DESCRIPTION

Field of the invention

The present invention relates to force measurement. In particular, the present invention relates to measurement of mechanical forces such as, for example, transversal forces (pressure) and rotational forces (torsion) applied to an optical fiber.

More in detail, the present invention relates to measurement of forces by means of optical transducers. Even more in detail, the present invention relates to measurement of forces by means of optical transducers and/or sensors comprising optical fibers.

Finally, the present invention relates to a method and a system for measuring forces, said system comprising optical transducers and/or sensors and/or optical fiber sensors.

Description of the state of the art

In the recent past numerous attempts have been made to overcome the typical defects of electronic systems for force measurement.

In particular, in recent years considerable efforts have been dedicated to the development of optical transducers as substitutes for electronic transducers.

The optical transducers known in the art are based on the consideration that forces, in particular mechanical forces such as pressure or torsion forces, can be measured and/or detected by evaluating the effects caused on the light transmitted through an optical path by a force acting, whether directly or indirectly, upon said optical path.

In particular, the principle of operation of most optical transducers known in the art exploits the variation occurring in the photocurrent detected at the output of an optical path as a function of the optical signal attenuation generated by the variation in the optical connection or by signal interference, which are controlled by the force to be detected. It is in fact always possible to establish a relationship between the photocurrent detected at the output of an optical path and the mechanical stress acting upon the optical path. In particular, some known optical transducers utilize the variation occurring in the polarization of optical fibers due to microstresses induced and caused by an external force applied thereto.

However, also the known optical transducers based on polarization change suffer from defects or problems that limit the use thereof to a few applications.

Moreover, most of the results of the measurements taken by means of these known optical transducers are not as reliable as desired. Finally, fabrication and assembly of these known optical transducers have proven to be rather complex, and hence quite costly, because they require very tight mechanical tolerances.

Most drawbacks or defects that affect the optical transducers known in the art can be traced back to the fact that optical transducers are based on interference of two orthogonal polarization modes, and hence require special, highly birefringing fibers and very precise mechanical enclosures. More in detail, the known optical transducers are based on the detection of polarization variations in the light beam transmitted through a birefringing optical fiber.

The application of a mechanical stress to the optical fiber produces a considerable number of interference fringes, and a phase measurement system is used to determine the change in terms of birefringence induced by the mechanical stress, so that it is then possible to go back to the mechanical stress value.

It is thus possible to establish a relationship between the optical signal at the output of the optical path and the forces acting upon the optical path.

It follows, therefore, that it is possible to establish a relationship between the power and/or intensity of the optical signal at the output of the optical path and the force acting upon the optical path.

Furthermore, since the optical signal at the output of the optical path can be converted into a current and/or voltage measurement, it will also be possible to establish a relationship between the measured current or voltage and the force acting upon the optical path.

Although the force detection principle may appear to be rather general, it has nevertheless proven to be very reliable for detecting and/or measuring forces, in particular mechanical forces such as, for example, pressure and torsion forces.

Document EP 1 748 284 Bl describes an optical transducer for detecting forces acting upon the transducer.

The transducer comprises an optical path adapted to transmit optical signals, wherein the optical path comprises sensor means adapted to modify the transmission of the optical signals along the optical path as a result of a force acting upon them; the optical path further comprises polarization scrambling means adapted to render the polarization of the optical signals entering the sensor means parallel to a first predefined direction, the sensor means being also adapted to modify the polarization of the optical signals entering the sensor means as a result of a force acting upon them; the transducer further comprises polarizing means adapted to collect the optical signals exiting the sensor means and having an axis of polarization parallel to a predefined direction, so as to allow only those optical signals which have a polarization that is parallel to the axis of polarization to exit the polarizing means.

With reference to Figure la, a first example of a known optical transducer will now be described.

In Figure la, reference numeral If identifies an optical path (e.g. an optical fiber) adapted to receive, transmit and emit an optical signal.

In particular, in Figure la the optical signal entering the optical path If is identified by reference numeral 2a, whereas the optical signal exiting the optical path is identified by reference numeral 2b.

The optical transducer 1 of Figure la comprises also polarization scrambling means la, sensor means lb and polarizing means lc.

