ABSTRACT
This chapter discusses how a pulse of light has a finite dimension. Destructive interference is the reason why the pulse intensity decays among its constituent waves. A transform-limited pulse occurs when all the wavelengths are coherent and interfere constructively at the peak of the pulse. Mathematicians use different approaches to approximate a function: one of the most common ones is the Taylor or power series. Interestingly, the second term of the expansion, in addition to being called dispersion, is also known as chirp. The reason for the name is that when the pulse has a large second-order spectral phase, the frequencies in the pulse disperse in time; therefore, the instantaneous spectrum of the pulse is changing in time. The average or carrier frequency of a positively chirped pulse sweeps from low to high; one can imagine a bird chirping. For a negatively chirped pulse, the carrier frequency sweeps from high to low.
