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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 11 — Jun. 3, 2013
  • pp: 13052–13061
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Investigation on LD end-pumped passively Q-switched c-cut Nd: YVO4 self-Raman laser

Shuanghong Ding, Meiqin Wang, Shiwu Wang, and Wenhui Zhang  »View Author Affiliations


Optics Express, Vol. 21, Issue 11, pp. 13052-13061 (2013)
http://dx.doi.org/10.1364/OE.21.013052


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Abstract

The compact LD end-pumped passively Q-switched c-cut Nd:YVO4/Cr4+:YAG self-Raman laser is realized, and its output performance is investigated in detail. The maximum average output power at 1178nm is 800mW with the pulse repetition frequency of 44kHz and pulse width of 2.6ns, and the first Stokes conversion efficiency is 10.1%. The outputs of fundamental and first Stokes laser are found to be linearly polarized along the diagonals of the rectangular cross section of the c-cut Nd:YVO4 crystal, and the polarization mode competition is observed in the outputs of fundamental and first Stokes laser.

© 2013 OSA

1. Introduction

Stimulated Raman scattering (SRS) process in crystals becomes the increasingly important way to generate new laser spectral lines. Actively Q-switched Raman lasers have been widely investigated [1

1. S. T. Li, X. Y. Zhang, Q. P. Wang, X. L. Zhang, Z. H. Cong, H. J. Zhang, and J. Y. Wang, “Diode-side-pumped intracavity frequency-doubled Nd:YAG/BaWO4 Raman laser generating average output power of 3.14 W at 590 nm,” Opt. Lett. 32(20), 2951–2953 (2007). [CrossRef] [PubMed]

6

6. C. L. Du, L. Zhang, Y. Q. Yu, S. C. Ruan, and Y. Y. Guo, “3.1W laser-diode-end-pumped composite Nd:YVO4 self-Raman laser at 1176nm,” Appl. Phys. B 101(4), 743–746 (2010). [CrossRef]

]. However, active Q-switching techniques employ acousto-optic or electro-optic modulators, which require external driving devices, and passive Q-switching techniques have the advantages of simplicity, compactness and low cost. Cr4+:YAG crystal has been proved to be a suitable passive Q-switcher for 0.9~1.2µm wavelength.

Some rare earth doped laser crystals are found to be Raman-active, for example Nd:KGd(WO4)2, Yb:KLu(WO4)2, Nd:PbWO4, Nd:YVO4, Nd:GdVO4, Yb:YVO4 and Nd:SrMoO4 [7

7. A. A. Kaminskii, K. I. Ueda, H. J. Eichler, Y. Kuwano, H. Kouta, S. N. Bagaev, T. H. Chyba, J. C. Barnes, G. M. A. Gad, T. Murai, and J. Lu, “Tetragonal vanadates YVO4 and GdVO4–new efficient χ(3)–materials for Raman lasers,” Opt. Commun. 194(1-3), 201–206 (2001). [CrossRef]

17

17. Y. X. Shi, Y. Zheng, J. Y. Peng, and T. L. Lu, “Passively Q Switched a-cut Nd:YVO4 Self-Raman Laser,” Laser Phys. 22(5), 904–906 (2012). [CrossRef]

]. These crystals can be utilized to realize the self-Raman lasers, in which the laser crystal also acts as Raman medium. Self-Raman lasers can generate new laser lines with very compact, stable and simple structure.

With the self-Raman configuration and passively Q-switched element, compact, stable and pulsed Raman lasers can be realized. This kind of lasers can generate new laser lines with high peak intensity and narrow pulse width, which have wide range of applications. In 2004, Y. F. Chen realized the passively Q-switched c-cut Nd:YVO4/Cr4+:YAG self-Raman lasers, the maximum output power of first Stokes at 1178.6nm is 125mW, and the conversion efficiency is 6.3% [11

11. Y. F. Chen, “Efficient subnanosecond diode-pumped passively Q-switched Nd:YVO4 self-stimulated Raman laser,” Opt. Lett. 29(11), 1251–1253 (2004). [CrossRef] [PubMed]

]. In the same year, Y. F. Chen reported a diode-pumped passively Q-switched c-cut Nd:GdVO4/Cr4+:YAG self-Raman laser, and the self-Raman laser produced the average output power of 140mW at 1175.6 nm [12

12. Y. F. Chen, “Compact efficient self-frequency Raman conversion in diode-pumped passively Q-switched Nd:GdVO4 laser,” Appl. Phys. B 78(6), 685–687 (2004). [CrossRef]

]. In 2005, Yb:YVO4/Cr4+:YAG self-Raman laser was realized to generate the 1119.5nm laser with pulse energy of 3.6μJ at a repetition rate of 25 kHz [13

13. V. E. Kisel, A. E. Troshin, N. A. Tolstik, V. G. Shcherbitsky, N. V. Kuleshov, V. N. Matrosov, T. A. Matrosova, and M. I. Kupchenko, “Q-switched Yb3+:YVO4 laser with Raman self-conversion,” Appl. Phys. B 80(4-5), 471–473 (2005). [CrossRef]

]. The passively Q-switched Yb:KLu(WO4)2 self-Raman laser with 0.4 W output power at 1137.0 nm was reported in 2006 [14

14. J. Y. Wang, H. J. Zhang, Z. P. Wang, W. W. Ge, J. X. Zhang, and M. H. Jiang, “Growth, properties and Raman shift laser in tungstate crystals,” J. Cryst. Growth 292(2), 377–380 (2006). [CrossRef]

]. In 2009, Basiev et al. realized the passively Q-switched SrMoO4:Nd3+/LiF:F-2 self-Raman laser, and average output power of 0.25W was obtained at 1163 nm [15

