ABSTRACT
A new implementation of heterodyning to measure spectral shifts of fiber Bragg gratings is described. A pair of closely matched phase-shifted gratings is used, and the separation of their transmission peaks is measured as a heterodyne beat frequency when illuminated with spectrally filtered incoherent light. This method can potentially offer high (<1 pm) resolution, fast (µs) measurement, and strain-temperature discrimination.
ABSTRACT
A new optical fiber head for Photonic Doppler Velocimetry (PDV) made from a combination of fiber types [multimode (MM) and single-mode (SM)] and lenses is described. The input laser beam is delivered by a SM fiber and imaged onto the target by simple optics, including an imaging lens centered inside a larger lens, whose role is to collect and image the back reflected light into the MM collection fiber. The large core of the MM fiber enhances the collection efficiency and also reduces its dependence on the target angle. Transmission through the MM fiber reduces the heterodyne fringe visibility considerably, but the Fourier analysis still enables very accurate resolution of the fringe frequency (and hence the velocity). The new PDV head with 20 GHz bandwidth was tested in a dynamical shock wave experiment to measure velocities of â¼3 km/s (â¼3.9 GHz), and the results agreed very well with measurements by a standard SM PDV.
ABSTRACT
We present results for the mechanical characterization of a bisphenol-A acrylate-based polymer optical fiber (POF) manufactured using a novel light polymerization spinning (LPS) process. The particular manufacturing process allows the development of POFs having unique mechanical characteristics, which result from an exceptionally low Young's modulus. The lower Young's modulus enables optical sensors for measuring stress or pressure with improved sensitivity and potentially a higher tunable mechanical range than conventional POFs. Moreover, properties such as the storage modulus variations with respect to the temperature and humidity were studied. Fiber Bragg gratings (FBGs), were inscribed in the POF using the plane-by-plane femtosecond laser, direct-write method for selective FBG mode excitation, and were characterized for changes to temperature, pressure, and relative humidity. The response of FBGs in this LPS-POF for all the three aforementioned measurands was several times higher than that measured for conventional POFs.
ABSTRACT
A novel, generic approach for fiber-optical sensing in rapidly rotating structures is presented. This approach does not require optical ingress via the central rotation axis. In this work, strain sensing at rotation speeds of up to 5000 rpm is demonstrated, and higher speeds should be possible. We demonstrate measurement of the rotation-induced strain in a rotating body at 500 rpm intervals up to 5000 rpm, and results agree very well with predictions.
ABSTRACT
The design of optical fiber based heads offering high accuracy and bandwidth for use in VISAR (velocity interferometer system for any reflector) experiments measuring ballistic velocities is described. A new, expanded, model for predicting the distance-dependent collection efficiency of the heads is presented. The model is shown to agree very well with experimental results, both within and outside the "depth of field". Various optical heads are demonstrated, to suit different experimental setups and conditions. Designs offering options for high bandwidths, accurate prealignment, and large stand-off distances are discussed. Results from a typical VISAR experiment are presented, verifying that our designs yield high-quality data.
ABSTRACT
We study the beam quality at the output of a multimode step-index optical fiber with a small number of modes following injection from a single-mode input fiber. It is shown that the output beam quality is optimized when the injecting fiber is offset from the center of the receiving core. Theoretical results demonstrating this phenomenon are in good agreement with an experimental demonstration. These results may have practical consequences in beam-coupling schemes and in applications emphasizing the importance of beam quality.
