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Abstract
We have experimentally demonstrated passively Q-switched (PQS) stimulated Raman scatting (SRS) emissions in an a-cut YVO4 Raman crystal in a coupled cavity, using a Yb:YAG/YAG/Cr4+:YAG/YAG composite crystal to generate the PQS fundamental laser, for the first time, to our best knowledge. At the incident pump power of 6.30 W, the Stokes average output power of 0.42 W was achieved. Due to the cascaded Raman effect, four first-Stokes lines operating at 1092.6 nm, 1125.4 nm, 1135.1 nm and 1157.0 nm and three second-Stokes lines operating at 1210.6 nm, 1263.2 nm and 1290.1 nm were generated simultaneously. It is noted that the spectral broadening of the fundamental field was observed. With the inclusion of an etalon in the fundamental cavity to suppress the spectral broadening, the average output power of the Stokes lines increased to 0.49 W and the maximum pulse energy was enhanced up to 84.30 μJ, corresponding to the diode-to-Stokes conversion efficiency of 7.78%.
© 2017 Optical Society of America
1. Introduction
Recently, Yb-doped yttrium aluminium garnet (Yb:YAG) has been more attractive in the passively Q-switched (PQS) all solid-state lasers [1,2] owing to its properties, such as the simple electronic level structure, long upper-state energy storage lifetime, low quantum defect and broad absorption and emission spectra. Based on thermal diffusion bonding technology, the composite crystal composes a laser crystal and a saturable absorber (SA) has been experimentally demonstrated [3,4]. For diode-pumped PQS crystalline lasers, composite crystals with the host material of yttrium aluminium garnet such as Yb:YAG/Cr4+:YAG [5], Yb:YAG/Cr4+,Yb:YAG [6] and Yb:YAG/Cr4+:YAG/YAG [7,8] have been reported their good laser performance. The composite crystals are more compact and could be chilled together which can reduce the air gap effects between the separate Yb:YAG crystal and the saturable absorber. To improve the PQS laser performance, in this work, we employ a composite Yb:YAG/YAG/Cr4+:YAG/YAG crystal with a sandwich structure by sandwiching a Cr4+:YAG crystal between two undoped YAG crystals to generate the fundamental field.
Stimulated Raman scatting (SRS) based on the third-order nonlinear optical process has been widely reported [9,10], as an efficient approach to extend the spectral range of the conventional all solid-state crystalline lasers. Compared with PQS Raman lasers which are pumped by diode-pumped Nd-doped crystal laser [11–14], these are pumped by diode-pumped Yb-doped crystal laser [15,16] have relatively low Raman frequency conversion efficiency, which mainly results from the broad emission spectrum of Yb-doped crystal laser. On the other hand, the high pulse energy of diode pumped Yb-doped crystal fundamental laser [5–7] could make the phenomenon of cascaded Raman frequency conversion possible in principle, which will hamper the efficient Stokes conversion. Therefore, harnessing an etalon in a coupled cavity to control the spectral broadening of fundamental and Stokes fields was both experimentally and theoretically demonstrated its effect on enhancing the effective Raman conversion [17].
In this paper, a laser diode (LD) pumped PQS Yb:YAG/YAG/Cr4+:YAG/YAG composite crystal laser is exploited to pump an a-cut YVO4 crystal to generate intracavity SRS frequency conversion in a coupled cavity for the first time. The initial transmission (T0) of the SA is 95%. When employing a coupled cavity structure, four first-Stokes emissions and three cascaded second-Stokes emissions can emit simultaneously. At the pump power of 6.30 W, the average output power of Stokes laser is 0.42 W with the pulse width of 2.43 ns and the pulse repetition frequency (PRF) of 5.52 kHz, corresponding to the single pulse energy of 75.41 μJ and the conversion efficiency of 6.67%. The spectral broadening of the fundamental laser was observed in the PQS Raman coupled cavity, so an etalon is inserted in the fundamental cavity to limit the spectral broadening. After adding an etalon, the spectral broadening of both the fundamental field and Stokes field was effectively controlled, and hence the performance of Raman laser was enhanced. The average Stokes output power can be up to 0.49 W at an incident pump power of 6.30 W and the diode-to-Stokes conversion efficiency increase to 7.78%. The pulse width and the PRF are 2.21 ns and 6.15 kHz, respectively. The maximum value of the single pulse energy could be 84.30 μJ.