15. T. T. Basiev, M. E. Doroshenko, L. I. Ivleva, I. S. Voronina, V. A. Konjushkin, V. V. Osiko, and S. V. Vasilyev, “Demonstration of high self-Raman laser performance of a diode-pumped SrMoO4:Nd3+ crystal,” Opt. Lett. 34(7), 1102–1104 (2009). [CrossRef] [PubMed]

]. In the same year, passively Q-switched output from a a-cut Nd:YVO4/YVO4/Cr:YAG self-Raman laser was reported, and the maximum Raman output of 570mW was obtained at 1176 nm [16

16. T. Omatsu, A. Lee, H. M. Pask, and J. Piper, “Passively Q-switched yellow laser formed by a self-Raman composite Nd:YVO4/YVO4 crystal,” Appl. Phys. B 97(4), 799–804 (2009). [CrossRef]

]. In 2012, Y. X. Shi et al. obtained the maximum average output power of 347mW at 1176nm with the passively Q-switched a-cut Nd:YVO4/Cr4+:YAG self-Raman laser [17

17. Y. X. Shi, Y. Zheng, J. Y. Peng, and T. L. Lu, “Passively Q Switched a-cut Nd:YVO4 Self-Raman Laser,” Laser Phys. 22(5), 904–906 (2012). [CrossRef]

].

Nd:YVO4 crystal is an excellent laser medium for LD pumping with many advantages, such as large absorption coefficient, large stimulated emission cross section, and wide absorption bandwidth. At the same time, Nd:YVO4 crystal is good Raman crystal, and can be obtained easily. In 2001, Kaminskii et al. predicted that YVO4 crystals were the promising Raman media, and the Raman gain coefficient in the NIR spectral region is estimated to be about 4.5cm/GW [7

7. A. A. Kaminskii, K. I. Ueda, H. J. Eichler, Y. Kuwano, H. Kouta, S. N. Bagaev, T. H. Chyba, J. C. Barnes, G. M. A. Gad, T. Murai, and J. Lu, “Tetragonal vanadates YVO4 and GdVO4–new efficient χ(3)–materials for Raman lasers,” Opt. Commun. 194(1-3), 201–206 (2001). [CrossRef]

]. In 2004, Chen et al. realized the efficient and compact solid-state self-Raman lasers with Nd:YVO4 crystals [10

10. Y. F. Chen, “Compact efficient all-solid-state eye-safe laser with self-frequency Raman conversion in a Nd:YVO4 crystal,” Opt. Lett. 29(18), 2172–2174 (2004). [CrossRef] [PubMed]

, 11

11. Y. F. Chen, “Efficient subnanosecond diode-pumped passively Q-switched Nd:YVO4 self-stimulated Raman laser,” Opt. Lett. 29(11), 1251–1253 (2004). [CrossRef] [PubMed]

].

Nd:YVO4 crystal crystallizes in a D4h tetragonal space group of the zircon type, and its unique optical axis is along the four-fold symmetric axis c. The π-polarized (electric field∥c) transition has much larger stimulated emission cross section (σ// = 25 × 10−19cm2) than the σ-polarized (electric field⊥c) one (σ = 6.5 × 10−19cm2) for the emission at 1.06 μm. However, high laser emission cross section limits energy storage capacities in the passively Q-switched regime. When a Nd:YVO4 crystal is cut along the c axis, i.e., c-cut, the effective stimulated-emission cross section is σ instead of σ//. A c-cut Nd:YVO4 crystal is more appropriate for passive Q-switching operation [11

11. Y. F. Chen, “Efficient subnanosecond diode-pumped passively Q-switched Nd:YVO4 self-stimulated Raman laser,” Opt. Lett. 29(11), 1251–1253 (2004). [CrossRef] [PubMed]

, 18

18. A. E. Siegman, Lasers (University Science, Mill Valley, Calif., 1986), p.1024.

, 19

19. G. H. Xiao and M. Bass, “A generalized model for passively Q-switched lasers including excited state absorption in the saturable absorber,” IEEE J. Quantum Electron. 33(1), 41–44 (1997). [CrossRef]

]. For intracavity Raman lasers, the normalized Raman gain is larger for the laser medium with smaller stimulated-emission cross section [20

20. S. Ding, X. Zhang, Q. Wang, J. Zhang, S. Wang, Y. Liu, and X. Zhang, “Numerically modeling of passively Q-switched intracavity Raman lasers,” J. Phys. D Appl. Phys. 40(9), 2736–2747 (2007). [CrossRef]

].

In this paper, with c-cut Nd:YVO4 crystal of 20mm in length as the laser and Raman media, we realize the efficient, compact LD end-pumped passively Q-switched self-Raman laser. The temporal, polarization, spectrum and spatial features of the self-Raman laser are studied experimentally in detail. Its optimized conditions are analyzed, and the most efficient Raman conversion is obtained under the conditions of Cr4+:YAG with initial transmission of T0 = 89% and output couplers with reflectivity of RS = 90% at 1178nm. The maximum average output power at 1178nm obtained is 800mW with respect to the incident pump power of 7.9W, and the corresponding conversion efficiency is 10.1%. To the best of our knowledge, this is the highest average output power ever obtained from passively Q-switched self-Raman lasers. With c-cut Nd:YVO4 crystal as laser medium, laser output should be unpolarized. However, the outputs of fundamental and Stokes laser are found to be linearly polarized along the diagonals of the rectangular cross section of c-cut Nd:YVO4 crystal in the experiments, and the polarization mode competition is observed in the outputs of fundamental and first Stokes laser.

