MXPA04012076A - Measuring system of passive networks of fiber optic sensors. - Google Patents

Abstract

The present system allows sensor networks of fiber optic to be measured in passive optic networks by means of a semi-conductor tunable laser operating in a continual wave regime. More particularly, the tunable laser (2) along with its controller (3) and through a circulator (4) sends signals of a determined wavelength to the passive network (1) of the optic sensors during a T1 period of time; after which the wavelength is changed, thereby receiving the signals of the sensors network reflected in an electro-optic converser (6) by means of a removed band filter (5) that rejects the "0ff" band.

Description

SYSTEM OF MEASUREMENT OF PASSIVE NETWORKS OF OPTICAL FIBER SENSORS OBJECT OF THE INVENTION The present invention relates to a measurement system specially designed for fiber optic sensor networks, whose optical reflectivity is a function of the parameter measured by the sensor, through the use of tunable lasers of continuous wave half conductor.

The system is designed to interrogate a group of sensors connected to a remote measurement device through a passive optical network.

More specifically, the object of the invention affects two particular aspects, on the one hand obtaining a method of interrelation of passive fiber optic sensor networks that can use as a light source a single tunable semi-duct laser, which does not require optical modulators external to the laser neither tunable optical filters, and that allows to obtain spatial resolution of nominally equal sensors, and on the other hand obtain a topology of interconnection of optical fiber sensors with which the uncertainty in the measurement of its parameter can be eliminated for temperature variations in the sensors or for drifts in the frequency calibration of the measuring laser.

FIELD OF THE INVENTION This invention has its application in the following fields: - Personal security. Given that one of the applications of passive fiber optic sensors is the monitoring of parameters related to safety, such as the deformation of structures in buildings, the system proposed constitutes an advance in increasing the safety of the occupants of the buildings. buildings Urban control. The proposed system allows the monitoring of the structural safety of buildings by a controlling entity, such as a municipal or autonomous authority, different from the occupants of the building, and without the need for collaboration. The system can be applied even to uninhabited buildings and without connection to the electric power network. Telecommunications The proposed system can be applied in urban environments, using urban fiber optic infrastructure, buildings or premises that house communications equipment, and IT management infrastructure, assets that telecommunications operators usually have. Construction Industry. Given the possible application of the measurement system for the monitoring of structural parameters of buildings, the system also finds a field of use in obtaining historical evolution of medium and long term of these parameters, which can be used by the actors of the Construction Industry.

BACKGROUND OF THE INVENTION Optometric sensor interrogation systems based on reflectometric techniques are described broadly in the article by Kersey and collaborators "Fiber Grating Sensors", published in the IEEE Journal of Light bird Technology, vol. 15, no. 8, August 1997, pp. 1442-1463. Among other systems, the article describes one named by its authors as "WDM / TDM interrogation for parallel networks", which consists of the sending to the light pulse sensors of different wavelengths, and the measurement of the echoes returned to the measuring equipment by the different sensors in the network. Also in the same master article describes a procedure for the elimination of some undesired effects, such as uncertainty about the measurements produced by temperature variations in the sensors, or by drifts in wavelength of the light source of measurement.

The procedures described in the article have found various forms of implementation in commercial or experimental measurement equipment. However, the particular DM / TDM procedure (time division multiplexing / wavelength multiplexing) for parallel networks has not been sufficiently exploited commercially because of the difficulty of obtaining narrow pulses over time with high spectral purity. In fact, and as far as the inventor is aware, the only WDM / TDM systems published to date require the simultaneous use of a pulsed optical source, which generates optical pulses of great optical spectral width, and a tunable optical filter, which is tuned in reception to the particular wavelength that you want to measure at each moment. There are also systems that use optical signals of continuous wave and high spectral purity that are modulated in frequency and amplitude by an external modulator. The former suffer from the defect of being able to measure networks with low optical floor loss margins, since only a fraction of the optical power of the broadband source is used for the measurement, and they require a tunable external optical filter that raises the complexity and price of the measurement equipment. The latter are complex, because they require an external optical modulator, which in the case of amplitude modulation is limited by the extinction ratio of the modulator, and in the case of frequency by the maximum modulation speed that can be applied to the modulator.

DESCRIPTION OF THE INVENTION The mediating system proposed by the invention allows to interrogate sensor networks with TDM / WDM techniques without using optical modulators or tunable optical filters, solving the fundamental problem derived from the absence of such modulators or optical filters, consisting of the difficulty of modulating lasers simultaneously in intensity and wavelength, since, as is known, when a laser is modulated in intensity, its output wavelength is also varied.

By means of the system of the invention only the wavelength is modulated, but achieving the same effect as if the intensity were modulated by impulses.

Specifically, the system consists in using as a interrogation signal a signal of constant amplitude modulated in discrete displacements of wavelength, in which the resulting optical signal is composed of frequencies of two wavelengths, one with a short duration, which defines the length of the wavelength. wave to which it is measured, and another of long duration, at a wavelength outside the range of measurement of reflections.