The polarization scrambling means la, the sensor means lb and the polarizing means lc may comprise, or even be defined by, one or more loops laa, lbb and lcc, respectively, of the optical fiber If

Finally, in Figure la reference numeral 5 identifies a force and/or a mechanical stress (e.g. a pressure force or a rotation force) acting upon the transducer 1, in particular upon the sensor means lb of the transducer 1.

The expression "polarization scrambling means" refers to means suitable for modifying the polarization of an optical signal 2a (e.g. a luminous signal) entering the polarization scrambling means la (after having entered the optical path or optical fiber If), so as to render the polarization of the optical signal exiting the polarization scrambling means parallel to a predefined axis or a predefined direction.

Furthermore, the expression "polarizing means" refers to means suitable for emitting optical signals with a predefined polarization.

For example, the polarizing means lc of Figure la may comprise an axis of polarization parallel to a predefined direction, so as to allow only those optical signals which have a polarization parallel to said axis of polarization to exit the polarizing means lc, while any optical signals having a polarization that is not parallel to the axis of polarization of the polarizing means will be either absorbed or reflected.

It should therefore be appreciated that the polarizing means lc included in the optical transducer 1 of Figure la behave like some sort of optical filter, because only those optical signals which have a polarization parallel to the axis of polarization can propagate along the polarizing means lc and be emitted, whereas any optical signals having a polarization that is not parallel to the axis of polarization of the polarizing means lc will be absorbed, and therefore neither transmitted/propagated along the polarizing means nor emitted from the polarizing means lc.

If it is appreciated that the optical signal 2a entering the optical path If normally comprises different components having as many different characteristics, in particular as many different wavelengths and polarizations, it can also be appreciated that only those components entering the polarizing means lc which have a polarization that is parallel to the axis of polarization will be propagated and transmitted through the polarizing means lc and then emitted.

The expression "sensor means" refers to means adapted to modify the transmission of optical signals entering the sensor means lb.

In particular, the expression "sensor means" indicates means adapted to modify the polarization of optical signals entering the sensor means lb as a function of a force (e.g. pressure or torsion, or even a mechanical stress) acting upon the sensor means.

For example, in the case wherein the sensor means lb comprise one or more loops lbb of an optical fiber, it will be appreciated that the forces acting upon the sensor means (and hence upon one or more of said loops of optical fiber, whether directly or indirectly) will be able to deform or even damage one or more loops of the optical fiber; the microdeformations ensuing in the fiber as a result of the mechanical stress acting upon the fiber will produce variations in the polarization of the optical signals transmitted and propagated through the sensor means. The variations in the polarization of the optical signals can be related to the forces or stresses acting upon the sensor means lb.

The principle of operation of the optical transducer 1 represented in Figure la can be summarized as follows. In the absence of any applied forces 5, the polarization scrambling means la are programmed to define the optical signals 2b exiting the optical transducer 1; in other words, through the polarization scrambling means la, the polarization of the optical signal 2a entering the optical path If will be modified until the polarization of the optical signal exiting the polarization scrambling means la will be parallel to a predefined direction. For example, the polarization of the optical signal exiting the polarization scrambling means la may be made parallel to the axis of polarization of the polarizing means lc. Alternatively, still by means of the polarization scrambling means la, the polarization of the optical signal exiting the polarization scrambling means la may be made orthogonal to the axis of polarization of the polarizing means lc. In the former case, i.e. when the polarization of the optical signal exiting the polarization scrambling means la is made parallel to the axis of polarization of the polarizing means lc, the optical signal 2b exiting the optical path If (from the polarizing means lc) will essentially correspond to the optical signal 2a at the input of the optical path If or, in other words, the intensity of the output optical signal 2b will substantially correspond to that of the input optical signal 2a, except for negligible power losses due to inevitable imperfections of the optical path. It follows, therefore, that the maximum or most intense optical signal 2b will be collected at the output of the optical transducer 1. On the contrary, in the case wherein the polarization of the optical signal 2a entering the optical transducer 1, and hence also the one exiting the polarization scrambling means la, is made orthogonal to the axis of polarization of the polarizing means lc, still assuming that no force or stress 5 is applied to the transducer 1 (to the sensor means lb), then essentially no signals, or the weakest signals 2b, will be collected or received at the output of the optical transducer 1.

