Fiber loop ringdown for physical sensor development: pressure sensor

Multichannel continuous-wave fiber cavity ringdown gas sensing utilizing frequency-shifted interferometry

We present a multichannel continuous-wave (CW) fiber cavity ringdown (FCRD) gas sensing method based on frequency-shifted interferometry (FSI). This scheme detects gas concentration by measuring the intensity decay rates of continuous light from different ringdown cavities in the spatial domain, unlike conventional FCRD techniques, which measure the decay rates of pulse light in the time domain. This method shares one CW light source, one slow detector, and one slow data collector. In order to illustrate the theory, acetylene gas concentration measurement in a two-channel FSI-FCRD system was experimentally conducted in the range of 0%-1%. A linear relation was established between concentration and absorption loss, which is proportional to the intensity decay rate, and the measurement resolutions of 3.871%/dB and 3.658%/dB were achieved, respectively. The results reveal that the proposed system has the advantages of low cost, high sensitivity, high precision, and good stability in multichannel gas detection.

Continuous-Wave Fiber Cavity Ringdown Pressure Sensing Based on Frequency-Shifted Interferometry

We present a continuous-wave fiber cavity ringdown (FCRD) pressure-sensing method based on frequency-shifted interferometry (FSI). Compared with traditional CRD or FCRD techniques, this FSI-FCRD scheme deduces pressure by measuring the decay rate of continuous light exiting the fiber ringdown cavity (RDC) in the spatial domain (i.e., the CRD distance), without the requirement for optical pulsation and fast electronics. By using a section of fiber with the buffer layer stripped in the fiber RDC as the sensor head, pressures were measured within the range from 0 to 10.4 MPa. The sensitivity of 0.02356/(km∙MPa) was obtained with a measurement error of 0.1%, and the corresponding pressure resolution was 0.05 MPa. It was found that the measurement sensitivity can be improved by enlarging the interaction length of the sensor head. The results show the proposed sensor has the advantages of simple structure, low cost, high sensitivity, and high stability in pressure detection.

Hybrid Structure Multichannel All-Fiber Current Sensor

We have experimentally developed a hybrid-structure multi-channel all-fiber current sensor with ordinary silica fiber using fiber loop architecture. According to the rationale of time division multiplexing, the sensor combines parallel and serial structures. The purpose of the hybrid-structure multi-channel all-fiber current sensor is to get more information from the different measured points simultaneously. In addition, the hybrid-structure fiber current sensor exhibited a good linear response for each channel. A three-channel experiment was performed in the study and showed that the system could detect different current positions. Each channel could individually detect the current and needed a separate calibration system. Furthermore, the three channels will not affect each other.

Microwave-assisted frequency domain measurement of fiber-loop ring-down system

A new frequency domain measurement method for a fiber-loop ring-down system is proposed in this Letter. Compared to traditional time domain measurement, this method uses a microwave modulated continuous wave (CW) laser as a light source, making full use of the duty cycle to achieve enhanced measurement efficiency. By measuring the amplitude modulation over a frequency span, this technique can be used to determine the ring-down time in the frequency domain, which will then be used to calculate the loss in the ring.

Magnetic field sensor using the fiber loop ring-down technique and an etched fiber coated with magnetic fluid

The fiber loop ring-down spectroscopy technique is introduced into the evanescent-field-based sensing scheme in order to create a new type of fiber-based magnetic field sensor. As a consequence, the sensitivity and stability of the magnetic field sensing system are significantly enhanced. The sensor head is constructed using a section of a single-mode fiber with its cladding partially etched. The process of fiber etching is described in detail, and the relationship between the diameter of the etched fiber and the etching time is experimentally investigated. After adopting the appropriate size of the etched fiber, the final experimental results show that the magnetic field strength has a well-defined linear relationship with the inverse of the ring-down time τ over a range of 30 mT with a sensitivity of 95.5 ns/mT.

Real time interrogation technique for fiber Bragg grating enhanced fiber loop ringdown sensors array

A novel fiber Bragg grating aided fiber loop ringdown (FLRD) sensor array and the wavelength-time multiplexing based interrogation technique for the FLRD sensors array are proposed. The interrogation frequency of the system is formulated and the interrelationships among the parameters of the system are analyzed. To validate the performance of the proposed system, a five elements array is experimentally demonstrated, and the system shows the capability of real time monitoring every FLRD element with interrogation frequency of 125.5 Hz.

Double-looped Mach-Zehnder interferometer for achieving multiple ring-down interferograms

We put forward a double-looped Mach-Zehnder interferometer for acquiring continuous ring-down interferograms with two fiber-loop cavities with slightly different optical path lengths. Each group of pulses through the sample and reference loops creates a ring-down pulse train with equal time intervals in Vernier fashion, and interferes with each other to produce multiple ring-down interferograms successively by scanning of a delay line. The system requires a scanning range of only a few millimeters to obtain multiple ring-down interferograms. In a proof-of-concept demonstration, the intrinsic losses of two loops are estimated. The measured combined-loss of both loops is compared to the sum of the loop losses measured separately with a conventional fiber-loop ring-down system. The result obtained using the proposed system exhibits a difference of only 0.06 dB with that of the reference system.

