RESUMEN
A two-port polarization-independent tunable spectral slicing filter at the 1530 nm wavelength regime is presented. The design utilizes an asymmetric interferometer with a sparse index grating along its arms. The sparse grating makes it possible to select equally spaced frequency channels from an incident WDM signal and to place nulls between them to coincide with the signal comb frequency. The number of selected channels and nulls between them depends on the number of coupling regions used in the sparse grating. The free spectral range depends on the spacing between the coupling regions. The Z-transform method is used to synthesize the filter and determine the spectral response. The operation of a device with six coupling regions is demonstrated, and good agreement with theoretical predictions is obtained. A 3 dB bandwidth of â¼1 nm and thermal tuning over a range of â¼13 nm are measured.
RESUMEN
Field tests in desert terrain of a distributed sensor system for detecting and locating intruders based on the phase-sensitive optical-time-domain reflectometer (phi-OTDR) are described. The sensing element is a single-mode telecommunications fiber in a 4.5 mm diameter cable buried in a trench filled with loose sand. Light pulses from a continuous-wave Er:fiber Fabry-Perot laser with a narrow (<3 kHz) instantaneous linewidth and low (few kilohertz per second) frequency drift are injected into one end of the fiber, and the orthogonal polarizations of the backscattered light are monitored with separate receivers. Localized phase changes in the optical carrier are sensed by subtracting a phi-OTDR trace from an earlier stored trace. High sensitivity and consistent detection of intruders on foot and of vehicles traveling down a road near the cable line was realized over a cable length of 8.5 km and a total fiber path of 19 km in real time.
RESUMEN
An optical binary switch for aircraft applications is demonstrated. A fiber Fabry-Perot interferometer (FFPI) bonded to a cantilever is used as the sensing element. A white-light interferometry system with two bulk Michelson interferometers sharing the same motor-driven translation stage is utilized to monitor the elongation of the FFPI. The system exhibits excellent linearity as a force sensor; the experimental results are in good agreement with theoretical calculated values. With a properly set threshold value, the system produces a binary output.
RESUMEN
Bragg grating reflectors etched in amorphous silicon overlay films have been integrated with Ti:LiNbO3 optical waveguides to obtain a narrow (0.05 nm) reflectance spectrum with a > 20 dB dip in the transmittance spectrum. These results were realized at a wavelength of 1542.7 nm for TE polarization on an x-cut, y-propagating substrate with gratings etched to a depth of approximately 93 nm in a 105 nm thick silicon film over a length of 12.5 mm. The reflectance in the channel waveguides is found to be strongly dependent on the depth of the etched grating. The effect of the Bragg waveguide loss factor on the transmittance and reflectance spectra is investigated by using a model for contradirectional coupling that includes an attenuation coefficient. The values for coupling constants kappa and amplitude attenuation constants alpha of samples etched for different time durations to control the grating depths are obtained from the model through the use of the depth of the dips in the transmittance spectra and the spectral widths of the reflectance peaks. It is concluded that the corrugated Si overlay film increases the insertion loss by approximately 2.7 dB, and the loss is not significantly affected by the grating depth.
RESUMEN
A distributed sensor system for detecting and locating intruders based on a phase-sensitive optical time-domain reflectometer (phi-OTDR) that utilizes polarization discrimination is described. The sensing element is a single-mode telecommunications fiber in a 3 mm diameter cable buried along a monitored perimeter in a 20-46 cm deep, 10 cm wide trench in clay soil. Light pulses from a continuous-wave Er fiber Fabry-Perot laser with a narrow (< 3 kHz) instantaneous linewidth and low (a few Kilohertz per second) frequency drift are injected into one end of the fiber, and the orthogonal polarizations of the backscattered light are monitored with separate receivers. Localized phase changes in the optical carrier are sensed by subtraction of a phi-OTDR trace from an earlier stored trace. In field tests with a monitored length of 12 km, detection of intruders on foot as far as 4.5 m from the cable line was consistently achieved.
RESUMEN
The effect of design and fabrication parameters on the polarization dependence of the splitting ratio in directional couplers produced in LiTaO3 by Zn diffusion has been investigated experimentally at a wavelength of 1558 nm. The directional couplers featured various combinations of waveguide width, separation gaps between waveguides, bending angle, and diffusion conditions. In each case the coupling region was 3.5 mm long. Of particular interest was the identification of parameter sets for which the sum of power splitting ratios from TE and TM inputs equals unity at the output, as needed for electro-optic tunable filters with relaxed beam-splitter requirements.
RESUMEN
Bar-code patterns were produced upon gold-coated glass spheres 8 mm in diameter with a novel projection photolithography exposure system that included a computer-controlled rotation stage-and-shutter arrangement. The patterns extended in an equatorial band about the entire 360 degrees periphery of the sphere. To obtain uniform thickness of the photoresist layer, we dip coated the spheres and removed the excess with an absorption pad. The widths of the bars in the pattern were in the range 21.85 +/- 0.6 microm at the equator, and the relative error in the angular position of the pattern features was less than 0.025 degrees. The patterned spheres are suitable for use in optically interrogated flying spot magnetic heading sensors.
