Physics Asked by David jorge Nordmann on April 16, 2021
I’ll explain my problem :
Given a Gaussian pulse with central wavelength 550 nm and a HMFW of 300 nm, and let’s assume the pulse is transform-limited. I need to get the pulse through 1mm of quartz.
I expect group delay dispersion (second order dispersion) to spread in time the wavelengths from each other making the red end arrive sooner then the green then the blue end.
I have correctly calculated the group velocity delay GVD, TOD (third order dispersion) and FOD (fourth order dispersion) for the quartz using the peak wavelength as the central wavelength for the Taylor expansion of the spectral phase.
Now my problem is that I’m having difficulty calculating the arrival time-shift with respect to the peak wavelength for GVD TOD and FOD of the quartz.
I’m able to calculate the time-broadening of the pulse but not the time-shift for each wavelength.
Can anybody explain to me how to calculate this? Thanks for the time.
PS: The reason I need to know the time delay for each wavelength it’s because I need to theoretically predict a FROG- frequency resolved optical gating.
The wikipedia article on Dispersion includes the following comment under high-order dispersion :
...The effects [of high order dispersion] can be computed via numerical evaluation of Fourier transforms of the waveform, via integration of higher-order slowly varying envelope approximations, by a split-step method (which can use the exact dispersion relation rather than a Taylor series), or by direct simulation of the full Maxwell's equations rather than an approximate envelope equation.
Your problem is discussed in section 2.2 of the article Effect of high order dispersion in slow-light propagation in photonic crystal waveguides. The difficulty is that dispersion is defined in the phase/frequency domain, whereas time delay is calculated in the space/time domain, so an accurate calculation requires the application of the Fourier Transform (equation 5 in this source, p 1664). The authors comment that :
When third order dispersion is included, the pulse envelope can be approximated using the Airy function [20]. This implies, that for a specific z, the pulses are deformed asymmetrically in time. The temporal asymmetry of the pulses can be a measure for the TOD in the photonic material.
When also higher-order dispersive term are included, Eq. (5) is no longer analytically solvable. Then, Eq. (5) needs to be solved numerically. In such a numerical solution, the full dispersion relation k(ω) is used as input. Hence the calculation is not limited by the accuracy of the Taylor expansion, since all dispersive orders are included, if present.
The effects of high-order dispersion on time delay are illustrated in figure 8. Even with the large effective refractive index of the photonic crystal waveguide ($n=frac{c}{v_g}=8.62$, bottom of page 1670) the effect on delay is very small. In your case $n$ is significantly smaller (about 1.46).
Quartz is not a highly non-linear material, and a 1mm length of crystal is not particularly long. Even with an ultrashort pulse (less than 10 fs) the effects of high-order dispersion will be incredibly small.
Unless your interest is theoretical, I suggest that inclusion of high-order dispersion in your calculation will have no noticeable impact on your predicted FROG trace, and will be swamped (no pun intended!) by the other approximations you make.
Answered by sammy gerbil on April 16, 2021
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