Optical frequency domain reflectometry (OFDR)

Optical frequency domain reflectometry measures where light is reflected by using frequency rather than time. A tunable laser sweeps its frequency linearly at a rate $\gamma$ (in Hz/s), and its output is split between the device under test and a reference path. Light reflected from a point at distance $z$ returns after a delay $2nz/c$, during which the laser has moved on in frequency, so when it is mixed with the reference it produces a beat at $f_b = 2 n z \gamma/c$. Each reflection along the device appears as a beat frequency proportional to its distance, and a Fourier transform of the recorded heterodyne signal turns the frequencies into a map of reflectivity against position. The same principle is used by swept-source OCT and FMCW lidar.

Resolution is set by the sweep span, not by a pulse width. Two reflections can be told apart when they are separated by $\Delta z = c/(2n\,\Delta F)$ for a total sweep $\Delta F$: a 1 THz sweep, about 8.0 nm at 1550 nm, resolves 0.10 mm in fiber with a group index of 1.468. That is four orders of magnitude finer than typical OTDR resolution, which is limited by the pulse duration. The maximum range is set by the laser's coherence length and by how finely the detector signal is sampled, so OFDR typically covers meters to tens of meters, occasionally more with narrow-linewidth lasers. Sweep nonlinearity would blur the result, so instruments record an auxiliary interferometer alongside the measurement and resample the data at equal frequency steps. Coherent detection also gives high sensitivity: OFDR can see the Rayleigh backscatter of ordinary fiber, far below the level of a connector reflection.

The high resolution suits short, complex paths. In photonic integrated circuits and component assemblies, OFDR locates reflections at facets, splices, connectors and waveguide defects, and measures loss along a waveguide from the slope of its backscatter. The Rayleigh backscatter pattern of a fiber is a fixed random fingerprint; comparing it with a reference measurement reveals local shifts caused by strain or temperature, which gives distributed sensing with millimeter resolution over tens of meters, used for structural and aerospace testing. Polarization-resolved OFDR also measures birefringence and polarization-mode dispersion along a device.

References: W. Eickhoff, R. Ulrich, Appl. Phys. Lett. 39, 693 (1981); B. J. Soller, D. K. Gifford, M. S. Wolfe, M. E. Froggatt, Opt. Express 13, 666 (2005).