Fiber loop ringdown sensor for potential real-time monitoring of cracks in concrete structures: an exploratory study

A fiber loop ringdown (FLRD) concrete crack sensor is described for the first time. A bare single mode fiber (SMF), without using other optical components or chemical coatings, etc., was utilized to construct the sensor head, which was driven by a FLRD sensor system. The performance of the sensor was evaluated on concrete bars with dimensions 20 cm × 5 cm × 5 cm, made in our laboratory. Cracks were produced manually and the responses of the sensor were recorded in terms of ringdown times. The sensor demonstrated detection of the surface crack width (SCW) of 0.5 mm, which leads to a theoretical SCW detection limit of 31 μm. The sensor's response to a cracking event is near real-time (1.5 s). A large dynamic range of crack detection ranging from a few microns (μm) to a few millimeters is expected from this sensor. With the distinct features, such as simplicity, temperature independence, near real-time response, high SCW detection sensitivity, and a large dynamic range, this FLRD crack sensor appears promising for detections of cracks when embedded in concrete.

High-current-sensitivity all-fiber current sensor based on fiber loop architecture

In this paper, we demonstrate a novel all-fiber current sensor using ordinary silica fiber. The sensor employs a fiber solenoid as a current sensor head, which improves the current sensitivity by allowing optical signals to traverse the sensor head repeatedly. Theory and experiment prove that the improvement in sensitivity increases periodically with the number of repetitions of optical signals circulating round the loop.

Absorption detection using optical waveguide cavities

Cavity ring-down spectroscopy is a spectroscopic method that uses a high quality optical cavity to amplify the optical loss due to the light absorption by a sample. In this presentation we highlight two applications of phase-shift cavity ring-down spectroscopy that are suited for absorption measurements in the condensed phase and make use of waveguide cavities. In the first application, a fiber loop is used as an optical cavity and the sample is introduced in a gap in the loop to allow absorption measurements of nanoliters of solution at the micromolar level. A second application involves silica microspheres as high finesse cavities. Information on the refractive index and absorption of a thin film of ethylene diamine on the surface of the microresonator is obtained simultaneously by the measurements of the wavelength shift of the cavity mode spectrum and the change in optical decay time, respectively.

Chemical sensing using fiber cavity ring-down spectroscopy

Waveguide-based cavity ring-down spectroscopy (CRD) can be used for quantitative measurements of chemical concentrations in small amounts of liquid, in gases or in films. The change in ring-down time can be correlated to analyte concentration when using fiber optic sensing elements that change their attenuation in dependence of either sample absorption or refractive index. Two types of fiber cavities, i.e., fiber loops and fiber strands containing reflective elements, are distinguished. Both types of cavities were coupled to a variety of chemical sensor elements, which are discussed and compared.

Sensitivity enhancement of fiber loop cavity ring-down pressure sensor

We present a theoretical and experimental study on sensitivity enhancement of a fiber-loop cavity ring-down pressure sensor. The cladding of the sensing fiber is etched in hydrofluoric acid solution to enhance its sensitivity. The experimental results demonstrate that the pressure applied on the sensing fiber is linearly proportional to the difference between the reciprocals of the ring-down time with and without pressure, and the relative sensitivity exponentially increases with decreasing the cladding diameter. When the sensing fiber is etched to 41.15 microm, its sensitivity is about 36 times that of nonetched fiber in the range of 0 to 32.5 MPa. The measured relative standard deviation of the ring-down time is about 0.15% and, correspondingly, the least detectable loss is about 0.00069 dB.

Fiber loop ringdown - a time-domain sensing technique for multi-function fiber optic sensor platforms: current status and design perspectives

Fiber loop ringdown (FLRD) utilizes an inexpensive telecommunications light source, a photodiode, and a section of single-mode fiber to form a uniform fiber optic sensor platform for sensing various quantities, such as pressure, temperature, strain, refractive index, chemical species, biological cells, and small volume of fluids. In FLRD, optical losses of a light pulse in a fiber loop induced by changes in a quantity are measured by the light decay time constants. FLRD measures time to detect a quantity; thus, FLRD is referred to as a time-domain sensing technique. FLRD sensors have near real-time response, multi-pass enhanced high-sensitivity, and relatively low cost (i.e., without using an optical spectral analyzer). During the last eight years since the introduction of the original form of fiber ringdown spectroscopy, there has been increasing interest in the FLRD technique in fiber optic sensor developments, and new application potential is being explored. This paper first discusses the challenging issues in development of multi-function, fiber optic sensors or sensor networks using current fiber optic sensor sensing schemes, and then gives a review on current fiber optic sensor development using FLRD technique. Finally, design perspectives on new generation, multi-function, fiber optic sensor platforms using FLRD technique are particularly presented.

Multichannel-fiber ringdown sensor based on time-division multiplexing

A multichannel-fiber ringdown sensor based on the principle of time-division multiplexing is described. The purpose of multiplexing is to obtain the information of one or more fiber ringdown sensor heads simultaneously under the premise of cost neutrality. To illustrate the principle, we study the typical dual-channel fiber ringdown force sensor in our experiment. It shows that the pressures at two different points can be measured simultaneously. In addition, we find by measurement that our sensors have good linear responses above the response threshold of the applied force.