AC


	© Stefan Marzenell




		Multiphoton interferometric autocorrelation (AC) techniques

We use photodiodes to measure intensity and interferometric autocorrelations at different wavelengths. The advantages of direct Two- (2PA) and Three- (3PA) Photon absorption in photodiodes are:

Only one element for 2PA or 3PA and its detection
No phasematching required
Coverage of large wavelength range
Online measurements


	We have used different photodiodes for each wavelength range. These are shown in the table below. The source and its possible wavelength can also be seen in the experimental setup of the OPO system.

Photodiode	2PA	Source	3PA	Source
GaAsP	680 - 1360 nm	Ti:Sa	1380 - 2040 nm	CTA
Si	1100 - 2100 nm	CTA	2200 - 3300 nm	Signal AgGaSe₂
InGaAs	1800 - 3500 nm	Signal AgGaSe₂	(3600 - 5400 nm)
InAs	3800 - 7600 nm	Idler AgGaSe₂	(7600 - 11400 nm)
SiC	400 - 840 nm	Ti:Sa	(840 - 1200 nm)


	We have made a comparison between 2PA (Si) and 3PA (GaAsP) measurements at a wavelength of 1.58 µm. We have measured the same pulse duration for each autocorrelation. A 3PA measurement with a sech²-shaped pulse of duration 95 fs is shown below. The upper and lower envelope in red and the intensity autocorrelation in blue are calculated with the cited pulse duration.




		To characterize to AgGaSe₂-OPO it was necessary to measure all three involved pulses (pump, signal, and idler) at the same time. It was of special interest to determine pulse duration and pulse chirp as a function of resonator length as this influences the power and therefore the self phase modulation as well as dispersion because the wavelength changes to stay resonant with the fixed pump repetition rate. Examples for signal and idler wave, respectively, are shown below.





		The measurement of the idler wave was performed with a peltier cooled InAs photodiode.

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