2. Experimental arrangement

Figure 1
Fig. 1 Experimental setup of the diode-pumped passively Q-switched c-cut Nd:YVO4 /Cr4+:YAG self-Raman laser
shows the experimental setup of the diode-pumped passively Q-switched c-cut Nd:YVO4/Cr4+:YAG self-Raman laser. The pump source is a 30W fiber-coupled 808nm laser diode with a core diameter of 400μm and a numerical aperture of 0.22. A focusing lens system (1:1 magnification) with a coupling efficiency of 95% is used to reimage the pump beam into the laser crystal. The resonator has a linear configuration. The rear mirror M1 is a concave mirror. It is coated for high reflection (HR) at 1066nm (R>99.8%) and 1178nm (R>99.5%), and high transmission (HT) at 808nm (T>90%). The plane output coupler M2 is coated for high reflection at 1066nm (R>99.8%), and two output couplers (OC) with different reflectivities at 1178nm (RS = 80% and 90%) are used in our experiment. The laser material is a 0.3%-Nd-doped c-cut Nd:YVO4 crystal with a dimension of 3 × 3 × 20mm3, and both faces are AR coated at 1066nm (R<0.2%), 1178nm (R<0.2%), and 808nm (R<3%). The c-cut Nd: YVO4 crystal acts as the Raman active material at the same time. The saturable absorbers Cr4+: YAG are AR coated at 1066nm (R<0.2%). Nd:YVO4 and Cr4+:YAG crystals are wrapped with indium foil and held in water-cooled copper blocks. The water temperature is controlled to be 23°C throughout the experiment. The saturable absorber is put close to the output coupler to obtain small cavity mode size in it, which is beneficial to the passively Q-switched criterion [19

19. G. H. Xiao and M. Bass, “A generalized model for passively Q-switched lasers including excited state absorption in the saturable absorber,” IEEE J. Quantum Electron. 33(1), 41–44 (1997). [CrossRef]

]. The overall laser cavity length is approximately 30mm. The distance between M1 and Nd:YVO4 crystal is about 3mm, and Cr4+:YAG crystal is apart from Nd:YVO4 crystal and M2 by 4mm and 2mm, respectively. A dichroic mirror separates the output of 1178nm from those of 808nm and 1066nm. The average output power is measured by a power meter (Ophir Laserstar). The pulse temporal behavior is recorded by an Agilent digital oscilloscope (DSO7104A, 1GHz Bandwidth) with a fast PIN photodiode.

3. Experimental results and discussion

3.1 Output power at 1178nm

Figure 2
Fig. 2 Average output power (a) and pulse energy (b) at 1178nm with respect to the incident pump power for Cr4+:YAG of initial transmission 89% and 86%, and M2 of RS = 80% and 90%, respectively
shows the average output power (a) and pulse energy (b) at 1178nm with respect to the incident pump power for Cr4+:YAG of initial transmission T0 = 89% and 86%, and M2 of RS = 80% and 90%, respectively. The most efficient Raman conversion is obtained under the conditions of T0 = 89% and RS = 90%, and the first Stokes average output power is much higher than the other three lines as shown in Fig. 2(a). The maximum average output power obtained is 800mW with respect to the incident pump power of 7.9 W, and the corresponding conversion efficiency is 10.1%. This is the highest average output power ever obtained from LD pumped passively Q-switched Nd:YVO4 self-Raman lasers. The average output power increases with incident pump power, and finally drop down beyond the incident pump power of 8.8 W with T0 = 89% and RS = 90%, which is due to unstable resonator caused by thermal lensing in the laser medium. The pulse energy decreases slightly with incident pump power as shown in Fig. 2(b). The average pulse energy is about 18μJ with T0 = 89% and RS = 90%, and maximum pulse energy is over 30μJ with T0 = 86% and RS = 90%. The Stokes pulse energy decreases with increasing the pump power, and there are two main reasons. One is the thermal lens effect in the laser crystal, which reduces the laser mode volume, resulting in a decrease of the pulse energy. The other is the pump-induced bleaching of the saturable absorber. With increasing the incident power, the laser medium could not completely absorb the pump power, and the remaining pump power, to some extent, could bleach the saturable absorber, resulting in a significant decrease of the pulse energy [21

21. M. A. Jaspan, D. Welford, and J. A. Russell, “Passively Q-Switched Microlaser Performance in the Presence of Pump-Induced Bleaching of the Saturable Absorber,” Appl. Opt. 43(12), 2555–2560 (2004). [CrossRef] [PubMed]

].

3.2 Temporal characteristics

Pulse repetition frequency (a) and pulse width (b) at 1178nm with respect to the incident pump power is shown in Fig. 3
Fig. 3 Pulse repetition frequency (a) and pulse width (b) at 1178nm with respect to the incident pump power for Cr4+:YAG of T0 = 89% and 86%, and M2 of RS = 80% and 90%, respectively
for Cr4+:YAG of T0 = 89% and 86%, and M2 of RS = 80% and 90%, respectively. As the incident pump power increases, the PRF of the Raman laser grows, and increases from 8.9 kHz to 46.4 kHz and from 4.5 kHz to 29.5 kHz for Cr4+:YAG of T0 = 89% and 86%, respectively. The PRF for T0 = 89% is higher than that for T0 = 86%, and the output coupling rate has a little effect on PRF.