2. Experimental setup
The schematic diagram for the coupled-cavity Raman laser in a YVO4 crystal pumped by a PQS Yb:YAG crystal laser was depicted in the Fig. 1. A fiber-coupled 940 nm LD with the core diameter of 100 μm and the numerical aperture (N.A.) of 0.22 was employed as the pump source. Two lenses with the focal lengths of 25.4 mm and 50 mm were used to focus the pump light into the composite crystal. In the coupled-cavity, the fundamental cavity was composed of the pump mirror (M1) and the output coupler (OC) with a physical cavity length of 62 mm and the Stokes cavity was composed of the intracavity mirror (IM) and the output coupler (OC) with a physical cavity length of 38 mm. When moving the IM out from the cavity, the coupled cavity would become a co-cavity where the fundamental field and the Stokes field shared the same resonator.
The Yb:YAG/YAG/Cr4+:YAG/YAG composite crystal was thermal diffusion bonded by four segments. The gain medium was a 5.0 at.% doped Yb:YAG crystal with a length of 4 mm and a transverse cross section of 4 × 4 mm2. The SA was a Cr4+:YAG with a T0 of 95% and a length of 0.5 mm which was diffusion bonded by two 2 mm length undoped YAG crystals on its two surfaces. Both end facets of the composite crystal are anti-reflectivity (AR) coated at the range of 800-1200 nm (T>99%). The composite crystal used in this paper could make the cavity more compact. The Raman active crystal was an a-cut YVO4 crystal with a length of 30 mm and a cross section of 3 × 3 mm2 with AR coating at the range of 1000-1400 nm (T>99%) on both two surfaces. These two crystals were mounted in thermoelectric cooled copper holders and the temperature was maintained at 14 °C. The pump mirror (M1) was a plate mirror with an AR coating at the pump wavelength (T>95%) and a high reflectivity (HR) coating at the range of 1000-1400 nm (R>99.8%).
The transmission spectra for the used IM and OC were depicted in the Fig. 2. The transmissions for the IM at wavelengths of 1031 nm, 1050 nm, 1093 nm and 1291 nm were 96.8%, 95.4%, 0.8% and 0.3% and at the range of 1120-1280 nm were lower than 0.09%. The transmissions for the OC at wavelengths of 1093 nm, 1126 nm, 1135 nm, 1157 nm, 1210 nm, 1263 nm and 1291 nm were 2.7%, 11.7%, 12.4%, 14.5%, 19%, 37.3% and 49.7% and at the range of 1000-1060 nm were lower than 0.05%. The OC was a concave mirror with a radius-of-curvature of 100 mm.
3. Experimental results and discussion
3.1 Free running of PQS fundamental laser
Firstly, the performance of the PQS fundamental laser was evaluated by employing a Yb:YAG/YAG/Cr4+:YAG/YAG composite crystal, as shown in the Fig. 3. The pump mirror was a concave mirror with a radius-of-curvature of 300 mm and it was coated with AR at the pump wavelength (T>95%) and HR at the range of 1000-1060 nm (R>99.8%). The flat OC had a part-reflectivity (PR) coating at 1030 nm (TOC = 20%). The overall length of the simple linear cavity was as short as 20 mm. Figure 3(a) depicted the average output power depending on the incident pump power. The lasing threshold was 2.71 W for the fundamental laser. At the incident pump power of 9.34 W, the maximum average output power of 2.98 W was obtained, corresponding to the diode-to-fundamental conversion efficiency of 31.9%. The slope efficiency was up to 45.1%. Compared with the former work reported by our group [7], the maximum average output power achieved in this paper was 11.5% higher than that when using a Yb:YAG/Cr4+:YAG/YAG composite crystal. Therefore, the two undoped YAG crystals bonded between the Yb:YAG crystal and Cr4+:YAG crystal could improve the performance of the PQS fundamental laser. It is because the undoped crystals can be acted as a heat sink to reduce the thermal lensing effect [18].