In the presence of a force or stress acting upon the transducer (upon the sensor means lb), on the contrary, the transmission of the optical signals along the optical path If will be modified as a result of the force or stress 5. In particular, the polarization of the optical signals exiting the polarization scrambling means will change because of the force 5, e.g. due to microdeformations produced in the sensor means lb (e.g. in a portion of one or more loops lbb).

This means that at least some components of the optical signal exiting the sensor means lb (and then entering the polarizing means lc) will have a different polarization compared to the corresponding components of the optical signal exiting the polarization scrambling means; in other words, the polarization of at least some components of the optical signal exiting the sensor means lb will be different from the polarization of the corresponding components exiting the polarization scrambling means la because, as explained, polarization has been made parallel to a predefined direction. In brief, the polarization of at least some components of the optical signal exiting the sensor means lb will no longer be parallel to the predefined direction.

It follows that, in the case wherein the polarization of the optical signal exiting the polarization scrambling means la has been made parallel to the axis of polarization of the polarizing means lc, the polarization of at least some components of the optical signal exiting the sensor means lb will no longer be parallel to the axis of polarization of the polarizing means lc. It follows that also the intensity of the optical signal 2b exiting the polarizing means lc (the optical path If) will be lower than the intensity of the optical signal 2a entering the optical path If and exiting the polarization scrambling means la. The difference between the intensities of the optical signals 2b and 2a will depend on the intensity of the force 5 applied to the sensor means lb, so that it will be possible to relate the difference between the output signal 2b and the input signal 2a to the intensity of the force 5. Likewise, in the case wherein the polarization of the optical signal exiting the polarization scrambling means la is made orthogonal to the axis of polarization of the polarizing means lc, at least some components of the optical signal exiting the sensor means lb will have a polarization that will no longer be orthogonal to the axis of polarization of the polarizing means lc because of the force or stress 5 acting upon the sensor means lb. It will then be possible to collect the optical signal 2b exiting the optical transducer 1 (the polarizing means) and establish a relationship between the intensity or power of the signal 2b and the intensity of the force or stress 5 acting upon the transducer 1.

The polarization scrambling means la may comprise a first portion of the optical fiber If; in particular, the polarization scrambling means may comprise one or more loops laa of the optical fiber If Likewise, the sensor means lb may comprise a second portion of the optical fiber If; in particular, the sensor means lb may comprise one or more loops lbb of the optical fiber If

With reference to Figure lb, the following will describe one example of the electric diagram of a measuring device; in Figure lb, any parts and/or features already described with reference to Figure la are identified by the same reference numerals.

In Figure lb, reference numerals 6a and 6b identify an optical signal emitter device and an optical signal receiver device, respectively. The device 6a generates and/or emits optical signals that enter the optical path If and propagate or are transmitted through the optical path defined by the polarization scrambling means la, the sensor means lb and the polarizing means lc, and may be partially or totally absorbed by the polarizing means lc. The resulting optical signals at the output of the polarizing means lc are received and/or collected by the receiver device 6b. For example, the device 6a may comprise a current or voltage generator connected to a laser light source; likewise, the receiver device 6b may comprise an amplified photodetector with a photodiode followed by a low-noise amplifier. Anyway, different solutions may be adopted for the purpose of generating optical signals to be sent along the optical path If and receiving optical signals at the output of the optical path If

With this type of transducers, it is only essential that the current or voltage signals are converted into optical signals 2a to be inputted to the transducer 1, and that the optical signals 2b exiting the transducer 1 are converted into either current signals or voltage signals, so that the resulting current and/or voltage signals can be processed in order to detect the force 5 acting upon the transducer 1.

Therefore, a need is felt for simpler solutions allowing for a further reduction of the implementation costs.

Object and summary

It is therefore the object of the present invention to propose a method and a system for measuring forces by means of an optical transducer.

