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Design and construction of a

mid-IR SPIDER apparatus

09/10/2012Malte Christian BrahmsImperial College London

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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

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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

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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

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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?

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