If you cannot find the PDF legally, or you want a modern supplement, consider these industry standards:
That said, Keiser and Agrawal are harder. Gowar remains the friendly introduction. optical communication system by john gowar pdf
Gowar’s original edition (circa 1993) does not cover coherent optical transmission or the explosion of DWDM (Dense Wavelength Division Multiplexing). Use his foundation to understand modern texts. Once you understand Gowar’s explanation of a Fabry-Perot laser, moving to a DFB (Distributed Feedback) laser is easy. If you cannot find the PDF legally, or
Gowar is famous for his hand-drawn style figures. They explain dispersion and modal cut-off better than paragraphs of text. Redraw them in your notebook. That said, Keiser and Agrawal are harder