Fiber-optic assay apparatus based on phase-shift interferometry

A kit, comprising:

• an optical assembly comprising:
• an optical fiber having a tip;
• an optical element removably attached to the tip of the optical fiber and configured for receiving a beam of light from the optical fiber,
• said optical element comprising a transparent material, a first reflecting surface, and a second reflecting surface, said second reflecting surface separated from the first reflecting surface by the transparent material, said first and second reflecting surfaces separated by at least 50 nm,
• wherein said first reflecting surface binds a layer of analyte binding molecules positioned so that interference between a beam of light reflected from the first reflecting surface into the optical fiber and a beam of light reflected from the second reflecting surface into the optical fiber varies as analyte binds to the layer of analyte binding molecules, and said second reflecting surface comprises a layer of material having an index of refraction greater than the refractive index of said transparent material;
• and
• instructions for binding said layer of analyte binding molecules to said first reflecting surface.