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Корреляционные методики измерения коротких импульсов терагерцового излучения Alexej Semenov German Aerospace Center. Outline. Коррелляция и автокорреляция Нелинейность и интерференция в автокорреляционных измерениях электромагнитных полей - электрическое поле - интенсивность

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  1. Корреляционные методики измерения коротких импульсов терагерцового излучения Alexej SemenovGerman Aerospace Center

  2. Outline • Коррелляция и автокорреляция • Нелинейность и интерференция в автокорреляционных измерениях электромагнитных полей - электрическое поле - интенсивность - Crosstalk • Получение коротких терагерцовых импульсов • Результаты измерений

  3. Fluorescent Correlation Spectroscopy Magde, D., Elson, E., and Webb, W.W. (1972) Phys. Rev. Lett. 29, 705

  4. Autocorrelation function Wz Wx,y Fluorescent Correlation Spectroscopy N – average number of the molecules in the focal volume D – diffusion coefficient

  5. Diffusion coefficient Fluorescent Correlation Spectroscopy

  6. Different light - time correlation of photons Thermal sources, gas discharge (natural light) - bunched photons (Bose statistics, strong fluctuation) Lasers (coherent light) - random photons (Poisson distribution, low fluctuation) Single photon sources (fluorescence, quantum dot) - anti-bunched photons

  7. Correlation function with a single photon detector Time correlation of photons Tkoh is the measure for the degree of coherence in thermal light sources C. Zinoni et al., APL 2007

  8. Hanbury-Brown/Twiss-Experiment Finite response time and/or dead time of a single photon detector brought up the HBT method Time correlation of photons

  9. Outline Коррелляция и автокорреляция Нелинейность и интерференция в автокорреляционных измерениях электромагнитных полей - электрическое поле - интенсивность - Crosstalk Получение коротких терагерцовых импульсов Результаты измерений Folie 9

  10. Femtosecond pulse lasers How to measure the pulse duration? Autocorrelator Interferometric autocorrelation SHD – second harmonic generator (non-linear optical crystal) D – any slow detector

  11. Interferometric autocorrelation Interferometric autocorrelation Two ultra-short pulses (a) and (b) with their respective interferometric autocorrelation (c) and (d). Because of the phase present in pulse (b) due to an instantaneous frequency sweep (chirp), the fringes of the autocorrelation trace (d) wash out in the wings. Note the ratio 8:1 (peak to the wings), characteristic of interferometric autocorrelation traces.

  12. P Fast optical detectors How to measure the response time of the detector? Use the nonlinearity V(P) of the detector response and do not forget to eliminate interference Interferometer V L – femtosecond pulse laser P – polarizer V – slow voltmeter D –detector under study A. Semenov et al., JLTP 1996 Folie 12

  13. P Intensity autocorrelation YBCO superconducting detector and Ti-Sapphire laser Interferometer V P. Probst et al., PRB <2012> Folie 13

  14. Intensity autocorrelation Intensity autocorrelation Two ultra-short pulses (a) and (b) with their respective intensity autocorrelation (c) and (d). Because the intensity autocorrelation ignores the temporal phase of pulse (b) that is due to the instantaneous frequency sweep (chirp), both pulses yield the same intensity autocorrelation. Here, identical Gaussian temporal profiles have been used, resulting in an intensity autocorrelation width twice as long as the original intensities. Note that an intensity autocorrelation has a background that is ideally half as big as the actual signal. The zero in this figure has been shifted to omit this background Folie 14

  15. Fast optical detectors Use the mutual current drain of two identical detectors How to measure the linear response time? L. Shi et al., APL 1992 Crosstalk correlation

  16. Crosstalk correlation Crosstalk correlation

  17. Outline Коррелляция и автокорреляция Нелинейность и интерференция в автокорреляционных измерениях электромагнитных полей - электрическое поле - интенсивность - Crosstalk Получение коротких терагерцовых импульсов Результаты измерений Folie 17

  18. THz Synchrotron Radiation Synchrotron radiation

  19. Signal appearance Bending magnet J. Feikes et al., PR ST AB 2011

  20. Synchrotron radiation Typical values

  21. Coherent synchrotron radiation

  22. reference orbit:L = 240 m longitudinal bunch length longitudinal bunch length intensity vs. number of electrons intensity vs. number of electrons hn hn normal user optics sz > 5 mm Dt > 35 ps a= 7·10-3 sz > l sz > l v  c DL bunch, Dp hn hn low alpha optics sz 1 mm Dt < 7 ps a 10-4 szl szl Coherent THz Radiation from a Synchrotron momentum compaction factor: Dp/p a = DL/L a fs2 10 ps Single electron 1 ps window THz -pulse

  23. MLS data sheet Synchrotron

  24. Outline Коррелляция и автокорреляция Нелинейность и интерференция в автокорреляционных измерениях электромагнитных полей - электрическое поле - интенсивность - Crosstalk Получение коротких терагерцовых импульсов Результаты измерений Folie 24

  25. Problems • Radiation pulses in the range 0.1 – 1 THz • Pulse duration 10 – 20 ps • Available detectors Slow – semiconductor bolometers (linear) Fast – superconducting electron bolometers (linear) Fast – superlattice detector (non-linear) • Beam size a few millimeters & detector size a few micrometers

  26. Antennensimulation Au-Antenne (100nm) auf Saphir S11=-18 dB bei f = 0,95 THz

  27. Antennen + Filter Layout

  28. Gesamtstruktur

  29. Antennen + Filter S-Parameter im THz-Bereich Signal wird gut inAntenne eingekoppeltund nur wenig reflektiert • S11 = S22 = -43 dB bei 0,95 THz • S21 = S12 = -32 dB sowie S31 = S32 = -24 dB bei 0,95 THz

  30. Martin-Puplett Interferometer Input 1 Input 2 Output

  31. Typical autocorrelation signal Beam parameter: 629 MeV, 480 kV, 7.05 kHz, 100mA beam current Streak camera: t (FWHM) = 26ps Detector signals seem to overlap over the whole scan length Negative autocorrelation signal Neither the peak at 0 nor the whole response corresponds with the streak camera measurements Period of about 20 ps Peak at zero shorter than the other peaks Combination from crosstalk correlation and field correlation

  32. Field detector

  33. Field autocorrelation Two ultra-short pulses (a) and (b) with their respective field autocorrelation (c) and (d). Note that the autocorrelations are symmetric and peak at zero delay. Note also that unlike pulse (a), pulse (b) exhibits an instantaneous frequency sweep, called chirp, and therefore contains more bandwidth than pulse (a). Therefore, the field autocorrelation (d) is shorter than (c), because the spectrum is the Fourier transform of the field autocorrelation (Wiener-Khinchin theorem).

  34. Autocorrelation with superlattice detector S. Winnerl et al., APL 1998 Combination from field and intensity correlation

  35. Thank you

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