The normalized rate equations of passively Q-switched intracavity Raman lasers were deduced, and numerical analysis were carried out in [20

20. S. Ding, X. Zhang, Q. Wang, J. Zhang, S. Wang, Y. Liu, and X. Zhang, “Numerically modeling of passively Q-switched intracavity Raman lasers,” J. Phys. D Appl. Phys. 40(9), 2736–2747 (2007). [CrossRef]

]. According to the definitions in [20

20. S. Ding, X. Zhang, Q. Wang, J. Zhang, S. Wang, Y. Liu, and X. Zhang, “Numerically modeling of passively Q-switched intracavity Raman lasers,” J. Phys. D Appl. Phys. 40(9), 2736–2747 (2007). [CrossRef]

], the normalized Raman gain M for the experiment shown in Fig. 1 is calculated to be 24, and saturable absorber synthetic parameter α is 23. The normalized initial population inversion density N and normalized first Stokes loss K for different T0 and RS are estimated as shown in Table 1

Table 1. The normalized parameters for passively Q-switched c-cut Nd:YVO4/Cr4+:YAG self-Raman lasers

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, and the optimum values of normalized Raman gain Mopt are obtained to achieve the maximum integral of the normalized first Stokes photon density for different conditions as given in Table 1. Figure 5
Fig. 5 Temporal profiles of first Stokes pulses generated with the pump power of 5W for T0 = 89% and RS = 90% in (a); T0 = 89% and RS = 80% in (b); T0 = 86% and RS = 90% in (c); T0 = 86% and RS = 80% in (d).
shows temporal profiles of first Stokes pulses generated with the pump power of 5W for T0 = 89% and RS = 90% in (a); T0 = 89% and RS = 80% in (b); T0 = 86% and RS = 90% in (c); T0 = 86% and RS = 80% in (d). As shown in Figs. 5(a)-5(c), the satellite pulses of first Stokes laser are present. With the larger RS and smaller T0, the multi-pulse phenomenon is more serious. There are two reasons for this phenomenon. One is the long upper level lifetime (3 μs~4 μs) of the slow recovery saturable absorber Cr:YAG crystal, which provides the preconditions for the emergence of satellite pulses. The other is that too large Raman gain will lead to satellite pulses of the fundamental and first Stokes, according to the numerical modeling of passively Q-switched intracavity Raman lasers [20

20. S. Ding, X. Zhang, Q. Wang, J. Zhang, S. Wang, Y. Liu, and X. Zhang, “Numerically modeling of passively Q-switched intracavity Raman lasers,” J. Phys. D Appl. Phys. 40(9), 2736–2747 (2007). [CrossRef]

]. When the SRS threshold is reached, the intracavity Stokes photon densities go up sharply while the fundamental photon densities decrease rapidly. However, the population inversion is not depleted completely, and another fundamental pulse will occur later. When the SRS threshold is reached again, the satellite Stokes pulse emerges. For our experiments, normalized Raman gain M = 24 is larger than Mopt given in Table 1, and the satellite pulses of first Stokes were observed in the experiment as shown in Fig. 5. In Fig. 5(a), the main pulse is much larger than the satellite pulses, and conversion efficiency of first Stokes is highest for T0 = 89% and RS = 90% as shown in Fig. 2, which means the Raman laser works under the good conditions. In Fig. 5(b), a very small satellite pulse presents, which indicates the Raman laser works just slightly above Mopt, and conversion efficiency is low. In Fig. 5(c), satellite pulse phenomenon is serious, and the number of satellite pulses is four, and their energies are high, which indicates the Raman laser works far above Mopt, with which the conversion efficiency of Raman laser is low. In Fig. 5(d), the number of the satellite pulses is 2, and pulse width of the main pulse is much smaller than that in Fig. 5(c), which means the output coupling rate is too high to maintain the efficient Raman conversion inside the resonator. In conclusion, when the normalized Raman gain M is moderately larger than Mopt as with T0 = 89% RS = 90%, the efficient and stable Raman laser is realized as shown in Fig. 2 and Fig. 4. The working conditions given in Figs. 5(b), 5(c) and 5(d) are far from optimized ones.

3.3 Polarization characteristics

The unique optical axis of Nd:YVO4 crystal is along the four-fold symmetric axis c. Perpendicular to this axis are the two indistinguishable a and b axes. With c-cut Nd:YVO4 crystal as laser medium, the stimulated emission cross section is same for electric field ∥a and ∥b, and the output laser should be unpolarized. However, we get the linearly polarized fundamental and first Stokes outputs, and also observe the polarization mode competition in our experiment.

In the experiment, the c-cut Nd:YVO4 crystal has a rectangular cross section of 3X3mm2, which are cut along a and b optical axes, respectively. The water cooled copper blocks including Nd:YVO4 crystal and indium foil is depicted in Fig. 6
Fig. 6 Water cooled copper blocks including Nd:YVO4 crystal and indium foil
. In the experiment, the outputs of fundamental and Stokes laser are found to be linearly polarized along the diagonals of the rectangular cross section, named horizontal and vertical directions in Fig. 6.

Table 2

Table 2. Polarized average output powers at 1066nm of passively Q-switched c-cut Nd:YVO4/Cr4+:YAG laser

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gives horizontally and vertically polarized average output powers at 1066nm of passively Q-switched c-cut Nd:YVO4/Cr4+:YAG laser for different pumping power with Cr4+:YAG of T0 = 86%, output coupling rate of R = 90%@1066nm, and the laser cavity length of 30mm. For most of the times, the polarization of the fundamental is along horizontal direction, however, polarization mode competition are observed under certain pumping power, e.g. 7.7 W as given in Table 2.

