Sergei N. Khotiaintsev

An optical-fibre refractometric liquid-level sensor for liquid nitrogen

This paper presents a prototype optical-fibre refractometric liquid-level sensor which can access the static and dynamic levels of liquid nitrogen in large cryogenic systems. This version of the sensor is intended for use in various tests of cryogenic systems such as used in rocketry, during the design stage. The sensor employs a vertical linear array of 140 highly sensitive optical-fibre refractometric transducers. The liquid-level measurement range is 1.6 m with a resolution of up to 5 mm. The optical transducers are multiplexed using a matrix-type optical-fibre network which employs a robust level tracking algorithm. This type of sensor can be used for measuring the level of other cryogenic liquids such as liquid hydrogen.

Novel optical fiber refractometric transducer employing hemispherical detection element

We present the results of a theoretical numerical analysis of a novel fiber optic refractometric transducer. It consists of a hemispherical glass detection element and a pair of the multimode optical fibers attached symmetrically to the elements flat equatorial plane. The internal reflection of light from the elements spherical surface is sensitive to the refractive index of the surrounding medium. We exploit several internal reflections in series and the focusing of the beam by the elements spherical surface to achieve enhanced sensitivity to the refractive index of the surrounding medium and reduced intrinsic optical loss in the transducer. We present the data on the effect of transducers parameters on its transmission function. We show that the transducer can operate in a relatively wide range of the refractive index of the surrounding medium (we show this for the interval n = 1.0 to n = 1.45). We also present experimental data confirming our theoretical results.

Linear and steplike characteristics in an optical fiber refractometric transducer with hemispherical detection element

We present the results of a theoretical numerical analysis of transmission characteristics of a fiber optic refractometric transducer with a hemispherical glass detection element. In this transducer, the internal reflection of light from the elements spherical surface depends on the refractive index of the surrounding medium. We examine the effects of the transducers geometrical and optical parameters and its refractive index on the transmission function, its nonlinearity, and the transducers sensitivity to the refractive index of the surrounding medium. We show that through a proper choice of the transducers material and geometrical parameters, it is possible to obtain a transmission function of any necessary span over a wide interval of the refractive index of the surrounding medium (from n = 1.0 to 1.7), and to modify the form of the transmission function from a linear one to a steplike one in virtually the same device. This permits us to use the proposed transducer for two contrasting applications: assessing the refractive index, and discriminating between two liquids or between air and a liquid, as in the detection of liquids, level measurement, etc. In addition, it is possible to adjust the transducer input range to the refractive index of a particular fluid (or fluids) of interest.

Structural health monitoring of concrete elements with embedded arrays of optical fibers

An optical-fiber sensor system for structural health monitoring of concrete elements such as beams and columns is presented. The system employs arrays of conventional optical fibers embedded in the concrete elements as crack sensors. Twelve types of optical fibers as well as several embedding techniques have been tested for this role. The survival rate of optical fibers embedded in concrete could be as high as 80%. The loss of fibers during the embedding process was acceptable provided that the number of fibers in the array had redundancy. The optical transmission of all fibers in the array was monitored in a time-division multiplexed mode at a high repetition rate, in the kHz range. The monitoring scheme allowed a quasi-continuous data acquisitions of large optical fiber arrays. A sharp decrease in the optical transmission of one or more optical fibers was a clear indicator of the development of cracking in the element subjected to flexural loads. The system was successful in detecting not only the initiation but also the propagation of cracks in concrete elements subjected to incremental flexural loading. In this work, the relation between the mechanical properties of the optical fibers and their behavior for the described application is discussed. Also, considerations towards a rational design of the system are proposed. The damage detection system may be used for the mapping and monitoring of cracks in concrete elements. The simplicity of the operation and relatively low cost of the proposed system make it a great candidate for applications in structural health monitoring of critical elements in civil infrastructure.

Design of Long-Period Fiber Grating Refractometric Sensors With Linear Response by a Genetic Algorithm

A new method for the design of optical fiber refractometric sensors based on nonuniform long-period fiber gratings (LPFGs) is presented. A specialized genetic algorithm (GA) with properly designed operators is a fundamental element of this method and performs the theoretical synthesis of a necessary nonuniform LPFG period profile. Such a profile yields a linear response of the sensor to the refractive index of the external medium. A distinctive feature of the designed LPFG is a linear gradient of the core and cladding refractive index along the grating length. Also, a new mathematical model of nonuniform LPFGs is presented, the model permits to take into account the effect of some LPFG sections of a lower and others of a higher cladding refractive index than that of the external medium. The application of both the design method and the mathematical model is illustrated with a numerical example.

Optical Intensity-Type Refractometer for Remote Measurements Via Fiber-Optic Cables

We present a fiber-optic refractometer with intensity-type transducer which can operate remotely via long fiber-optic cables. This version of the refractometer is intended for accessing the concentration of aqueous solutions of sodium chloride in the range from zero to about 1450/00 (1.327 <; n <; 1.353 at λ = 940 nm and 25°C), which is of significant practical interest for industrial users. We address the relationship between the span and linearity of the intensity-type optical transducer with spherical working surface which we employ in the refractometer. We optimized the transducer for the specified measurement range and achieved good sensitivity and non-linearity as small as 0.05. We significantly reduced the negative effect of the unstable optical loss of the fiber-optic cables, variation of the optical source power and photo-detector response on the readings of the refractometer by using an optical bridge configuration followed by an analogue optoelectronic signal processing unit. The refractometer has a parasitic variation of its output as low as 1.5% under a total variation of the optical source power and a fiber-optic cable loss of 20 dB. Potential applications of this refractometer include the in-situ characterization of various liquids such as the sodium chloride salt-brine solutions in food industry.

