John Gowar is a Professor Emeritus of Electronic Engineering at the University of Southampton, UK. He has extensive experience in the field of optical communications and has published numerous papers and books on the subject.
If you prefer a hard copy for your shelf, used versions are often quite affordable:
: Operation and limitations of light sources, modulators, and detectors.
: Examination of electromagnetic wave propagation in step-index and graded-index fibers, as well as single-mode fibers. Components and Hardware
Unfortunately, I couldn't find a free PDF download of "Optical Communication Systems" by John Gowar that is legally available. The book is a copyrighted material, and downloading it without permission may infringe on the author's and publisher's rights.
However, before downloading the PDF version, ensure that you are doing so from a legitimate source. Some websites may offer pirated copies of the book, which can be illegal and potentially harm your computer.