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1
Design and construction of a
mid-IR SPIDER apparatus
09/10/2012Malte Christian BrahmsImperial College London
09/10/2012
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Contents
• Purpose of SPIDER• Working principle• Design of mid-IR SPIDER• User’s manual• Outlook
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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)09/10/2012
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Purpose
• SPIDERs available commercially • Why build one?– Unusual wavelength in mid-IR– Specific variable wavelength SPIDER
needed for TOPAS
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Contents
• Purpose of SPIDER• Working principle• Design of mid-IR SPIDER• User’s manual• Outlook
09/10/2012
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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:
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Frequency Shear
Time delay
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Working principle
Interferogram:
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Working principle
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Working principle
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Contents
• Purpose of SPIDER• Working principle• Design of mid-IR SPIDER• User’s manual• Outlook
09/10/2012
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Design
• Two problems to solve:– Time delay– Frequency shear
• Solutions:– Delay: Split mirror• Allows variable delay
– Shear: SFG in BBO with chirped pulse
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Design – Time delay
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Split mirrors
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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.
1500nm09/10/2012
Stretched ancilla
Test pulse replicas
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Design – Frequency shearing
Chirping the ancillary pulseGrating Compressor:
09/10/2012Image: 2010 J. Phys. B: At. Mol. Opt. Phys. 43 103001 , p.5
Walmsley et al.: The role of Dispersion in ultrafast optics, Rev. Sci. Instrum., Vol. 27,
No.1, Jan 2001, p. 7
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Design – Recombination
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Sum-frequency generation• Focus delayed pulse-pair and chirped
ancillary pulse into BBO crystal
Image: 2010 J. Phys. B: At. Mol. Opt. Phys. 43 103001 p. 25
SHG + Original
SHG + Original
SFG
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Design – Constraints
• On delay τ: Spectrometer resolution and shear– Fringe spacing 2π/τ must be sufficiently large
τ cannot be too large– τ must 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
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T: Reconstruction window
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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
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Design - Constraints
• Resolution:
• Shear size:
• Lower limit:
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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
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Design – Setup
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Spectrometer
A1A2
A3 f=100mm
f=100mm
D-mirror
D-mirror
BS
(400-1100nm)
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Contents
• Purpose of SPIDER• Working principle• Design of mid-IR SPIDER• User’s manual• Outlook
09/10/2012
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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
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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
• τ determined by software
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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)
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User’s manual – Procedure
1. Take reading without ancillary pulse (calibration)– Block after beam splitter
2. Take SPIDER signal– Unblock ancillary, take trace
3. Analyse• OR: Use Tobi’s TwinSPIDER (live
operation)– Ask Tobi for more advice
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User’s Manual – Software
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Outlook
• This week or next: Use on TOPAS• Later: Test on few-cycle pulses• Intended for long-term use on TOPAS
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Thank you!
Questions?
09/10/2012