The raw materials: diffractive optical elements and single mode fibers from a materials perspective are all quite easy to manufacture. The primarily limitation with miniaturization is the single-mode fibers, which are limited by the optical wavelength you are using and the index of the fiber. For a conventional silica optical fiber, this is probably around ~100 nm diameter at a minimum. Newer materials can definitely change this 2-3x, but I'm not aware of anything more fundamental.

So in general this would be something that you would potentially be able to see in cars, but unlikely consumer electronics or handhelds without a modification in the operational principle (eg time-multiplexing to reduce the required number of fibers).

My personal opinion is that competing on low-power and small-scale is a lost cause for photonic computing. In terms of absolute energy efficiency and absolute miniaturization, photonics will never win. But at larger energy scales and larger systems, photonics can reach a regime where higher parallel throughput will dominate.