A Glan prism is placed in front of the fast PIN photodiode. When the transmission axis of the polarizer is rotated, two different sets of pulse trains are recorded with the oscilloscope in the fundamental laser output. With the transmission axis nearly parallel to the vertical direction, two pulse trains of 1066nm are recorded simultaneously as shown in Fig. 7(a)
Fig. 7 Pulse trains of 1066nm output of passively Q-switched laser in (a); pulse trains at 1178nm of passively Q-switched self-Raman laser, horizontally polarized pulse train in (b) and vertically polarized one in (c).
for pump power of 6.9W with Cr4+:YAG of T0 = 86%, output coupling rate of R = 90%@1066nm, and the laser cavity length of 30mm. The vertically polarized pulse train has high intensity, and horizontally polarized one has low intensity in Fig. 7(a). The polarized mode competition between two polarization directions is obvious. The polarized mode competition is also present in the first Stokes output. Pulse trains at 1178nm output of passively Q-switched self-Raman laser are shown in Figs. 7(b) and 7(c). For pump power of 5.9W with Cr4+:YAG of T0 = 86% and output coupling rate of RS = 90%@1178nm, horizontally polarized pulse train is in (b), and vertically polarized one in (c). The horizontal polarization is advantageous to the vertical one.

In our experiment, the LD pumping beam is tested to be unpolarized. With the polarization interference method, c-cut Nd:YVO4 crystal used in the experiment is verified to be perfect uniaxial crystal. We attribute the unusual polarization phenomenon to thermally induced birefringence for the moment. The laser medium is subject to convective cooling by the surrounding water, causing non-uniform temperature distribution, furthering resulting in thermal stress and stress birefringence. The temperature, thermal stress, and thermal strain distribution in laser medium are solved both analytically and numerically in [23

23. Z. Li, X. Huai, Y. Tao, and Z. Guo, “Analysis of thermal effects in an orthotropic laser medium,” Appl. Opt. 48(3), 598–608 (2009). [CrossRef] [PubMed]

]. It is found that the distributions of temperature gradient, thermal stress and thermal strain are not axisymmetric on the rectangular cross section of laser medium. The temperature gradient is smallest along the two diagonals of the rectangle, and the thermally induced birefringence leads to the laser output polarized along these two directions. In the experiment, water cooled copper blocks comprise of two parts as shown in Fig. 6, and clasp laser crystal between them, whose contact area is covered with thermally conductive silicone. The thermally conductive conditions of vertical and horizontal directions are different, and the horizontal polarization is found to be advantageous to the vertical one in the experiments. The unusual polarization phenomenon caused by thermally induced birefringence will be studied in more detail. We believe that higher efficient passively Q-switched c-cut Nd:YVO4/Cr4+:YAG self-Raman laser can be realized by eliminating this unusual polarization phenomenon.

3.4 Spectrum characteristics

The spectrum information of the intracavity Raman laser was registered with an optical frequency analyzer (ANDO, AQ-6317B, 600nm-1750nm) with the resolution of 0.5nm. Figure 8
Fig. 8 (a) Spectrum of passively Q-switched c-cut Nd:YVO4/Cr4+:YAG self-Raman laser output under the pump power of 4.3W for T0 = 89% and RS = 90%, (b) the spectrum of fundamental laser corresponding to (a)
depicts spectra of passively Q-switched c-cut Nd:YVO4/Cr4+:YAG self-Raman laser under the pump power of 4.3W for T0 = 89% and RS = 90%. As shown in Fig. 8, four spectrum lines were observed in the Raman laser output, which are registered in 1066.7nm, 1097.3nm, 1129.5nm and 1178.8nm, respectively. The line of 1178.8nm is the most intensive, and the corresponding SRS shift is calculated to be 889.7cm−1 relative to the fundamental laser registered in 1066.7nm, which is in accordance to the reported Raman shift 890cm−1 of the totally symmetric A1g optical vibration modes of tetrahedral VO42--ionic groups [7

7. A. A. Kaminskii, K. I. Ueda, H. J. Eichler, Y. Kuwano, H. Kouta, S. N. Bagaev, T. H. Chyba, J. C. Barnes, G. M. A. Gad, T. Murai, and J. Lu, “Tetragonal vanadates YVO4 and GdVO4–new efficient χ(3)–materials for Raman lasers,” Opt. Commun. 194(1-3), 201–206 (2001). [CrossRef]

]. The lines of 1097.3nm and 1129.5nm are very weak, and they are first and second Stokes lines caused by the interaction of fundamental laser with the optical vibration mode of 259.8cm−1 [7

7. A. A. Kaminskii, K. I. Ueda, H. J. Eichler, Y. Kuwano, H. Kouta, S. N. Bagaev, T. H. Chyba, J. C. Barnes, G. M. A. Gad, T. Murai, and J. Lu, “Tetragonal vanadates YVO4 and GdVO4–new efficient χ(3)–materials for Raman lasers,” Opt. Commun. 194(1-3), 201–206 (2001). [CrossRef]

]. As shown in Fig. 8(b), besides the fundamental laser line registered in 1066.7nm (originates from the R2 component of the 4F3/2 level, and terminates at the Y3 component of the 4I11/2 level.), another weaker line in 1062.8nm is observed, which originates from the R1 component of the 4F3/2 level, and terminates at the Y2 component of the 4I11/2 level.