Optical-fibre sensor system for monitoring the performance of the gas?propellant centrifuge separator of a spacecraft

An optical-fibre sensor system is presented for monitoring void fraction distribution in a spacecrafts gas and propellant centrifuge separator. The system could be used at the separator development stage or for monitoring, during ground tests, the elements of the spacecraft propulsion system. Our sensor system employs an array of point optical-fibre refractometric transducers installed in the form of several linear radial arrays on the separator rotating blades. We employed a small-size hemispherical optical detection element as the transducer and we optimized its parameters through numerical ray-tracing. The aim is to minimize the effect of the thin film of liquid that forms on the transducers surface in this application. The features of this sensor system are: (1) an efficient matrix-type multiplexing scheme, (2) the installation of the main optoelectronic unit of the sensor in a hermetically sealed container inside the separator tank located on the rotating shaft and (3) the spark-proof and explosion-proof design of the sensor circuits and elements. The sensor is simple, reliable, low-cost and is capable of withstanding the factors involved during operation of the propulsion system such as cryogenic temperatures and chemically aggressive liquids. The novel elements and design concepts implemented in this sensor system can also find applications in other sensors for spacecraft propulsion systems and also in a variety of optical-fibre sensors used in scientific research and industry.

Nonlinear Analyses of the Parasitic Backward-Wave Oscillation Power in the Magnetically Focused Pulsed Helix Traveling-Wave Tube Amplifier in the Absence of the Amplified Signal

This paper analyzes parasitic backward-wave oscillation power as a function of interaction length and focusing magnetic field parameters in a generic helix traveling-wave tube (TWT) amplifier in the absence of amplified signal. The permanent periodic magnetic (PPM) focusing of the electron beam, the relatively narrowband (less than one octave) TWT, the electron gun with no control grid, the pulsed full beam current cutoff operating mode of the TWT, and the accelerating voltage pulse wider than the amplified RF pulse are considered. Under such conditions, the parasitic backward-wave oscillation can build up at the leading and trailing edge and at the top of the accelerating pulse before and after the input RF signal is applied when the instantaneous accelerating voltage provides for the synchronism condition. The parasitic backward-wave oscillation, although nondesirable in general, can be tolerated if its power does not exceed some allowable level. In this paper, the nonlinear (large-signal) theory of beam-wave interaction in the TWT in the specified case is developed. The theory accounts for the interaction of the multiple harmonics of the backward wave of the slow-wave circuit with the electron beam that alternatively changes the direction of its rotation on each half period of the focusing magnetic field. A system of equations, which makes accessible the start oscillation length and the starting Pierce relative velocity parameter as a function of the electrical parameters of the TWT, the PPM focusing field period, and the magnetic flux density distribution in the large-signal regime, is obtained. A particular numerical example reveals the relation between interaction length, PPM focusing field period, and parasitic backward-wave oscillation power. The approach permits one to design a TWT having the maximum possible interaction length under the allowable parasitic backward-wave oscillation power. Also, the results demonstrate that the focusing magnetic field parameters have a significant effect on the interaction of the rotating electron beam with the backward wave in the nonlinear regime, as they have in the linear regime

Linear two-dimensional analysis of parasitic backward-wave oscillation in a monofilar-helix traveling wave tube

We investigated the attainable stability of the monofilar-helix traveling wave tube (TWT) with respect to the parasitic backward-wave oscillation under periodic magnetic focusing. We accounted for the effect of the electron beam rotation in the nonhomogeneous focusing magnetic field, magnetic field amplitude and periodicity, and the specific coupling of the electron beam with the synchronous harmonics of the electromagnetic wave existing on the slow-wave circuit. This specific coupling takes place in the monofilar-helix TWT only and decreases its stability with respect to the parasitic backward-wave oscillation in comparison to other types of helix TWT. For the purpose of this analysis, we developed further the linear two-dimensional (2-D) multiwave theory of the helix TWT described in our previous work . In this theory, we account for space charge fields by means of waveguide excitation model based on the direct solution of Maxwell equations. Differently from , the present version of our theory allows for accounting of the coupling of the synchronous 0th spatial harmonic (forward employed for the amplification) and -1st (parasitic backward) harmonic of the circuit wave with the electron beam in the monofilar-helix TWT. By numerical modeling, we obtained quantitative data on the starting conditions for the parasitic backward-wave oscillation in the typical monofilar-helix TWT in the intersection point of the dispersion curves of these two harmonics. The described coupling mechanism does not exist in the bifilar-helix TWT. Therefore, the TWT with the bifilar helix is more stable with respect to the parasitic backward-wave oscillation than the TWT having the monofilar helix. These theoretical results correlate with the existing experimental evidence. The results of the present study, in particular, explain why the backward-wave oscillation suppression by tailoring of the focusing magnetic field has a significant effect in the bifilar-helix TWT but does not work in the monofilar-helix TWT.

Fiber optic refractometric sensors using a semi-ellipsoidal sensing element.

We present theoretical and experimental results for a fiber optic refractometric sensor employing a semi-ellipsoidal sensing element made of polymethyl methacrylate. The double internal reflection of light inside the element provides sensitivity to the refractive index of the external analyte. We demonstrate that the developed sensor, operating at a wavelength of 632 nm, is capable of measurement within a wide range of refractive indices from n=1.00 to n=1.47 with sensitivity over 500 dB/RIU. A comparison of the developed sensor with two more complex refractometric sensors, one based on tapered optical fiber and the other based on suspended-core microstructure optical fiber, is presented.