Fig. 3 (a) The average output power of the fundamental laser with respect to the incident pump power; (b) the pulse width and the PRF with respect to the incident pump power; (c) the emission spectrum of the fundamental laser at the incident pump power of 9.34 W.
A fast photodiode (Thorlabs, DET08CL/M) was used to record the fundamental pulse profiles, whose output signal was connected to an Agilent digital oscilloscope (DSO90604A with the electrical bandwidth of 6 GHz). Figure 3(b) showed the pulse width and the PRF as a function of the pump power. The pulse width and the PRF were 10.54 ns and 22.2 kHz at the incident pump power of 9.34 W, respectively, corresponding to the pulse energy of 134.2 μJ and the peak power of 12.73 kW. The center wavelength of the fundamental laser was measured by a spectrometer (Solar Laser System, S100) with a resolution of 0.3 nm, to be 1029.8 nm when the pump power was 9.34 W, as depicted in Fig. 3(c).
3.2 Free running of PQS Raman laser
The experimental configuration for the free running PQS Raman laser was depicted in the Fig. 1 and the Raman output performance was investigated and shown in the Fig. 4 for two cases (case 1: coupled cavity versus case 2: co-cavity). The lasing threshold for the case 1 was 1.94 W which was lower than that for the case 2, 2.01 W. The average output power of Stokes radiation increased linearly with the pump power and the output power for the case 1 was 0.42 W at an incident pump power of 6.30 W which was higher than that for the case 2. This might be attributed to that the coupled cavity makes the Stokes cavity more compact than that in a co-cavity. In addition, the IM can prevent the Stokes light propagating back to the composite crystal. Therefore, the losses for the Stokes laser in a coupled cavity would be lower than these losses in a co-cavity. The diode-to-Stokes conversion efficiency for the case 1 was 6.67% while that for the case 2 was only 5.51%. Similar, as shown in the Fig. 4(b), the pulse width of the PQS Stokes output for the case 2 was slightly smaller than that for the case 1, whereas the PRF was slightly higher. The pulse width and the PRF of the pulse Raman laser for the case 1 were 2.43 ns and 5.52 kHz, respectively. We calculated that the single pulse energy was 75.41 μJ.
Fig. 4 (a) The average output power of Stokes laser and (b) the pulse width and the PRF for a free running PQS Raman laser with respect to the incident pump power.
The spectral characteristics of the PQS cascaded Raman laser at the incident pump power of 6.30 W were measured by an optical spectrum analyzer (ANDO AQ6317C) with a 0.1 nm resolution and were depicted in the Fig. 5. The wavelengths of fundamental field, first-Stokes field, second-Stokes filed and corresponding detail Raman shift were listed in the Table. 1. As shown in Figs. 5(a) and 5(a’), in the high-Q fundamental resonator, fundamental laser at wavelengths of 1030.6 nm and 1048.2 nm could resonate simultaneously for the coupled cavity, while there were two fundamental wavelength peaks centered at 1030.8 nm and 1049.6 nm for the co-cavity. Note that the spectral widths of the fundamental field were broad remarkably at a high incident pump power (the full widths at half maximum (FWHM) were 1.2 nm at 1030.7 nm and 2.8 nm at 1049.7 nm at an incident pump power of 6.30 W). The phenomenon of cascaded Raman effect was evitable because of the high intracavity fundamental circling intensity and single pulse energy.
Fig. 5 (a) The output spectra of fundamental laser for the case 1; (b) the cascaded Raman laser spectra for the case 1 at the incident pump power of 6.30 W; (a’) the output spectra of fundamental laser for the case2; (b’) the cascaded Raman laser spectra for the case 2 at the incident pump power of 6.30 W;
Figure 5(b) depicted the Stokes emissions at the incident pump power of 6.30 W in a coupled cavity. We found that up to seven Stokes lines could be derived from the PQS dual-wavelength Yb:YAG laser. For the first-order Stokes waves, laser at 1135.1 nm and 1125.4 nm was generated from 1030.6 nm while Stokes lines at 1157.0 nm and 1092.6 nm were converted from the fundamental laser at 1048.2 nm. For the second-order Stokes emissions, in addition to the SRS frequency conversion from the primary first-Stokes line at 1157.0 nm to 1290.1 nm and 1210.6 nm, emission at a wavelength of 1263.2 nm could also be created from the secondary first-Stokes line at 1135.1 nm. Figure 5(b’) depicted the Stokes emissions at the incident pump power of 6.30 W in a co-cavity. Four first-Stokes lines centered at 1092.9 nm, 1125.7 nm, 1135.3 nm and 1157.1 nm could also be generated. However, only one cascaded Stokes line was created at the wavelength of 1210.6 nm. This may be attributed to that the longer Stokes cavity in the co-cavity introduces more losses for the Stokes laser, hence increase the thresholds of cascaded Raman emissions.