3.5 Spatial characteristics

The spatial features of laser output are studied with laser beam analyzer BeamOn-IR-1550 (Duma Optronics Ltd., Israel). Figure 9
Fig. 9 Spatial profile of fundamental laser in (a) of passively Q-switched c-cut Nd:YVO4/Cr4+:YAG laser under the pump power of 4.8W for T0 = 89% and output coupling rate of R = 90%@1066nm, spatial profile of first Stokes laser in (b) of passively Q-switched c-cut Nd:YVO4/Cr4+:YAG self-Raman laser output under the pump power of 6.2W for T0 = 89% and RS = 90%.
shows the spatial profile of fundamental laser in (a) of passively Q-switched c-cut Nd:YVO4/Cr4+:YAG laser under the pump power of 4.8W for T0 = 89% and output coupling rate of R = 90%@1066nm, and spatial profile of first Stokes laser in (b) of passively Q-switched c-cut Nd:YVO4/Cr4+:YAG self-Raman laser output under the pump power of 6.2W for T0 = 89% and RS = 90%. M2 factor of fundamental laser beam is measured to be 1.0 under the pump power of 4.8W for T0 = 89% and output coupling rate of R = 90%@1066nm, and perfect fundamental laser beam of TEM00 mode is generated as shown in Fig. 9(a). For passively Q-switched c-cut Nd:YVO4/Cr4+:YAG self-Raman laser under the pump power of 6.2W for T0 = 89% and RS = 90%, M2 factor of the first Stokes laser beam is measured to be 2.8 as shown in Fig. 9(b). For passively Q-switched c-cut Nd:YVO4/Cr4+:YAG self-Raman laser, because the output coupler M2 is coated for high reflection at 1066nm (R>99.8%), fundamental laser of high order spatial mode and TEM00 mode will oscillate simultaneously under the pump power of 6.2W resulting in the M2 factor of the first Stokes laser to be 2.8.

4. Conclusion

For compact LD end-pumped passively Q-switched c-cut Nd:YVO4/Cr4+:YAG self-Raman laser, when the normalized Raman gain M is moderately larger than Mopt as with Cr4+:YAG of initial transmission T0 = 89% and output couplers of reflectivity RS = 90% at 1178nm, the efficient and stable Raman laser is realized. The maximum average output power at 1178nm obtained is 800mW with respect to the incident pump power of 7.9W, the pulse repetition frequency is 44kHz, pulse width is 2.6ns, pulse energy is 18μJ, and peak power intensity is 7.0kW. The corresponding conversion efficiency for LD pump power to first Stokes laser is 10.1%. With c-cut Nd:YVO4 crystal as self-Raman medium, the outputs of fundamental and Stokes laser are found to be linearly polarized along the diagonals of the rectangular cross section in the experiments, and the polarization mode competition is observed in the output of fundamental and first Stokes laser. The mechanism of the unusual polarization phenomenon is explained from the thermally induced birefringence aspect. It is predicted that higher efficient passively Q-switched c-cut Nd:YVO4/Cr4+:YAG self-Raman laser can be realized by eliminating the unusual polarization phenomenon. M2 factor of the first Stokes laser beam is measured to be about 2.

Acknowledgments

This research is supported by the National Natural Science Foundations of China (Grant No.10974168), and the Science and Technology Program of the Shandong Higher Education Institutions of China (Grant No.J09LA06).

References and links

1.

S. T. Li, X. Y. Zhang, Q. P. Wang, X. L. Zhang, Z. H. Cong, H. J. Zhang, and J. Y. Wang, “Diode-side-pumped intracavity frequency-doubled Nd:YAG/BaWO4 Raman laser generating average output power of 3.14 W at 590 nm,” Opt. Lett. 32(20), 2951–2953 (2007). [CrossRef] [PubMed]

2.

Z. H. Cong, X. Y. Zhang, Q. P. Wang, Z. J. Liu, X. H. Chen, S. Z. Fan, X. L. Zhang, H. J. Zhang, X. T. Tao, and S. T. Li, “Theoretical and experimental study on the Nd:YAG/BaWO4/KTP yellow laser generating 8.3 W output power,” Opt. Express 18(12), 12111–12118 (2010). [CrossRef] [PubMed]

3.

X. H. Chen, X. Y. Zhang, Q. P. Wang, P. Li, S. T. Li, Z. H. Cong, G. H. Jia, and C. Y. Tu, “Highly efficient diode-pumped actively Q-switched Nd:YAG-SrWO4 intracavity Raman laser,” Opt. Lett. 33(7), 705–707 (2008). [CrossRef] [PubMed]

4.

Y. F. Chen, “High-power diode-pumped actively Q-switched Nd:YVO4 self-Raman laser: influence of dopant concentration,” Opt. Lett. 29(16), 1915–1917 (2004). [CrossRef] [PubMed]

5.

S. Ding, X. Zhang, Q. Wang, F. Su, P. Jia, S. Li, S. Fan, J. Chang, S. Zhang, and Z. Liu, “Theoretical and experimental study on the self-Raman laser with Nd:YVO4 crystal,” IEEE J. Quantum Electron. 42(9), 927–933 (2006). [CrossRef]

6.

C. L. Du, L. Zhang, Y. Q. Yu, S. C. Ruan, and Y. Y. Guo, “3.1W laser-diode-end-pumped composite Nd:YVO4 self-Raman laser at 1176nm,” Appl. Phys. B 101(4), 743–746 (2010). [CrossRef]

7.

A. A. Kaminskii, K. I. Ueda, H. J. Eichler, Y. Kuwano, H. Kouta, S. N. Bagaev, T. H. Chyba, J. C. Barnes, G. M. A. Gad, T. Murai, and J. Lu, “Tetragonal vanadates YVO4 and GdVO4–new efficient χ(3)–materials for Raman lasers,” Opt. Commun. 194(1-3), 201–206 (2001). [CrossRef]

8.

T. Omatsu, Y. Ojima, H. M. Pask, J. A. Piper, and P. Dekker, “Efficient 1181nm self-stimulating Raman output from transversely diode-pumped Nd3+:KGd(WO4)2 laser,” Opt. Commun. 232(1-6), 327–331 (2004). [CrossRef]

9.

