I have an optical fiber (1 meter long, 100 microns diameter) orthogonal to a glass coverslip, and the ability to control the position of this fiber in the vertical axis. A microscope objective sits below and is looks at the tip of the fiber through the coverslip. A laser focuses light through the fiber. On the fiber tip, I've attached a fairly delicate structure.

What I want to do is lower the fiber onto the surface of the coverslip (see attached drawing) and know exactly when contact has been made, or at least when the fiber is less than a few microns away. I've thought of several methods, but I'd appreciate comments on the feasibility of each method or alternative methods I could use. Ideally, the method would be simple and not require the use of tools that cost more than a few thousand dollars. I also have extensive microelectronics cleanroom experience so feel free to suggest methods that involve various coatings.

Some ideas include 1) Vibrating the coverslip and detecting vibration in the fiber 2) Coating the coverslip with something fluorescent, shining the laser through the fiber, and determining when fluorescent intensity is maximum 3) Measuring stress in the coverslip when the fiber touches it (might damage the structure) 4) Coating the fiber in something conductive, using conductive ITO coated coverslips, delivering a current through the fiber and measuring current in the cover slip (the structure at the tip is slightly conductive).