RESUMEN
An erbium-doped fiber laser that emits a series of spectrally scanned pulses is used to monitor an array of fiber Bragg grating (FBG) sensors. The cavity for this Fabry-Perot laser is formed by two spectrally selective reflectors: a rotating mirror-grating combination for scanning the reflectance peak wavelength and a fiber Fabry-Perot interferometer (FFPI) with a periodic reflectance spectrum. During a scan of the rotating mirror, the laser produces a set of Q-switched pulses over the 1522-1568-nm spectral range at each of the FFPI reflectance peak wavelengths. This laser is used to simultaneously demonstrate wavelength-division multiplexing of FBGs with reflectance peaks in different spectral regimes and time-division multiplexing of FBGs with overlapping spectra. The spectral location of the FBG peaks was determined to an accuracy of 1.4 pm.
RESUMEN
The monitoring of interferometer fiber optic sensors using a laser that is scanned over a wide frequency range is investigated. The interrogation technique is based on the principle that if the light-source frequency varies linearly with time, the optical signal reflected or transmitted is intensity modulated at a frequency that is proportional to the optical path difference (OPD) in the interferometer. Fourier components in the detected optical output signal then correspond to the OPDs of any interferometers that have contributed to this modulation. The temporal position of a peak in the power spectrum of this signal is proportional to the OPD of the interferometer that is responsible for that peak. A fine tuning of the OPD value is determined from the phase of the corresponding Fourier component. Experimentally, an Er:fiber laser scanned over a 48-nm range centered at 1540 nm was used to monitor intrinsic fiber Fabry-Perot interferometers (FFPIs). Variations in the laser scan rate were compensated with the optical signal modulated by a reference FFPI held at a constant temperature. The OPD measurement resolution was 3.6 nm, and the dynamic range was 1.3 x 10(7). The temperature was measured from 20 degrees C to 610 degrees C with a 0.02 degrees C resolution, and multiplexing of three of the sensors arranged in series was demonstrated.
RESUMEN
A novel monitoring system for a fiber Fabry-Perot interferometer (FFPI) temperature sensor has yielded a resolution of 0.013 degrees C (0.0025 fringe). Light from a broadband source passes through a scanned Michelson interferometer and is reflected from a FFPI to produce a fringe pattern, the temporal position of which is proportional to a change in the optical length of the fiber interferometer. A second Michelson interferometer with a distributed-feedback laser source is used to correct for variations in the translation rate of the motor-driven scanning mirror. Coherence multiplexing of three such sensors has also been demonstrated.
RESUMEN
A fiber Fabry-Perot interferometer (FFPI) sensor is formed with broadband (~3 nm, 3-dB bandwidth) fiber Bragg grating (FBG) mirrors. Repetitively modulating a distributed-feedback laser produces chirping that modulates the reflectance of the FFPI. Because the reflectance of the FBG mirrors varies with optical frequency, the fringes in the sensor reflectance modulation are distinguishable, making it possible to extend the sensor dynamic range versus that of a FFPI sensor with conventional wavelength-dependent mirrors. An ambient temperature is determined in the range from 25 to 170 degrees C with a resolution of 0.005 degrees C.
RESUMEN
The effect of design and fabrication parameters on the dependence on polarization of the splitting ratio in directional couplers produced by Ti diffusion in LiNbO3 has been investigated experimentally at a wavelength of 1.54 microm. The parametric study was carried out at a diffusion temperature of 1025 degrees C, which was found to suppress outdiffusion effects. The directional couplers were produced by use of various combinations of waveguide separation, Ti film thickness, and diffusion time. Of particular interest was the identification of parameter sets for which the sum of TE and TM splitting ratios equals unity, as required for electro-optic and acousto-optic tunable filter designs with relaxed beam-splitter requirements. Directional couplers that closely match this criterion were obtained by diffusion of 3.5-mm-long, 7-microm-wide Ti strips separated by 11 microm for 11 h. It was found that a bending angle of 0.6 degrees for input and output waveguides produces lower transmission loss for both polarizations than a 1.0 degree bend angle (>1-dB loss reduction).
RESUMEN
A rotating machinery test rig was instrumented with fiber Fabry-Perot interferometer strain sensors for condition monitoring of rolling element bearings. Strain variations produced by ball passes were observed and analyzed in the time and frequency domain. Wavelength division multiplexing was utilized to simultaneously monitor the sensors with analog and digital readout systems--analog for high bandwidth and digital for high dynamic range and the monitoring of multiple sensors. The effects of imbalance on the shaft, changes in rotational speed, effects on the rotor system, and detection of bearing defects were investigated. Frequency peaks observed in the bearing sensor spectra closely matched predicted values. Imbalance and rotational speed tests showed good agreement with expected trends, and bearing defects were successfully detected.
RESUMEN
Pressure sensing in an internal combustion engine with an intrinsic fiber Fabry-Perot interferometer (FFPI) integrated with a spark plug is demonstrated for the first time. The spark plug was used for the ignition of the cylinder in which it was mounted. The FFPI element, protected with a copper/gold coating, was embedded in a groove in the spark-plug housing. Gas pressure inthe engine induced longitudinal strain in this housing, which was also experienced by the fiber-optic sensing element. The sensor was monitored with a signal conditioning unit containing a chirped distributed-feedback laser. Pressure sensitivities as high as 0.00339 radians round-trip phase shift per pounds per square inch of pressure were observed. Measured pressure versus time traces showed good agreement with those from a piezoelectric reference sensor mounted in the same engine cylinder.