Finding steepest regions of data for bandwidth extraction

I have the following data with power versus wavelength, which is a filter response from my simulation.

I want to be able to extract the bandwidth of the region indicated in Figure 3 (zoomed in portion around 1.45 micron).

For that, I imagine that I will first need to isolate the region of highest upward and downward slopes, as in Figure 2. Here I am doing this by imposing an arbitrary range from the maximum y-value point (as shown). What would be the best way to do this dynamically?

I can then repeat the same process for the isolated dataset and apply the reverse to find the points below (selected manually here).

Is this the best approach to finding the bandwidth from the region of interest? Please advise!