All these Raman shifts were in good agreement with the optical vibration modes of tetrahedral VO−3 ionic groups in the YVO4 crystal. Besides, no other high order Stokes emission was detected in this experiment. It is noted that the cascaded Raman laser could be generated once reaching the SRS threshold (see Table 1).
Table 1. The list of wavelengths of fundamental laser, first-Stokes laser and second-Stokes laser, and corresponding Raman shift.
3.3 Introducing an etalon to limit spectral broadening of fundamental spectra
From the experimental results above, we found that the spectral broadening of the fundamental laser is significant in this PQS intracavity cascaded Raman laser. The spectral broadening of the fundamental spectrum had a considerably negative impact on the conversion efficiency of SRS process because it would introduce a decrease of the effective Raman gain. The investigation into the effect of spectral broadening of the fundamental field on SRS performance had been reported in CW intracavity Raman lasers [17].
As shown in the Figs. 6(a)-(b), the fundamental spectra were depicted when increasing the incident pump power to 6.30 W for the case 1. To observe the spectral broadening clearly, we normalized the fundamental spectra to the area over ranges of 30 nm (1028 nm-1058 nm). Therefore, the decrease of the spectral peak height indicates the broadening of spectra [17]. Above the SRS threshold, with increasing the incident pump power, the spectral broadening of the fundamental field was observed remarkably. The spectral FWHMs of the fundamental laser were 1.0 nm at 1030.3 nm and 2.1 nm at 1049.1 nm at the incident pump power of 3.53 W, increasing to 1.2 nm at 1030.7 nm and 2.8 nm at 1049.7 nm at the pump power of 6.30 W. The red shift of the fundamental laser results from the spectrum property of Yb:YAG crystal which is temperature dependent. To limit the spectral broadening of the fundamental laser, an uncoated BK7 glass etalon with the thickness of 170 μm (corresponding to a free spectral range of 2.0 nm) was introduced in the fundamental cavity (case 3: with adding an etalon in the fundamental cavity) and the schematic configuration was shown in the Fig. 7. Note that the spectral broadening of the fundamental field could be well controlled after introducing the etalon (case 3) in the fundamental cavity, as shown in the Fig. 6(c). Though the fundamental laser around 1050 nm was split to two peaks centered at 1048.1 nm and 1050.1 nm due to the etalon effect, the spectral bandwidths were much narrower than these for the case 1 at the pump power of 6.30 W. The separated spectral ranges of central wavelengths were in good agreement with the free spectral range of the inserted etalon which was about 2.0 nm. The FWHM of each peak was approximately 0.8 nm.
Fig. 6 (a)-(b) The spectra of fundamental field for the case 1 at the incident pump power of 3.53 W and 6.30 W; (c) the spectra of fundamental field for the case 3 (inserting an etalon in the fundamental cavity) at the incident pump power of 6.30 W; selected primary first-Stokes spectra at the range of 1132-1162 nm at the incident pump power of 6.3 W for (d) case1: coupled cavity; (e) case3: inserting an etalon.
Fig. 7 The experimental setup of the coupled-cavity Raman laser when inserting an etalon in the fundamental cavity.
