Multiple Fabry-Perot Interferometer Research Articles

A metal-ceramic coaxial cable with multipoint Fabry-Pérot interferometers for monitoring distributed high temperature

Abstract A metal ceramic coaxial cable (MCCC) sensor containing multiple Fabry-Perot interferometers (FPIs) has been designed, fabricated, and evaluated for measuring distributed high temperatures. The MCCC sensor demonstrated here consists of stainless steel tube and wire conductors, alumina tube insulator, and three pairs of air gap reflectors distributed along the cable. The multipoint sensor operation is realized by the joint time-frequency domain technique, which combines the functions of time-domain reflectometry and frequency division multiplexing to enable the measurement of spatially distributed temperature. The 3-point MCCC-FPI sensor has been demonstrated for distributed temperature measurements between 250 and 550 °C and operability up to 1000 °C. The sensor offers high sensitivity of >0.18 MHz/°C with a measurement resolution of 1 Hz and reasonably small high temperature measurement error range of ±3 °C.

Multimode Fabry⁻Perot Interferometer Probe Based on Vernier Effect for Enhanced Temperature Sensing.

New miniaturized sensors for biological and medical applications must be adapted to the measuring environments and they should provide a high measurement resolution to sense small changes. The Vernier effect is an effective way of magnifying the sensitivity of a device, allowing for higher resolution sensing. We applied this concept to the development of a small-size optical fiber Fabry–Perot interferometer probe that presents more than 60-fold higher sensitivity to temperature than the normal Fabry–Perot interferometer without the Vernier effect. This enables the sensor to reach higher temperature resolutions. The silica Fabry–Perot interferometer is created by focused ion beam milling of the end of a tapered multimode fiber. Multiple Fabry–Perot interferometers with shifted frequencies are generated in the cavity due to the presence of multiple modes. The reflection spectrum shows two main components in the Fast Fourier transform that give rise to the Vernier effect. The superposition of these components presents an enhancement of sensitivity to temperature. The same effect is also obtained by monitoring the reflection spectrum node without any filtering. A temperature sensitivity of −654 pm/°C was obtained between 30 °C and 120 °C, with an experimental resolution of 0.14 °C. Stability measurements are also reported.

Distributed temperature sensing with unmodified coaxial cable based on random reflections in TDR signal

This paper presents a new method to utilize an unmodified coaxial cable for distributed temperature sensing. It compares the time domain reflectometry (TDR) signal before and after temperature change by cross-correlation, and the resultant time delay will indicate the region as well as the magnitude of the temperature change. The temperature change from 27 °C to 70 °C is monitored and a linear relationship between the cross-correlation value and temperature can be observed. Two-section temperature change monitoring is also demonstrated to show its distributed sensing capability. This method is more sensitive than the traditional TDR method. Compared to coaxial cable sensors based on multiple Fabry–Perot interferometers or coaxial cable Bragg gratings, the cable does not need to be modified before installation, and it is truly distributed sensing without dark zone.

High-Speed Interrogation of Low-Finesse Fabry–Perot Sensors Using a Telecom DFB Laser Diode

This paper presents a high-speed high-resolution low-finesse Fabry–Perot sensor interrogation system. The system utilizes a standard telecommunication distributed-feedback laser diode and an all-digital high-speed signal processing based on field-programmable-gate-array. The system can resolve multiple Fabry–Perot Interferometers (FPI) simultaneously, which might include multiple sensors, multiparameter sensors, or combinations of sensor(s) and reference FPIs. This opens up possibilities for adopting different measurement system configurations and compensation schemes. The system exhibits white-noise characteristics for a system bandwidth above 100 Hz, with spectral density of about 2 pm/√Hz when interrogating about 1 mm long low-finesse FPI. At an output sampling rate of 10 Hz, a sensor measurement length resolution of less than 50 pm was demonstrated for about a 600 μm long sensing cavity (corresponding to a strain resolution of about 70 nϵ). Proper use of simple all-fiber reference also provided low long-term drift. The system provides an up to 40 KHz sampling rate and was used to demonstrate dynamic measurement of strain variation on a piezo-electric stack and for interrogation of in-cylinder pressure variations within a high-rotation-rate miniature diesel engine. Fabry–Perot sensors with typical lengths between a few hundred and a few mm can be read-out with the current version of the proposed interrogator. The proposed system was built out of a small number of highly cost-efficient optoelectronics and electronics components (all used in FTTH) and thus, opens-up potential for new applications where the cost of interrogation is presently limiting.