W. Chen, Y. Inagawa, T. Omatsu, M. Tateda, N. Takeuchi, and Y. Usuki, “Diode-pumped, self-stimulating, passively Q-switched Nd3+: PbWO4 Raman laser,” Opt. Commun. 194(4-6), 401–407 (2001). [CrossRef]

10.

Y. F. Chen, “Compact efficient all-solid-state eye-safe laser with self-frequency Raman conversion in a Nd:YVO4 crystal,” Opt. Lett. 29(18), 2172–2174 (2004). [CrossRef] [PubMed]

11.

Y. F. Chen, “Efficient subnanosecond diode-pumped passively Q-switched Nd:YVO4 self-stimulated Raman laser,” Opt. Lett. 29(11), 1251–1253 (2004). [CrossRef] [PubMed]

12.

Y. F. Chen, “Compact efficient self-frequency Raman conversion in diode-pumped passively Q-switched Nd:GdVO4 laser,” Appl. Phys. B 78(6), 685–687 (2004). [CrossRef]

13.

V. E. Kisel, A. E. Troshin, N. A. Tolstik, V. G. Shcherbitsky, N. V. Kuleshov, V. N. Matrosov, T. A. Matrosova, and M. I. Kupchenko, “Q-switched Yb3+:YVO4 laser with Raman self-conversion,” Appl. Phys. B 80(4-5), 471–473 (2005). [CrossRef]

14.

J. Y. Wang, H. J. Zhang, Z. P. Wang, W. W. Ge, J. X. Zhang, and M. H. Jiang, “Growth, properties and Raman shift laser in tungstate crystals,” J. Cryst. Growth 292(2), 377–380 (2006). [CrossRef]

15.

T. T. Basiev, M. E. Doroshenko, L. I. Ivleva, I. S. Voronina, V. A. Konjushkin, V. V. Osiko, and S. V. Vasilyev, “Demonstration of high self-Raman laser performance of a diode-pumped SrMoO4:Nd3+ crystal,” Opt. Lett. 34(7), 1102–1104 (2009). [CrossRef] [PubMed]

16.

T. Omatsu, A. Lee, H. M. Pask, and J. Piper, “Passively Q-switched yellow laser formed by a self-Raman composite Nd:YVO4/YVO4 crystal,” Appl. Phys. B 97(4), 799–804 (2009). [CrossRef]

17.

Y. X. Shi, Y. Zheng, J. Y. Peng, and T. L. Lu, “Passively Q Switched a-cut Nd:YVO4 Self-Raman Laser,” Laser Phys. 22(5), 904–906 (2012). [CrossRef]

18.

A. E. Siegman, Lasers (University Science, Mill Valley, Calif., 1986), p.1024.

19.

G. H. Xiao and M. Bass, “A generalized model for passively Q-switched lasers including excited state absorption in the saturable absorber,” IEEE J. Quantum Electron. 33(1), 41–44 (1997). [CrossRef]

20.

S. Ding, X. Zhang, Q. Wang, J. Zhang, S. Wang, Y. Liu, and X. Zhang, “Numerically modeling of passively Q-switched intracavity Raman lasers,” J. Phys. D Appl. Phys. 40(9), 2736–2747 (2007). [CrossRef]

21.

M. A. Jaspan, D. Welford, and J. A. Russell, “Passively Q-Switched Microlaser Performance in the Presence of Pump-Induced Bleaching of the Saturable Absorber,” Appl. Opt. 43(12), 2555–2560 (2004). [CrossRef] [PubMed]

22.

S. Ding, X. Zhang, Q. Wang, J. Zhang, and S. Wang, “Temporal properties of the solid-state intracavity Raman laser using the traveling-wave method,” Phys. Rev. A 76(5), 053830 (2007). [CrossRef]

23.

Z. Li, X. Huai, Y. Tao, and Z. Guo, “Analysis of thermal effects in an orthotropic laser medium,” Appl. Opt. 48(3), 598–608 (2009). [CrossRef] [PubMed]

OCIS Codes
(140.3540) Lasers and laser optics : Lasers, Q-switched
(140.3550) Lasers and laser optics : Lasers, Raman
(140.3580) Lasers and laser optics : Lasers, solid-state

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: March 26, 2013
Revised Manuscript: May 12, 2013
Manuscript Accepted: May 12, 2013
Published: May 20, 2013

Citation
Shuanghong Ding, Meiqin Wang, Shiwu Wang, and Wenhui Zhang, "Investigation on LD end-pumped passively Q-switched c-cut Nd: YVO4 self-Raman laser," Opt. Express 21, 13052-13061 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-11-13052


