The total rise time or rise time budget is root – sum – square rise time of each contributor to the pulse rise time. It is defined by the equations in the picture attached.
Ttx is the transmitter rise time and can be calculated by measuring the rise time of the transmitter using a visualizer like in the image attached
Tmat is = group velocity dispersion rise time , tmat=D*L*gx where D is dispersion of optical link, l is length of fibre, Gx is half power spectral width of light source. All these variable are dependant on properties of the fibre and light source
Tmod is modal dispesion rise time given by tmod=440/Bm , where Bm =bandwidth of optical link (note there is no modal effect in single mode fiber)
Trx is receiver rise time which is defined by Trx=350/Brx
For more information on the equations and variables used refer to this link https://www.ques10.com/p/29974/what-is-rise-time-budget-analysis-derive-an-expres/