Characterization of Fabry-Perot interferometers and multi-etalon transmission profiles

Aims. Properly characterizing Fabry-Perot interferometers (FPI) is essential for determining their effective properties and evaluating the performance of the astronomical instruments in which they are employed. Furthermore, in two-dimensional spectrographs where multiple FPI are used in series, the actual distribution of plate separation errors will be crucial for determining the resulting transmission profiles. We describe techniques that address these issues utilizing the FPI of IBIS, a solar bidimensional spectrometer installed at the Dunn Solar Telescope. Methods. A frequency-stabilized He-Ne laser was used in three different optical layouts to measure the spatially-resolved transmission of the FPI. Analyzing the shape and wavelength shift of the observed profiles allows the characteristics of the cavity errors and the interferometer coating to be determined. Results. We have measured the spatial distribution of the large-scale plate defects, which shows a steep radial trend, as well as the magnitude of the small-scale microroughness. We also extracted the effective reflectivity and absorption of the coating at the laser line wavelength for both interferometers. Conclusions. These techniques, which are generally applicable to any Fabry-Perot interferometer, provide the necessary information for calculating the overall instrumental profile for any illuminated area of the interferometer plates. Accurate knowledge of the spectral transmission profile is important, in particular when using inversion techniques or in comparing observations with simulated data.

Advanced technology solar telescope multiple Fabry-Pérot interferometer telecentric optical design

We present four preliminary designs for a telecentric optical train supporting the Advanced Technology Solar Telescope (ATST) multiple Fabry-Perot interferometer (MFPI), which is to be used as an imaging spectrometer and imaging spectropolarimeter. The point of departure for all three designs is the F/40 telecentric image at the Coude focus of the ATST. The first design, representing the high-spectral-resolution mode of operation, produces an intermediate F/300 telecentric image within the triple etalon system and a 34-arcsec field of view (FOV). The second design, intermediate between high- and low-spectral-resolution modes of operation, produces an intermediate F/150 telecentric image at the etalons and a 1.1-arcmin FOV. The third and fourth designs each represent a low-resolution mode of operation, producing an F/82 telecentric image at the etalons and a 2-arcmin FOV. Each design results in good telecentricity and image quality. Departures from telecentricity at the intermediate image plane cause field-dependent shifts of the bandpass peak, which are negligible compared to the bandpass FWHM. The root mean square (rms) geometric spot sizes at the final image plane fit well within the area of a camera pixel, which is itself in accordance with the Nyquist criterion, half the width of the 28-µm-wide resolution element (as determined from the diffraction limit of the ATST). For each configuration, we also examine the impact that the Beckers effect (the pupil apodization caused by the angle-dependent amplitude transmittance of the MFPI) has on the image quality of the MFPI instrument.

Measurement of the differential thermal expansion and temperature dependence of refractive index in gradient-index glass

The thermal expansion and temperature dependence of the refractive index in gradient-index glass have been investigated. In an earlier paper [ P. O. McLaughlin D. T. Moore , “ Models for the Thermal Expansion Coefficient and Temperature Coefficient of the Refractive Index in Gradient Index Glass,” Appl. Opt.24, this issue ( 15Dec.1985)] these two thermal properties were modeled as functions of the local glass composition within the gradient-index region. In this paper measurements of αL and dn/dT are described. A modified multiple Fabry-Perot interferometer was designed and built into an environmental temperature chamber to measure these thermal properties in gradient-index glass in the 0–100°C range. The interferometer operated under computer control to measure optically the differential thermal expansion and change in refractive-index profile in several samples of gradient-index glass.