According to another characteristic of the invention, the optional use of a fixed eliminated band filter, which improves the signal-to-noise ratio in said measuring system, has been provided.

Finally and according to another characteristic of the invention, the effects of the influence of the temperature on the sensors are eliminated, by means of the use of compensating strings of sensors identical to the strings of measuring sensors.

DESCRIPTION OF THE DRAWINGS To complement the description that is being made and in order to help a better understanding of the characteristics of the invention, according to a preferred example of practical realization thereof, an assembly of drawings is included as an integral part of said description. With an illustrative and non-limiting character, the following has been represented: Figure 1 shows a diagram of the operation of the passive network measurement system of fiber optic sensors that constitutes the object of the present invention.

Figure 2 shows the scheme of connection of the sensors in the form of pairs of strings to the same optical divider, to achieve the elimination of the influence of the temperature in the measurement.

PREFERRED EMBODIMENT OF THE INVENTION As previously mentioned, Figure 1 shows the operation principle of the measurement system, for which the following steps are established: - The set of operating wavelengths is divided into two bands: the "on" band and the "off" band. The "on" band includes all those wavelengths in which the resonances, or reflections, of the optical sensors are found or can be found. In the "off" band, however, no resonance frequency of the sensors is found. To interrogate the network (1) at the wavelength belonging to the measurement band "on", it select the laser wavelength (2) to ?? by control (3), and the laser is maintained at that wavelength during time Ti. Ti defines the spatial resolution, res, of the measurement system in the sensor network by means of the expression: Ti < 2. res. n / c, where n: refractive index in optical fiber c: speed of light in vacuum. Once the Ti period has elapsed, the operating wavelength of the laser (2) is changed to? 2, being ¾, 2 a wavelength belonging to the "off" band, and it is maintained at that wavelength during a time interval of duration T2, where T2 must be greater than the time it takes the light to travel the fiber network of round trip. Once the intervals Ti and T2 have elapsed, the measurement can be carried out at another wavelength j, by simply repeating the first two steps, only specifying the first wavelength a ¾.

- The signals reflected in the sensor network are extracted from the fiber that connects the measuring equipment with the network through a circulator or coupler (4), they are transformed into electrics in an electro-optical converter and processed using conventional reflectometry techniques. - It is possible to install, optionally, an optical filter (5) that eliminates the wavelength ¾, 2 at the entrance of the electro-optical converter (6), or at the laser exit and before the circulator (4) . The filter increases the signal-to-noise ratio of the measurement system by eliminating retroreflections from the optical fiber itself during intervals 2.

As a complement to the structure described, the special connection of the sensors (S) shown in FIG. 2 is provided to compensate during the measurement process the undesired effect of the temperature variations in said sensors.

The usual procedure for compensation is to connect in series with the sensors an additional sensor, not subject to the variable or parameter that you want to measure, but at the same temperature as the rest of the sensors. In this way, by comparing the reflection wavelength of the measurement sensors with that of the additional sensor, the variation of the wavelength itself of the variation of the parameter can be extracted.

The connection procedure, which, as just said, is illustrated in Figure 2, consists of the following: - Associate each of the measurement sensors (S) with another reference sensor (S '), connected to it in parallel. The first sensor, called measurement, is exposed to the variations of the parameter to be measured, while the reference sensor is not, but is at the same temperature as the measurement. Deploy the sensors as pairs of strings: one measurement string (Si) (S2) - · · (Sn), and another reference (or 1) (or 2) ... (or n). All strings are nominally equal, and within each string the sensors resonate at different nominal wavelengths. - Connect all the strings, both the measurement ones and the reference ones, to different arms of the same optical splitter (7).

As is usual in the connection of strings of sensors, between the splitter (7) and each of the strings a fiber section is inserted that introduces a delay (8), so that the minimum difference of delays between different strings is higher than ?? / 2. These delays allow to discriminate between nominally equal sensors that are in different strings.

As can be deduced from the foregoing, for the application of the measuring system of the invention it is essential to use tunable semiconductor lasers.

Claims (1)

1. Claims Ia.- Measurement system for passive networks of fiber optic sensors, which using TDM / WDM techniques (time division multiplexing / wavelength multiplexing), is characterized in that it consists of using a constant amplitude signal as a questioning signal modulated in discrete displacements of wavelength, while the resulting optical signal is composed of sequences of two wavelengths, one with a short duration, which defines the wavelength at which it is measured, and another with a longer duration, with a wavelength outside the range of reflections measurement. 2 a.- Measurement system of passive networks of fiber optic sensors, according to claim Ia, characterized in that a fixed eliminated band filter that improves the signal-to-noise ratio is optionally used. 3a.- Measurement system of passive networks of fiber optic sensors, according to previous claims, characterized in that a string of reference sensors collaborates with each string of measurement sensors, with characteristics coinciding with those of the first and also coincident number, in order to eliminate the influence of temperature on the measurement sensors.