respectively, under an absorbed pump power of 6.6 W. The corresponding peak powers are 10.33 and 10.26 kW, respectively. Combined with the power considerable fundamental lasers at 1042.8, 1047.5 nm and 1043.4, 1046.9 nm, a total orthogonally polarized eight-wavelength Raman lasers are obtained. This work shows a simple and reliable intracavity coupled SRS for the realization of orthogonally polarized multi-wavelength Raman lasers, which is very attractive for the generation of broadband visible and THz radiation.
Funding
National Nature Science Foundation of China (51472240, 61078076, and 11304313); Material Genetic Engineering Program of Ministry of Science and Technology of China (Grant No. 2016YFB0701002); the Strategic Priority Research Program of the Chinese Academy of Science (Grant No. XDB20000000); Science and Technology Plan Cooperation Project of Fujian Province (2015I0007); Nature Science Foundation of Fujian Province (2015J05134, and 2016J01274); Nature Science Foundation of Jiangxi Province (20161BAB216132); Scientific Project of Jiangxi Education Department of China (GJJ150510); and State Key Laboratory of Structural Chemistry (20160012).
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