Counting signal processing and counting level normalization techniques of polarization-insensitive fiber-optic Michelson interferometric sensors

A counting signal processing technique of the fiber-optic interferometric sensor is proposed. The technique is capable of counting the numbers of the maximum and minimum of the output interferometric signal in a specific time duration, and it can be used as the basis to distinguish the sensing phase signal. It can also be used as a signal detector on applications such as intrusion detection. All sensors are subject to aging of the optical components and bending loss, and therefore the output signal of each sensor may vary with time. We propose a counting level normalization technique to compensate for these variations and to obtain the correct counting numbers.     

Methods of electronic signal processing in fiber-optic phase sensors

Translated from Izmeritel'naya Tekhnika, No. 1, pp. 31–33, January, 1992.     

Multiplexed,white-light interferometric fiber-optic sensor matrix with a long-cavity,Fabry-Perot resonator

A long-cavity, single-mode, fiber-optic Fabry-Perot resonator has been used to develop white-light fiberoptic multiplexed Michelson interferometers (MMIs). The MMI matrix is constructed and demonstrated. We also analyzed the capacity of the long-cavity Fabry-Perot resonator multiplexing technique. The experimental results of a 2 x 2 fiber-optic interferometric sensor matrix are given, and the effects of polarization are discussed.     

Damage monitoring of carbon fiber-reinforced plastics with Michelson interferometric fiber-optic sensors

This paper presents the application of a Michelson interferometric fiber-optic sensor for monitoring the damage of fiber-reinforced plastics. A Michelson interferometric fiber-optic sensor was mounted on the surface of unidirectionally aligned carbon fiber-reinforced epoxy composites. Response of the interference signal to either dynamic or static loading was investigated. Specimen being impacted, the optical interference signal dropped suddenly and then oscillated. The tensile test was performed with the measurement of optical interference signal, strain as well as acoustic emission. Both fast Fourier transform and digital filter processing of the optical interference signal were carried out to characterize the damage signal from the fiber-optic sensor. The optical interference signal whose frequency ranged from tens to hundreds Hz occurred when the specimen was damaged. It was shown that real-time information comparable to acoustic emission (AE) data could be obtained from Michelson interferometric fiber-optic sensor through a digital filtering technique. The Michelson interferometric fiber-optic sensor proved to be effective for monitoring the damage processes of the material studied.     

Time-and-spatial-multiplexing tree topology for fiber-optic Bragg-grating sensors with interferometric wavelength-shift detection

A combined time-and-spatial-division-multiplexed tree topology with eight fiber-optic Bragg-grating sensors operating at the 830-nm wavelength was constructed and tested for both quasistatic and periodic strain and temperature measurements. The system uses a interferometric wavelength-shift discriminator and incorporates a reference channel for thermal drift compensation in the output. Dynamic sensor sensitivity, as determined by primary noise sources, is evaluated, and numerical results are presented and compared with experimental results.     

Investigation of interferometric noise in fiber-optic gas sensors with use of wavelength modulation spectroscopy

We report on interferometric noise limitation of fiber-optic gas sensors with highly coherent lasers and wavelength modulation spectroscopy. Interference between signal wave and reflected waves causes signal fluctuation in the output, which limits the performance of the sensing system. Sensor resolution limited by interferometric noise is calculated for a fiber-optic gas sensor with the Q(6) absorption line of methane gas at approximately 1650 nm. The results are useful for system designers of this particular type of gas sensor.     

Large-scale remotely interrogated arrays of fiber-optic interferometric sensors for underwater acoustic applications

The fiber-optic interferometric acoustic sensor array has established itself as a potential alternative to the conventional sonar array based on electroceramic transducers. In this paper, we discuss all the aspects of a large-scale fiber-optic interferometric sensor array. We review the basic operating principles of the fiber-optic interferometric sensor, signal processing, and multiplexing techniques, we present results from a noise model for a full size system, and we determine the benefit of incorporating a remotely-pumped optical amplifier in the array. As a practical example we describe the design and construction of a prototype array with 96 hydrophones incorporating a remotely pumped erbium-doped fiber amplifier, called the fiber-optic bottom mounted array, which is based on a dense wavelength division and time division multiplexed architecture. These arrays have applications in military sonar and seismic surveying.     

Substitution-type voltmeters with digital signal processing

Translated from Izmeritel'naya Tekhnika, No. 6, pp. 53–55, June, 1993.     

