I did a quick search and found that photonics research at universities can be in a variety of departments from physics to applied physics to chemistry to engineering to subdepartments of engineering. If the photonics programs you're applying to are part of a physics department, you may have to take the PGRE anyway. If it's part of another department, you won't have to. But you can choose to take it anyway. There is a test date in April. If you get a good score, it could help your application, assuming the profs of that department know what a good PGRE score is. If you get a lousy score, you can just opt not to submit the score with your application.
Whether it's worth the time and effort to take it if you don't need to, I can't say. I really don't know if you can use the PGRE as the "main" basis for acceptance, since even for physics departments it's not the main part of the application. But if there's a chance you might have to take the PGRE anyway for some departments you're applying to, it might be worthwhile to take it.