Frequency Doubling Ole Bjarlin Jensen

Transcription

1 Frequency Doubling Ole Bjarlin Jensen DTU Fotonik, Risø campus Technical University of Denmark, Denmark (

2 Outline of the talk Quasi phase matching Schemes for frequency doubling Conversion efficiency Focusing Acceptance bandwidths Single-pass SHG External cavity SHG Experimental results using different lasers Conclusion Acknowledgements

3 Quasi phase matching The generated second harmonic is sum of contributions from the entire crystal Birefringent phase matching Quasi phase matching largest nonlinear tensor elements can be used Λ Λ Λ Λ No phase matching nω nω E L ω ( L) Eωd ( z)exp ω ω ) 0 [ i( k k z] πnω πnω πnω + = nω = n λω λω λω π k ω kω = 0, Λ = l c Λ Intensity / a.u BPM QPM 1 NPM ω Periods dz

4 Schemes for frequency doubling Single-pass SHG External cavity SHG Intracavity SHG

5 Conversion efficiency P P ω ω = η = P SHG ω P ω tanh η SHG P ω ( Including pump depletion) Thermal effects in the nonlinear material will limit the conversion efficiency due to phase mismatch through the crystal. Common crystals: LBO, BBO, BiBO, KTP, LiNbO 3, PPKTP, PPLN η SHG = 0.01 %/W cm

6 Focusing plane waves kl sin P ω ω d ηshg = 3 3 Pω n ε 0c kl For high efficiency k = 0 P l A = ω Small beam area (A) plane wave analysis not accurate! l A A more accurate analysis assumes focused Gaussian beams.

7 Focusing Gaussian beams 3 P ω ω d η SHG = = hbk ( ξ, σ ) Pω l 4 P πn ε c Focusing parameter: ω Normalized phase mismatch: 0 ξ = λl πnw 0 σ = z 0 k l SHG focusing function: h BK ( σ, B, κ, ξ, µ ) Optimized focusing is a tradeoff between high intensity and long interaction length 1 G. D. Boyd and D. A. Kleinman, J. Appl. Phys. 39, 3597, 1968.

8 Focusing Gaussian beams Optimal focusing: ξ =.84, h = Confocal focusing: ξ = 1, h = 0.8. Weak focusing: ξ << 1; tight focusing: ξ >> 1. Note that k 0. B = 0

9 Acceptance bandwidths The conversion efficiency will depend on the deviation from perfect phase matching. Three parameters will be important Wavelength Temperature Propagation angle η SHG sin c k L = k L sin k L A large deviation from phase matching will strongly reduce conversion efficiency. High power and/or strong thermal effects will alter the acceptance bandwidths. 1 M. M. Fejer et al, IEEE J. Quant. Electron. 8, 631, 199.

10 Acceptance bandwidths 10 mm long PPKTP FWHM = nm 809 nm FWHM = 1.1ºC FWHM = 0.6 nm 1064 nm FWHM = 4.7ºC

11 Single-pass SHG Good single-pass conversion efficiency is achieved using efficient nonlinear materials, optimal focusing and perfect phase matching. P ω = Pω tanh ηshgpω

12 External cavity SHG By enhancing the power using a resonant cavity, the SHG power and efficiency can increase dramatically. Assume R = 1 - L = 0.99 P circ = 100 P in, P SHG = P SHG,single-pass R P ω, circ = P ω, in (1 1 R(1 L)) Frequency locking is required to keep the laser at the cavity resonance or vice versa.

13 External cavity SHG Optimum coupling mirror depends on non-linearity and losses R opt = 1 Loss Loss 4 +η SHG Efficiency Γ vs. η SHG and losses Γ R input Loss η SHG η SHG = 0.01 %/W P = 1 W in ω P ΓP η in ω SHG 4(1 R input ) η SHG P in ω = 0 η SHG = 0.8 %/W P = 1 W

14 External cavity SHG Efficiency vs. η SHG in the crystal. η SHG = 0.01 %/W, 0.1 %/W and 0.8 %/W. Losses = %. Optimized coupling mirror. 0.8%/W 0.1%/W 0.01%/W 0.% 0.5% Efficiency vs. losses in the cavity. η SHG = 0.01 %/W. Losses = 0. %, 0.5 % and 1 %. R = 99 % coupling mirror. 1%

15 Experimental results SHG of single-mode diode lasers in nonlinear waveguides 488 nm 530 nm 1 A. Jechow et al, Opt. Lett. 3, 3035, 007. H. K. Nguyen et al, IEEE Phot. Technol. Lett. 18, 68, 006.

16 Experimental results SHG of broad area diode lasers 976 nm SHG to 488 nm Bulk PPLN Waveguide 1 A. Jechow et al, Appl. Phys. B, 89, 507, 007.

17 Experimental results SHG of tapered lasers single-pass f sf=3.1 f sf=3.1 f x=50 f=100 Grating BS PPKTP Second harmonic power (W) 0,08 0,04 0,00 0,016 0,01 0,008 0,004 Tapered amplifier Diagnostic beam 808 nm to 404 nm T= 38.8 o C η= 0.83%W -1 HWP 0,000 0,0 0,3 0,6 0,9 1, 1,5 1,8 Fundamental power (W) 976 nm to 488 nm 1 M. Chi et al, Opt. Express, 13, 10589, 005. M. Maiwald et al, Opt. Lett. 31, 80, 006.

18 Experimental results SHG of tapered lasers single-pass 106 nm DBR tapered laser (FBH)

19 Experimental results SHG of tapered lasers external cavity SHG nm to 404 nm Second harmonic power [mw] R. Le Targat et al, Opt. Com. 47, 471, 005. J. H. Lundeman et al, Opt. Express 16, 486, Circulating power [mw]

20 Experimental results 1 D. Georgiev et al, Opt. Express. 13, 677, 005. H. Furuya et al, Jap. J. Appl. Phys. 45, 6704, 006.

21 Experimental results 1088 nm to 544 nm 544 nm to 7 nm 1 P. Herskind et al, Opt. Lett., 3, 68, 007.

22 Experimental results Up to 18 W single-frequency green light at 53 nm.

23 Conclusions (I) Basic principles for frequency doubling described. Good beam quality, low spectral bandwidth and high fundamental laser power is mandatory for obtaining high frequency doubling efficiency. There exist an optimum focusing condition where the conversion efficiency is maximized. This optimum condition depends on both the laser and nonlinear crystal. The main parameters are Low/zero walk-off in the nonlinear crystal Perfect phase matching Low absorption Focusing optimized to crystal length Location of focus in the center of the nonlinear crystal

24 Conclusions (II) Single-pass frequency doubling is relatively simple to implement but the conversion efficiency is limited. External cavity frequency doubling puts more strict requirements on the laser parameters and the setup is more complicated. However, the conversion efficiency can be very high (> 80 %). High power diode lasers with good beam quality represent a strong candidate for future visible and UV laser systems based on frequency doubling.

25 Acknowledgements Peter E. Andersen, Jesper Holm Lundeman, Mingjun Chi, Birgitte Thestrup, Peter Jensen, Bjarne Sass and Christian Petersen All partners involved in the experiments.