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References

  1. S. T. Li, X. Y. Zhang, Q. P. Wang, X. L. Zhang, Z. H. Cong, H. J. Zhang, and J. Y. Wang, “Diode-side-pumped intracavity frequency-doubled Nd:YAG/BaWO4 Raman laser generating average output power of 3.14 W at 590 nm,” Opt. Lett.32(20), 2951–2953 (2007). [CrossRef] [PubMed]
  2. Z. H. Cong, X. Y. Zhang, Q. P. Wang, Z. J. Liu, X. H. Chen, S. Z. Fan, X. L. Zhang, H. J. Zhang, X. T. Tao, and S. T. Li, “Theoretical and experimental study on the Nd:YAG/BaWO4/KTP yellow laser generating 8.3 W output power,” Opt. Express18(12), 12111–12118 (2010). [CrossRef] [PubMed]
  3. X. H. Chen, X. Y. Zhang, Q. P. Wang, P. Li, S. T. Li, Z. H. Cong, G. H. Jia, and C. Y. Tu, “Highly efficient diode-pumped actively Q-switched Nd:YAG-SrWO4 intracavity Raman laser,” Opt. Lett.33(7), 705–707 (2008). [CrossRef] [PubMed]
  4. Y. F. Chen, “High-power diode-pumped actively Q-switched Nd:YVO4 self-Raman laser: influence of dopant concentration,” Opt. Lett.29(16), 1915–1917 (2004). [CrossRef] [PubMed]
  5. S. Ding, X. Zhang, Q. Wang, F. Su, P. Jia, S. Li, S. Fan, J. Chang, S. Zhang, and Z. Liu, “Theoretical and experimental study on the self-Raman laser with Nd:YVO4 crystal,” IEEE J. Quantum Electron.42(9), 927–933 (2006). [CrossRef]
  6. C. L. Du, L. Zhang, Y. Q. Yu, S. C. Ruan, and Y. Y. Guo, “3.1W laser-diode-end-pumped composite Nd:YVO4 self-Raman laser at 1176nm,” Appl. Phys. B101(4), 743–746 (2010). [CrossRef]
  7. A. A. Kaminskii, K. I. Ueda, H. J. Eichler, Y. Kuwano, H. Kouta, S. N. Bagaev, T. H. Chyba, J. C. Barnes, G. M. A. Gad, T. Murai, and J. Lu, “Tetragonal vanadates YVO4 and GdVO4–new efficient χ(3)–materials for Raman lasers,” Opt. Commun.194(1-3), 201–206 (2001). [CrossRef]
  8. T. Omatsu, Y. Ojima, H. M. Pask, J. A. Piper, and P. Dekker, “Efficient 1181nm self-stimulating Raman output from transversely diode-pumped Nd3+:KGd(WO4)2 laser,” Opt. Commun.232(1-6), 327–331 (2004). [CrossRef]
  9. W. Chen, Y. Inagawa, T. Omatsu, M. Tateda, N. Takeuchi, and Y. Usuki, “Diode-pumped, self-stimulating, passively Q-switched Nd3+: PbWO4 Raman laser,” Opt. Commun.194(4-6), 401–407 (2001). [CrossRef]
  10. Y. F. Chen, “Compact efficient all-solid-state eye-safe laser with self-frequency Raman conversion in a Nd:YVO4 crystal,” Opt. Lett.29(18), 2172–2174 (2004). [CrossRef] [PubMed]
  11. Y. F. Chen, “Efficient subnanosecond diode-pumped passively Q-switched Nd:YVO4 self-stimulated Raman laser,” Opt. Lett.29(11), 1251–1253 (2004). [CrossRef] [PubMed]
  12. Y. F. Chen, “Compact efficient self-frequency Raman conversion in diode-pumped passively Q-switched Nd:GdVO4 laser,” Appl. Phys. B78(6), 685–687 (2004). [CrossRef]
  13. V. E. Kisel, A. E. Troshin, N. A. Tolstik, V. G. Shcherbitsky, N. V. Kuleshov, V. N. Matrosov, T. A. Matrosova, and M. I. Kupchenko, “Q-switched Yb3+:YVO4 laser with Raman self-conversion,” Appl. Phys. B80(4-5), 471–473 (2005). [CrossRef]
  14. J. Y. Wang, H. J. Zhang, Z. P. Wang, W. W. Ge, J. X. Zhang, and M. H. Jiang, “Growth, properties and Raman shift laser in tungstate crystals,” J. Cryst. Growth292(2), 377–380 (2006). [CrossRef]
  15. T. T. Basiev, M. E. Doroshenko, L. I. Ivleva, I. S. Voronina, V. A. Konjushkin, V. V. Osiko, and S. V. Vasilyev, “Demonstration of high self-Raman laser performance of a diode-pumped SrMoO4:Nd3+ crystal,” Opt. Lett.34(7), 1102–1104 (2009). [CrossRef] [PubMed]
  16. T. Omatsu, A. Lee, H. M. Pask, and J. Piper, “Passively Q-switched yellow laser formed by a self-Raman composite Nd:YVO4/YVO4 crystal,” Appl. Phys. B97(4), 799–804 (2009). [CrossRef]
  17. Y. X. Shi, Y. Zheng, J. Y. Peng, and T. L. Lu, “Passively Q Switched a-cut Nd:YVO4 Self-Raman Laser,” Laser Phys.22(5), 904–906 (2012). [CrossRef]
  18. A. E. Siegman, Lasers (University Science, Mill Valley, Calif., 1986), p.1024.
  19. G. H. Xiao and M. Bass, “A generalized model for passively Q-switched lasers including excited state absorption in the saturable absorber,” IEEE J. Quantum Electron.33(1), 41–44 (1997). [CrossRef]
  20. S. Ding, X. Zhang, Q. Wang, J. Zhang, S. Wang, Y. Liu, and X. Zhang, “Numerically modeling of passively Q-switched intracavity Raman lasers,” J. Phys. D Appl. Phys.40(9), 2736–2747 (2007). [CrossRef]
  21. M. A. Jaspan, D. Welford, and J. A. Russell, “Passively Q-Switched Microlaser Performance in the Presence of Pump-Induced Bleaching of the Saturable Absorber,” Appl. Opt.43(12), 2555–2560 (2004). [CrossRef] [PubMed]
  22. S. Ding, X. Zhang, Q. Wang, J. Zhang, and S. Wang, “Temporal properties of the solid-state intracavity Raman laser using the traveling-wave method,” Phys. Rev. A76(5), 053830 (2007). [CrossRef]
  23. Z. Li, X. Huai, Y. Tao, and Z. Guo, “Analysis of thermal effects in an orthotropic laser medium,” Appl. Opt.48(3), 598–608 (2009). [CrossRef] [PubMed]

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