This is awesome! Presumably you can make this work with any interface that doesn't enforce the total internal reflectivity of a fiber optic cable, and therefore allows light to leak out. Instead of an air gap, have you tried experimenting with removing the cladding of the fiber optic cable, but keeping the core intact?
Alteratively, could you use a short segment of colored cladding that allows certain wavelengths to leak out more than others? I think that would allow you to encode each bend point as a different color-- which might require a different (more expensive) rx sensor, but could be useful for certain applications.
I did experiment with various ways of allowing light to escape but nothing came close to the properties of a total air gap. You can actually measure (relative) bend angle with it like a protractor since the attenuation is very linear!
There is already existing work that uses colored segments for something similar but those techniques are hard to do outside a well equipped lab.
Alteratively, could you use a short segment of colored cladding that allows certain wavelengths to leak out more than others? I think that would allow you to encode each bend point as a different color-- which might require a different (more expensive) rx sensor, but could be useful for certain applications.