Quasidistributed fiber-optic interferometric pickups

The operating principle of a quasidistributed fiber-optic pickup based on internodal interference in weakly guiding fiber waveguides is considered. The pickup can be used as both an independent measuring instrument and a base element in the fabrication of fiber-optic distributed measuring networks.Translated from Izmeritel'naya Tekhnika, No. 1, pp. 16–17, January, 1994.     

Analysis of a high-speed fiber-optic spectrometer for fiber-optic sensor signal processing

A novel high-speed fiber-optic spectrometer has been demonstrated in our previous work. The high-speed spectrum measurement is enabled by translating the spectral-domain signal into a time-domain signal through a dispersion element. We present a mathematical model that accurately describes the relationship between the optical spectrum to be measured and the dispersed time-domain signal. Based on the model, the effects of the key parameters on the performance of the spectrometer are investigated in detail using numerical simulation. The analysis is useful for the design and application of such spectrometers.     

Digital signal processing for an open-loop fiber-optic gyroscope

Open-loop fiber-optic gyroscopes (FOG's) are generally less stable than closed-loop FOG's, yet they offer simpler implementation. We propose a modification of the harmonic division algorithm for an open-loop FOG, which is more stable and also simpler for implementation than the original one. It is shown that when the analog signal is properly sampled and quantized, the performance of our algorithm reaches that of closed-loop FOG's. Our algorithm may be implemented by the use of off-the-shelf component, and does not require an integrated optics circuit.     

White-light interferometric fiber-optic pressure sensor

A novel configuration for a fiber-optic white-light interferometric sensor is presented which allows for absolute measurements of hydrostatic pressure with an improved operation range. The performance of two fibers (York bow-tie 800 and especially designed elliptical-core side-hole fiber) used as sensing elements was experimentally studied. The sensor itself was composed of two equal lengths of the fiber spliced at 90°. This structure assures temperature compensation and enables application of a Wollaston prism as a receiving interferometer. A step delay line made of crystalline quartz was used to increase the operation range of the sensor     

Amplitude fiber-optic sensors

Translated from Izmeritel'naya Tekhnika, No. 1, pp. 34, 39–40, January, 1992.     

New interferometric fiber-optic gyroscope with amplified optical feedback

A novel interferometric fiber-optic gyroscope with amplified optical feedback by an Er-doped fiber amplifier (EDFA) is proposed and theoretically investigated (the proposed gyroscope is named the feedback EDFA-FOG, FE-FOG in what follows). The FE-FOG functions like a resonant fiber-optic gyro (R-FOG) because of its multiple utilization of the Sagnac loop; however, it is completely different because a low-coherence light source is used. In addition, the gyro output signal is pulsed because the modulation frequency of the phase modulator placed in the Sagnac loop is selected to match the total round-trip time delay of the light, which includes the Sagnac-loop delay plus that of the feedback loop of the fiber amplifier. The sharpness of the output pulse can be adjusted by both the gain of an EDFA and the modulation depth of the phase modulator. When rotation occurs the peak position of the output pulse is shifted as a result of the Sagnac effect. The resolution of the rotation measurement depends on the sharpness of the output pulse. The techniques of both the open-loop and closed-loop methods are described in detail, which shows the great advantage of the proposed gyroscope over the to the conventional interferometric fiber-optical gyroscope (I-FOG).     

Investigation of interferometric noise in fiber-optic bragg grating sensors by use of tunable laser sources

Interferometric noise in fiber-optic grating sensors isinvestigated. Interference between a signal wave and reflectedwaves causes signal fluctuation in the output that limits thewavelength detection accuracy of the sensing system. Themeasurement error limited by interferometric noise is calculated forboth reflective-type and transmission-type sensors.     

Railroad bridge instrumentation with fiber-optic sensors

Fiber-optic sensors were installed on fatigue-critical components in the superstructure of a railroad bridge to monitor dynamic strains induced by trains crossing the bridge as well as to detect the onset of cracks. Each fiber Fabry-Perot interferometer (FFPI) strain gage was adhesively bonded to a stainless-steel strip to facilitate all-weather installation on the steel bridge members by spot welding. FFPI strain sensors were also installed on a rail at an approach to the bridge. Electrical resistive strain gages were colocated with the fiber-optic sensors on the bridge for the purpose of performance verification. In addition to the strain gages, fiber-optic continuity sensors for crack detection were bonded to the structure at critical locations. A telemetry system for transmitting the data over telephone lines was also installed at the bridge site. Dynamic response of the fiber-optic strain sensors is comparable with that of the electrical gages, and their performance has not degraded in the year since the initial installation.