I got my master's degree in Mechanical Engineering, but my research was on a measuring technique called LIBS (short for Laser-Induced Breakdown Spectroscopy). I assume that the photonics engineering field is similar. I had to learn a lot of optics and instrumentation along with the base load of ME classes. The photonics classes were interesting, but I did most of my calculations from the formulas in the vendor catalogs for my specific applications. A combination of Photonics/Mechanical can be utilized to detect mechanical measurements such as stresses, strains, etc. via optical means. Examples include: using Raman Spectroscopy to determine residual stresses, using inteferometry to determine internal stresses and thermal stresses, using Atomic Emission Spectroscopy to determine elemental composition and properties such as HHV and LHV of fuels.
... Sorry, I'm rambling. Anyways, the reason why they only hire PhD's is because most of the interest in Photonics is in either research or academia. The ones that graduate with their Master's, in my experience, have a choice to either endenture themselves to academia and research, or jump ship.