Search

A femtosecond optical parametric oscillator (OPO) has been used to characterize the transmission of a number of different all-silica photonic bandgap (PBG) fibers specially developed for gas sensing in the […]

We demonstrate methane sensing based on Fourier transform infrared spectroscopy using a hollow-core photonic bandgap fiber guiding in the mid-infrared and idler pulses from a femtosecond optical parametric oscillator. Transmission measurements are presented for several fibers, and sensing is demonstrated using a fiber whose bandgap overlaps the methane fundamental absorption lines. The gas filling process of the air core is described, and qualitative methane concentrations measurements to 1000ppm (parts in [10.sup.6]) are reported. Operation down to 50 ppm based on our current experiment is predicted. OCIS codes: 060.2370, 060.2390, 060.5295, 300.1030, 300.6340.

Experimental modal analysis of multifrequency vibration requires a measurement system with appropriate temporal and spatial resolution to recover the mode shapes. To fully understand the vibration it is necessary to be able to measure not only the vibration amplitude but also the vibration phase. We describe a multipoint laser vibrometer that is capable of high spatial and temporal resolution with simultaneous measurement of 256 points along a line at up to 80 kHz. The multipoint vibrometer is demonstrated by recovering modal vibration data from a simple test object subject to transient excitation. A practical application is presented in which the vibrometer is used to measure vibration on a squealing rotating disk brake. OCIS codes: 120.3930, 120.7280, 120.6160.

We describe the use of arrayed waveguide gratings (AWGs) in the interrogation of fiber Bragg gratings (FBGs) for dynamic strain measurement. The ratiometric AWG output was calibrated in a static deflection experiment over a [+ or -]200 [micro][epsilon] range. Dynamic strain measurement was demonstrated with a FBG in a conventional single-mode fiber mounted on the surface of a vibrating cantilever and on a piezoelectric actuator, giving a resolution of 0.5 [micro][epsilon] at 2.4 kHz. We present results of this technique extended to measure the dynamic differential strain between two FBG pairs within a multicore fiber. An arbitrary cantilever oscillation of the multicore fiber was determined from curvature measurements in two orthogonal axes at 1125 Hz with a resolution of 0.05 [m.sup.-1]. OCIS codes: 060.2370, 230.1480, 120.7280.

Cavities have been laser ablated in the ends of single-mode optical fibers and sealed by aluminized polycarbonate diaphragms to produce Fabry-Perot pressure sensors. Both conventional fibers and novel, multicore fibers were used, demonstrating the possibility of producing compact arrays of sensors and multiple sensors on an individual fiber 125 [micro]m in diameter. This high spatial resolution can be combined with high temporal resolution by simultaneously interrogating the sensors by using separate laser sources at three wavelengths. Shock tube tests showed a sensor response time of 3 [micro]s to a step increase in pressure. OCIS codes: 060.2370, 120.2230, 140.3390.

We demonstrate the sensitivity of Bragg gratings in a nmlticore fiber to transverse load. The Bragg peaks are split because of stress-induced birefringence, the magnitude of which depends upon the load and grating position relative to the load axis. Experiments show that a set of gratings in a four-core fiber can measure a load axis angle to [+ or -] 5[degrees] and a load magnitude to [+ or -] 15 N [m.sup.-1] up to 2500 N [m.sup.-1]. We consider alternative designs of multicore fiber for optimal load sensing and compare experimental and modeled data. OCIS codes: 060.2370, 260.1440, 230.1480.

The curvature- or bend-sensing response of long-period gratings (LPGs) UV inscribed in D-shaped fiber has been investigated experimentally. Strong fiber-orientation dependence of the spectral response when such LPGs are subjected to bending at different directions has been observed and is shown to form the basis for a new class of single-device sensor with vector-sensing capability. Potential applications utilizing the linear response and unique bend-orientation characteristics of the devices are discussed. OCIS codes: 060.2310, 060.2370.

We describe the characterization of the temperature and strain responses of fiber Bragg grating sensors by use of an interferometric interrogation technique to provide an absolute measurement of the grating wavelength. The fiber Bragg grating temperature response was found to be nonlinear over the temperature range -70 [degrees]C to 80 [degrees]C. The nonlinearity was observed to be a quadratic function of temperature, arising from the linear dependence on temperature of the thermo-optic coefficient of silica glass over this range, and is in good agreement with a theoretical model. OCIS codes: 060.0060, 060.2300, 060.2310, 060.2370, 060.2400, 230.1480.