In addition, we selected the primary first-Stokes area over ranges of 30 nm (1132 nm-1162 nm) and plotted the normalized Stokes spectra to this area. As depicted in the Fig. 6(d), in the coupled cavity, the two first-Stokes radiations had broad spectral bandwidths which are 1.5 nm and 3.0 nm centered at 1135.1 nm and 1157.0 nm at the incident pump power of 6.30 W. Whereas, the emission lines were narrowed by inserting an etalon and the height of the emission spectrum around 1155 nm was about double compared with that without an etalon shown in the Fig. 6(e). The introduced etalon not only controlled the spectral broadening of the fundamental field but narrowed the spectra of the first-Stokes radiations. Note that the Stokes spectrum around 1155 nm was split to two peaks also centered at 1156.1 nm and 1158.6 nm due to the corresponding split fundamental laser around 1050 nm. Another first-order Stokes line was at 1135.8 nm.
The average power scaling characteristic as a function of the incident pump power for the case 3 was depicted in the Fig. 8(a). Adding an etalon in the fundamental cavity led to a decrease in the SRS threshold which was 1.80 W and an increase in the average output power compared with these for the case 1. The maximum average output power was 0.49 W at the incident pump power of 6.30 W for the case 3 which was about 16.67% higher than that for the case 1. The slope efficiency for the case 3 was 11.30% while that without an etalon was 9.40%. The Fig. 8(b) showed the entire Raman spectrum for the case 3 when inserting an etalon in the coupled cavity. In addition to the three first-order Stokes lines mentioned above in the Fig. 6(e), three second-order Stokes lines centered at 1263.6 nm, 1288.8 nm and 1291.8 nm were generated based on the Raman shift of 893 cm−1 from three first-order Stokes lines. The FWHMs of these Stokes lines were less than 0.8 nm.
Fig. 8 (a) The average output power of Stokes lines for the case 1 and the case 3 as a function of the incident pump power; (b) the spectrum of output laser for the case3 at an incident pump power of 6.30 W.
Figure 9 compared the pulse width, the PRF, the diode-to-Stokes conversion efficiency (ηopt) and the pulse energy depending on the incident pump power for the case 1 and the case 3. As shown in the Fig. 9(a), the pulse width with adding an etalon for case 3 was slightly smaller than that without an etalon for the case 1. At the incident pump power of 6.30 W, the pulse width was 2.20 ns for the case 3 while it was 2.43 ns for the case 1. The PRF for the both cases increased with the incident pump power linearly. The PRF of 6.15 kHz at the pump power of 6.30 W for the case 3 was obtained which was higher than that of 5.52 kHz for the case 1. As shown in the Fig. 9(b), when the etalon was introduced, the diode-to-Stokes conversion efficiency were significantly higher, leading to 7.78% and the maximum pulse energy can be up to 84.30 μJ. In fact, the spectral broadening of fundamental field in an intracavity Raman laser can lead to the decrease of effective Raman gain [17]. Therefore, introducing an etalon to limit the spectral broadening of fundamental field in the intracavity Raman laser could enhance the performance of the Stokes frequency conversion.
Fig. 9 (a) The pulse width and the PRF; (b) the diode-to-Stokes conversion efficiency and the pulse energy for the case 1 and the case 3 with respect to the incident pump power
4. Conclusion
We reported a PQS Raman laser in a YVO4 crystal pumped by a diode-pumped composite Yb:YAG/YAG/Cr4+:YAG/YAG crystal laser using a coupled cavity resonator. For free running of Stokes laser, the maximum average output power of 0.42 W was obtained in the coupled-cavity, corresponding to the pulse width of 2.43 ns and the PRF of 5.52 kHz. The spectral broadening of the fundamental field was observed in this intracavity Raman laser, so an etalon was utilized to limit the spectral broadening and in this way, the average output power, the diode-to-Stokes conversion efficiency and the pulse energy could be enhanced. The average output power was 18% higher than that without adding an etalon, to be 0.49 W, corresponding to the pulse width of 2.20 ns and the PRF of 6.15 kHz. The diode-to-Stokes conversion efficiency was calculated to be 7.78% and the available maximum pulse energy was 84.30 μJ.
Funding
National Natural Science Foundation of China (61475067, 61605062), Guangdong Project of Science and Technology Grants (2014B010131004, 2015B090901014, 2016B090917002, 2016B090926004) and Guangzhou Union Project of Science and Technology Grants (201508010021,201604040006,201604040007).
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