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Speaker : Huang Chi Chung Teacher : Ja-ho Lin

Comparison on performance of acousto-optically Q-switched Nd:GdVO 4 and Nd:YVO 4 lasers at high repetition rates under direct diode pumping of the emitting level. Speaker : Huang Chi Chung Teacher : Ja-ho Lin. Outline. Introduction Theoretical analysis Experimental setup

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Speaker : Huang Chi Chung Teacher : Ja-ho Lin

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  1. Comparison on performance of acousto-opticallyQ-switched Nd:GdVO4 and Nd:YVO4 lasers athigh repetition rates under direct diode pumpingof the emitting level Speaker : Huang Chi Chung Teacher : Ja-ho Lin

  2. Outline • Introduction • Theoretical analysis • Experimental setup • Experimental results and discussion • Conclusion

  3. Introduction • The advantages of acousto-optically (A-O) Q-switching • Low modulation voltage • Low insertion losses, high repetition rate • Short pulse width and high peak power • Nd:GVO4 and Nd:YVO4 • widely used for their excellent physical and optical properties • When short width and high repetition rates are desired, owing to • high gain • limited upper-state lifetime • A large stimulated emission cross section and effective absorption coefficient • can provide higher gain • enhances Q-switching at high repetition rates • Modest upper-state lifetime leads to • the faster building up of the pulses to achieve short pulse width

  4. A major limitation in the scaling of a solid-state laser to high power • is the quantum defect between the pump and the laser emission wavelengths • has a major contribution to the heat generation in the laser material • The reduction of the quantum defect is an important issue and for Nd3+ laser materials . • This can be accomplished by direct pumping of the emitting level 4F3/2 • Maik Frede et al reported an end-pumped Nd:YAG laser with direct pumping into the upper laser level [18], • the maximum output power was 250W with an optical-optical efficiency of 57%. • Y. Sato realized a near quantum-defect slope efficiency in Nd:YVO4 laser under direct diode pumping [19] • 80% and 75% slope efficiency were obtained under Ti:sapphire and LD pumping at 880nm

  5. An efficient A-O Q-switched Nd:GdVO4 laser under 879-nm pumping was also reported, • a maximum average output power of over 4W was obtained at 100 kHz [7]. • In this paper, we do in detail some comparative studies on • the performance of A-O Q-switching Nd:GdVO4 and Nd:YVO4 lasers at high repetition rates operated at 1.06μm under direct diode pumping of the emitting level. • When higher repetition rates and shorter pulse width are desired • Nd:YVO4 shows superior laser pulse performance to Nd:GdVO4 • indicates that Nd:YVO4 crystal is a more favorable gain medium than Nd:GdVO4 crystal, owning to its larger stimulated emission cross-section.

  6. Theoretical analysis • Nd:GdVO4 and Nd:YVO4 are isomorph and have the same crystal structure. • Table 1 shows the thermal and laser properties of Nd:GdVO4 and Nd:YVO4 at room temperature (by Yoichi Sato et al)

  7. There is no remarkable difference on thermal properties of YVO4 and GdVO4. • They has measured the thermal conductivity by quasi-one-dimensional flash method • The stimulated emission cross-section is greatly different between Nd:GdVO4 and Nd:YVO4. • Nd:YVO4 is two more times greater than that of Nd:GdVO4. • σem·τ is an important parameter for lasers with high repetition rates. • At the same repetition rates, the higher product of σem·τ, the higher gain of each pulse • σem : the stimulated emission cross-section at 1.06μm • τ : the upper-state lifetime. • The σem·τ of Nd:YVO4 is two more times greater than that of Nd:GdVO4. • According to the simulation theoretically ,we find that Nd:YVO4 laser can obtain shorter pulse width than Nd:GdVO4 laser operated at high repetition rates • owning to larger emission cross-section related to the higher single-pulse gain.

