Transportation of Megawatt Millijoule Laser Pulses via Optical Fibers?

Transcription

1 Transportation of Megawatt Millijoule Pulses via Optical Fibers? J. Tauer, H. Kofler and E. Wintner Vienna University of Technology, Photonics Institute

2 Contents of laser Transportation of mj-ns-pulses via Conventional step-index fibers (SIF) Photonic band gap fibers (PBG) Hollow dielectric capillaries (HDC)

3 Ignition I The conventional spark plug should be replaced by a pulsed laser. I thr ~ W/cm² Typically ~100 µj@1ns MPE ~ 8-12 mj Pulse characteristics Energy ~ mj Length ~ 1 ns Depends on -temperature - pressure - fuel air eq.ratio

4 Ignition II The laser pulse is focused into the combustion chamber and ignites the mixture.

5 Ignition III Why do we want to replace the well known and established spark plug? Conventional spark reach its physical borders. enables: - Higher pressure higher engine efficiency - Combustion of leaner mixtures higher efficiency and lower NO X emissions - Enhanced lifetime due to the absence of any electrodes - Free positioning of the focal spot no quenching effects

6 of laser Concept 1 Concept 2 Concept 3: Advantages: oscillator is mounted remoted of the engine in order to reduce parasitic influences like temperature or vibration. The use of a multiplexer reduces the number of required laser oscillators.

7 Transportation of mj-ns-pules over Different fibers in use: (i) Conventional step-index fibers (SIF) (ii) Photonic band gap fibers (PBG fibers) (iii) Hollow dielectric capillaries (HDC)

8 Transportation of mj-ns-pules over Step-index fibers Stuat et.al., Phys.Rev.Let. 74 (12), 1995 Pulse characteristics: 10 1 ns Peak power > 10 MW Threshold intensity I thr ~5 GW/cm² required core diameter > 600 µm Problem: Focusing into the combustion chamber beam profile destroyed Damage threshold depends mostly on pulse duration: Φ τ thr α p I thr (1 ns) ~ 5 GW/cm² I thr (10 ns) ~ 2 GW/cm² Ops: fabrication, spot size, etc.!!

9 Transportation of mj-ns-pules via Step-index fibers Some achieved results: Pulse energy Pulse duration Core diameter Intensity Beam profile Reference 110 mj 10 ns 1500 µm 0.62 GW/cm² Multimode Schmidt-Uhlig (2001) 8 mj 8 ns 600 µm 0.35 GW/cm² Multimode Stankhiv, Wintner, et.al. (2004) 3 mj 15 mj 45 mj 6 ns 200 µm 400 µm 940 µm 1.60 GW/cm² 1.99 GW/cm² 1.09 GW/cm² Multimode El-Rabil (2007) No plasma formation after focussing recorded! Interesting Alternative: Bundle of fibers, Yilmaz et. Al. Optics Letters, 45 (27), Capillarities (700 µm), total diameter 5.4 mm Transmitted pulse energy ~ 200 mj Questions: bending, incouple loss, beam profile, etc. Resüme: SIF canbeusedfor pulse propagation, but not for iniciate an!

10 Transportation of mj-ns-pulses via of laser Transportation of mj-ns-pulses via Conventional step-index fibers (SIF) Photonic band gap fibers (PBG) Hollow dielectric capillaries (HDC)

11 Transportation of mj-ns-pulses via Problem: Damage of the fiber core at high intensities hollow core fibers Possible solutions: (i) Photonic Band Gap fiber (PBG fiber) (ii) Hollow dielectric capillarity (HDC) Source: Pittsburgh Supercomputing Center Source: Orban, Kofler, Tauer, Wintner, Vienna (2006)

12 Transportation of mj-ns-pulses via Photonic Band Gap fibers Cregan, et.al., Science, Vol. 285(3), 1999 Internal reflection on the 2- dimensional structure Advantage: very good beam quality of the transmitted laser beam

