Fiber-Optic Phase-Locked Loop Sensitive to Local Strain Only

Langley Research Center, Hampton, Virginia

The figure schematically depicts an apparatus for measuring strain at only one designated location on a structure. The apparatus is a fiber-optic phase-locked loop, wherein strain in a multimode optical fiber gives rise to a change in the phase of modulation of a laser beam that propagates along the fiber. The optical phase-locked loop makes it possible to perform sensing by use of a multimode optical fiber (in contradistinction to a single- or few-mode fiber as in some other fiber-optic sensor systems). Unlike in other fiber-optic-based sensor systems, the phase change in this system does not occur in response to strain integrated along the entire fiber-optic path. Instead, this system includes lead-in and lead-out strain-insensitive lengths of optical fiber connected to a short strain-sensitive length of optical fiber that is affixed to the structure at the desired measurement location. Strain in the short sensory length produces a phase change, but strain in the lead-in and lead-out portions does not.

The Frequency of the VCO in this optical phase-locked loop changes in response to strain in the short sensory length of optical fiber. Strain in the lead-in and lead-out lengths of fiber does not cause the frequency to change.

The output of a voltage-controlled oscillator (VCO) is used to modulate the laser light and to supply a reference signal to a double balanced mixer. After traveling along the strain-insensitive and strain-sensitive lengths of optical fiber, the modulated laser signal impinges on a photodetector, the output of which is amplified and mixed with the reference signal. A filter removes the radio-frequency component of the mixer output, passing only the DC or low-frequency component, which component constitutes a DC error voltage. The phases of the fiber-optic-propagated and reference signals are maintained at quadrature by feedback of the DC error voltage to the VCO. A change (caused by strain) in the phase of the modulation manifests itself as an error voltage and, by virtue of the feedback, is compensated by a change in the modulation frequency. The frequency is monitored by a counter.

A multimode optical fiber can be made more or less sensitive to strain through selection of the fiber core and cladding materials. Assuming that the fiber can be approximated as weakly guiding (meaning, essentially, that the index of refraction of the core exceeds that of the cladding by an amount «1), it can be shown that the condition for complete insensitivity to strain (zero phase shift in response to strain) is given by

n_core = (2/P_eff)^1/2,

where n_core is the index of refraction,

P_eff = [P₁₂-n_f(P₁₁-P₁₂)]/2,

n_f is the Poisson's ratio of the fiber, and P₁₁and P₁₂ are the strain-optic coefficients of the fiber.

The insertion of appropriate parameters in these equations leads to the conclusion that a strain-insensitive optical fiber is one in which the core has a very high index of refraction (4.5 is an approximate representative value). Germanium is one example of a material suitable for a multimode optical fiber with a very high index of refraction.

This work was done by Claudio O. Egalon and Robert S. Rogowski of Langley Research Center.

This invention has been patented by NASA (U.S. Patent No. 5,780,844). Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to