Design and construction of a mid-IR SPIDER apparatus

1. Design and construction of a mid-IR SPIDER apparatus 09/10/2012 Malte Christian Brahms Imperial College London

2. Contents • Purpose of SPIDER • Working principle • Design of mid-IR SPIDER • User’s manual • Outlook

3. Purpose • Pulse characterised completely by • Field as fct. of time or • Spectral amplitude and phase • Oscillations in on the order of fs • Temporal resolution required too high • Instead: Measure spectral amplitude and phase: Spectral Phase Interferometry for Direct Electric Field Reconstruction (SPIDER)

4. Purpose • SPIDERs available commercially • Why build one? • Unusual wavelength in mid-IR • Specific variable wavelength SPIDER needed for TOPAS

5. Contents • Purpose of SPIDER • Working principle • Design of mid-IR SPIDER • User’s manual • Outlook

6. Working principle • Based on two replicas of the test pulse • Displaced relative to each other • In time: Delay τ • In frequency: Shear Ω • In frequency domain: Time delay Frequency Shear

7. Working principle Interferogram:

8. Working principle

9. Working principle

10. Contents • Purpose of SPIDER • Working principle • Design of mid-IR SPIDER • User’s manual • Outlook

11. Design • Two problems to solve: • Time delay • Frequency shear • Solutions: • Delay: Split mirror • Allows variable delay • Shear: SFG in BBO with chirped pulse

12. Design – Time delay Split mirrors

13. Design – Frequency shearing • Use SFG with chirped ancillary pulse replica • Chirp: Carrier frequency depends linearly on time: • SFG with same original frequency at two different times  different output frequencies: • Obvious solution: Piece of glass • But: Most glass almost dispersion-free at ca. 1500nm Stretched ancilla Test pulse replicas

14. Design – Frequency shearing Chirping the ancillary pulse Grating Compressor: Walmsley et al.: The role of Dispersion in ultrafast optics, Rev. Sci. Instrum., Vol. 27, No.1, Jan 2001, p. 7 Image: 2010 J. Phys. B: At. Mol. Opt. Phys. 43 103001 , p.5

15. Design – Recombination Sum-frequency generation • Focus delayed pulse-pair and chirped ancillary pulse into BBO crystal SHG + Original SFG SHG + Original Image: 2010 J. Phys. B: At. Mol. Opt. Phys. 43 103001 p. 25

16. Design – Constraints • On delay τ:Spectrometer resolution and shear • Fringe spacing 2π/τ must be sufficiently large τcannot be too large • τmust be sufficiently large resolve peaks in FT • On shear Ω: • Small enough to satisfy the sampling theorem: • Satisfied in practice, usual: Ω=10% of bandwidth T: Reconstruction window

17. Design – Constraints • On chirp : • Only SFG with monochromatic field conserves shape of spectrum: • In time domain: • In frequency domain (if Eanc is monochromatic): • Need slow-varying, quasi-monochromatic frequency lower limit on

18. Design - Constraints • Resolution: • Shear size: • Lower limit:

19. Design – Constraints • With numbers: At 1300-2000nm for a ca. 40fs pulse • Both the constraints and the value are wavelength-dependent! • Chirp determined by compressor angle and separation • Can vary angle easily, use mostly • If necessary, enough space to move one grating

20. Design – Setup D-mirror A2 BS A1 D-mirror f=100mm Spectrometer A3 f=100mm (400-1100nm)

21. Contents • Purpose of SPIDER • Working principle • Design of mid-IR SPIDER • User’s manual • Outlook

22. User’s Manual – Practical issues • Calibration by simply adding ignores frequency-dependent behaviour (noise) in apparatus • Need to calibrate for this • Use signal without shear instead: • Depending on wavelength and spectrometer: • Measure calibration either in SHG (blue) or original (red) • SPIDER in SFG • For this SPIDER at 1300-2000nm: • Measure calibration and SPIDER signal in blue, first order

23. User’s Manual – Practical issues • Spectral amplitude: • Determined from calibration signal • Take magnitude instead of phase angle: • Determining shear Ω: • Measure compressor parameters or block one of sheared pair (spatially) • In practice: Use compressor parameters • τdetermined by software

24. User’s manual – Procedure • Good signal: • High fringe contrast • Well separated fringes • Near saturation • Needed for this: • Phase-matching and crystal position • Spatial overlap (focus on crystal) • Temporal overlap (compressor translation) • Right amount of delay • SHG beams blocked (use aperture A3)

25. User’s manual – Procedure • Take reading without ancillary pulse (calibration) • Block after beam splitter • Take SPIDER signal • Unblock ancillary, take trace • Analyse • OR: Use Tobi’s TwinSPIDER (live operation) • Ask Tobi for more advice

26. User’s Manual – Software

27. Outlook • This week or next: Use on TOPAS • Later: Test on few-cycle pulses • Intended for long-term use on TOPAS

28. Thank you! Questions?