13 Transportation of mj-ns-pulses via Photonic Band Gap fibers Some achieved results: Transmitted pulse energy Pulse duration Coupling efficiency Fiber Reference 0.75 mj 10 ns > 82 % 15 µm Tauer, Orban et.al. Las.Phys.Lett. 4(6), mj 65 ns? 8.2 µm Shepard et.al., Opt.Exp. 12(4), mj 30 ns 90 % 22 µm Michaille et. al., SPIE 5618, 2004 Chronologic Vienna University of Technology: 19 cell 1060nm HC PBG fiber, M² fiber out, length PBG fibre 8 mm 2003: 0.60 coupling efficiency 14% [coupling 532 nm 30%] 2006: 0.75 coupling efficiency 82% J.Tauer, F.Orban, H. Kofler, J. Tauer,A.B. Fedotov, I.V.Fedotov, V.P. Mitrokhin,A.M. Zheltikov and E. Wintner High-throughput of single high-power laser pulses by hollow photonic band gap fibers, Physics Letters, 4, No. 6, (2007)

14 Transportation of mj-ns-pulses via Photonic Band Gap fibers Resüme: Better propagation properties than SIF, but damage limit the transmitted energy; high costs;

15 Transportation of mj-ns-pulses via of laser Transportation of mj-ns-pulses via Conventional step-index fibers (SIF) Photonic band gap fibers (PBG) Hollow dielectric capillaries (HDC)

16 Transportation of mj-ns-pulses via Hollow dielectric capillary One step backward: 2-dimensional cladding-structure 1-dimensional structure Propagation is based on internal reflection Fresnel equations 1-dimensional structure (> 2 layers)

17 Transportation of mj-ns-pulses via Hollow dielectric capillarity The 1 st theoretical model assumes that the whole energy of the laser beam is on a cone which propagates over the fiber length by partial reflection. The transmission T writes as E ψ T r OUT = = EIN where r is the coefficient of reflection (Fresnel coefficient) and Ψ the number of reflections. Assuming only one cladding (simplest model) layer one recieves from geometrical aspects ψ = a l tanα Here, l denotes the fiber length, 2a the fiber diamater and α the incoupling angle.

18 Transportation of mj-ns-pulses via Hollow dielectric capillary Some results from the simulation (simplest model)

19 Transportation of mj-ns-pulses via Hollow dielectric capillary Comparison between theory and experiment T(theory) < T(experiment)? The energy distribution of the laser beam is e.g. Gaussian and not only a simple light cone.

20 Transportation of mj-ns-pulses via Hollow dielectric capillary Since no total internal reflection occurs, the radial losses are rather high. How can these radial losses be reduced? many layers; Coefficient of reflection: Ops! r(single layer) ~ 96% r(multi layer) Colorado State University (Willson, Yalin et.al.)

21 Transportation of mj-ns-pulses via Hollow dielectric capillary Résumé: (i) Fiber transmissions up to 80% CSU using multilayer HDC; dominating loss mechanism: radial losses, incoupling losses can be neglected; (ii) Transmission decreases with the length (for both single- and multilayer HDC) (iii) Fiber transmission dramatically reduced when the fiber is bent; (iv) Optical damage might occurs not on endfaces, but near to the start of the first bent; (v) Manufacturing is still a complex process and therefore the cost remain high compared to conventional fibers;

22 Transportation of mj-ns-pulses via of laser Transportation of mj-ns-pulses via Conventional step-index fibers (SIF) Photonic band gap fibers (PBG) Hollow dielectric capillaries (HDC)

23 : Transportation of mj-ns-pulses over The use of fibers for pulse propagation is very limited: -SIF: bad beam profile, damage at high intensities; -PBG fiber: good beam profile, but also damage at high intensities -HDC: beam quality is ok; Transportation of high energy pulses is possible; Fiber bending cause a drastic reduction of transmission bad overall-efficiency, but still the best candidate for laser!! Stöchiometric mixtures might be ignited with pulse energies around 1 1ns and for this prupose pulse transportation via any (SIF, PB and HDC) would be applicabe but our issues focus on lean mixture engines where pulse energies in the order of 10 mj are required.

24 Thank you to the attention! Contact: Johannes Tauer Gußhausstr Vienna Phone: +43/1/58801/ Mail: Web:

25 Appendix Vision of laser

26 Appendix Breakdown voltage and efficiency versus pressure

27 Appendix Minimum pulse energy versus pressure

28 Appendix Emissions

29 Appendix Damage PBF fiber

30 Appendix Advanced scheme HDC

31 Appendix Radial CSU

32 Appendix 2d vs. 1d cladding structure