  8. Q-switched system pulse width Dtp formulas: tc : the photon decay time tr: 2L’/c c: the velocity of light L’ : the optical length of the cavity T: the transmissivity of the output coupler L: the other loss of the cavity. ni, nfand nt: the initial population inversion density, the final population inversion density, and the population inversion density at threshold

  9. Theoretical pulse width versus repetition rates at absorbed pump power of 20W • The difference of the pulse width between Nd:GdVO4 and Nd:YVO4 laser • At relative lower repetition rates (lower than 30kHz) is not obvious • As the repetition rate increases, the pulse width of Nd:YVO4 laser is gradually shorter than that of Nd:GdVO4 laser Pulse width versus absorbed pump power at the repetition rate of 100kHz • It’s forecasted that the pulse performance of Nd:YVO4 laser will be superior to that of Nd:GdVO4 laser at very high repetition rates according to the theoretical simulation.

  10. Experimental setup • The a-cut Nd:GdVO4 and Nd:YVO4 crystals ( 4mm × 4mm × 8mm.) • both polished and AR-coated at both the pump and laser wavelength on two facets of each crystal • was wrapped with indium foil and mounted in a copper heat-sink cooled by flowing water at 18°C • had a same Nd3+ ion concentration of 0.5at.% and had a same • The output coupler M2: 35% transmissivity • The mirror M1:AR coating at 879nm and HR at 1064nm • A 879nm pumping source: high-power fiber-coupled diode-laser(NL-LDM-120-879 nLIGHT Inc.), • The A-O Q-switch (39041-50DSFPS, made by Gooch and Housego Inc.): • AR-coating at 1064nm on both facets and the power of the radio frequency driver was 50W at 41MHz.

  11. CW output power as a function of the incident pump power • The maximum multi-mode CW output powers for Nd:GdVO4 and Nd:YVO4 lasers • were 22.2W and 23.5W, respectively • In the range of linear output with respect to incident pump power, the maximum optical-to-optical efficiency and slope efficiency • 38.1% and 60.7%, for Nd:GdVO4 laser • 40.5% and 66.5%, for Nd:YVO4 laser

  12. Output power versus absorbed pump power For both Nd:GdVO4 and Nd:YVO4 lasers a) CW output power b) average output power • The optical-optical efficiency of CW output power to absorbed pump power • about 69% • The slope efficiency • about 75% • More than 20W average output power => the slope efficiencies of average output power • were nearly equal and above 67.6% At the repetition rate is 100kHz

  13. Pulse width versus repetition rate at the absorbed pump power of about 33W • At the same repetition rate from 30kHz to 100kHz, the pulse width of Nd:YVO4 laser • is obviously shorter than that of Nd:GdVO4 laser • The minimum pulse widths at the repetition rate of 100kHz • 15.6 ns for Nd:GdVO4 laser • 12.1 ns for Nd:YVO4 laser Temporal single pulse profile at 100kHz a) Nd:YVO4 laser b) Nd:GdVO4 laser

  14. Pulse width versus absorbed pump power at the repetition rate of 100kHz • As the absorbed pump power increases for either Nd:GdVO4 laser or Nd:YVO4 laser. • The pulse width decays approximately exponentially • At the same absorbed pump power, • Nd:YVO4 laser can obtained shorter pulse width than Nd:GdVO4 laser • As the theory expected • we also observed the difference on pulse width of Nd:GdVO4 and Nd:YVO4 lasers with high repetition rates in our experiments. • The results reveal that • Nd:YVO4 crystal has more capability to obtain shorter pulse width and higher peak power at high repetition rates than Nd:GdVO4 crystal, even the repetition rate is much higher than 100kHz.

  15. Conclusion • We compare the performance of A-O Q-switching Nd:GdVO4 and Nd:YVO4 lasers at the repetition rate of 100kHz • There is no remarkable difference on property of output power for both Nd:GdVO4 and Nd:YVO4 lasers. • Both Nd:GdVO4 and Nd:YVO4 lasers. • are proved to be very efficient and promising laser crystals. • when operated with high repetition rates, • Nd:YVO4 laser can obtain shorter pulse width than Nd:GdVO4 laser. • It’s concluded that Nd:YVO4 is a more favorable gain medium • are desired of the higher repetition rates and shorter pulse width, • owning to its larger stimulated emission cross section related to higher single-pulse gain. • We believe that • an efficient laser system with short pulse width at much higher repetition rates will be further realized by Nd:YVO4 crystal

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