Brief description of the drawings

Further advantages, objects, features and embodiments of the present invention are set out in the claims and will become apparent in the light of the following detailed description provided with reference to the drawings, wherein identical or corresponding parts are identified by the same reference numerals. In the drawings:

- Figures la and lb schematically represent the principle of operation of a known optical transducer and provide a schematic view of the electric layout of a measuring device making use of the known transducer of Figure la;

- Figures 2a,2b,2c,2d and 2e show a detail of the receiver according to the present invention and graphs illustrating the trend of the powers outputted by the receiver. Detailed description

The following description will illustrate various specific details aimed at fully understanding some examples of one or more embodiments. The embodiments may be implemented without one or more of such specific details or by using other methods, components, materials, etc. In other cases, known structures, materials or operations will not be shown or described in detail to avoid overshadowing various aspects of the embodiments. Any reference to "an embodiment" in this description will indicate that a particular configuration, structure or feature described with reference to that embodiment is comprised in at least one embodiment. Therefore, phrases such as "in one embodiment", which may be present in different parts of this description, will not necessarily be all related to the same embodiment. Furthermore, any particular configuration, structure or feature may be combined in one or more embodiments as deemed appropriate.

The references below are therefore used only for simplicity's sake and do not define the protection scope or extent of the embodiments.

While the present invention will be clarified below by means of a detailed description provided with reference to the drawings of some embodiments thereof, it must nevertheless be pointed out that the present invention is not limited to the embodiments described herein and represented in the drawings, since the embodiments described and illustrated herein exemplify the various aspects of the present invention, the scope of which is set out in the claims.

The present invention has proven particularly advantageous when used for detecting and/or measuring transversal and rotational forces, such as, for example, pressure and torsion forces. For this reason, the following will describe some examples wherein embodiments of an optical transducer according to the present invention are used for detecting and/or measuring pressure and torsion forces.

It should however be noted that the use of the optical transducers according to the present invention is not limited to detecting or measuring pressure or torsion forces; on the contrary, the optical transducers according to the present invention can also be used for measuring and/or detecting different forces acting thereupon. The present invention is therefore useful also for measuring all such forces, and the transversal (pressure) and/or rotational (torsion) forces described below will represent forces of any kind acting upon the transducers.

The state of the art is defined by patent EP 1 748 284 Bl, which describes a solution that stands out as an evolution providing simplification and, in perspective, a reduction in the costs to be incurred for implementing a measurement system.

The application scope is the same as described in patent EP 1 748 284 B l, but some modifications have been conceived which allow the costs of the overall system to be further reduced.

In particular, one embodiment of the measurement system according to the present invention uses an optical source on the source side, e.g. a single-mode laser. More in detail, said single-mode laser must not rigorously be one with known polarization. Therefore, the transmitter side can use a less complex source. This is because the polarization controller and/or the polarizer employed in the known solution described in patent EP 1 748 284 Bl are no longer necessary, and a simpler source can be used.

The sensitive fiber used for implementing the optical path and the sensor means remains unchanged from the known solution.

For example, in various embodiments the sensitive fiber is selected among single-mode optical fibers.

On the receiver side, instead of a polarizer, a polarizing beam splitter (PBS) device is used, i.e. a device having one input IN and two separate outputs Ul and U2.

In particular, with reference to Figure 2a, the splitter device PBS has one input IN, whereat the signal arrives from the optical path, which signal comprises two polarization components x and y along the two main axes of the fiber, and two outputs Ul and U2. On the first output branch Ul there is the component x, and on the second output branch U2 there is the component y of the signal.

More in detail, the two outputs Ul and U2 of the splitter device PBS are connected to two receiver devices, e.g. two photodiodes PD1 and PD2, which convert the optical signal into an electric signal. The two electric signals outputted by the photodiodes PD1 and PD2 can be processed in a simple manner, e.g. by means of a direct voltage or current measurement. The signal inputted to the splitter device PBS will have a total power Ρτοτ flowing along the fiber. The total power Ρτοτ is split between the two fundamental polarizations, i.e. the components x and y, which are parallel to the two main axes of the fiber. Therefore, each component x and y will have a corresponding power Px and Py, and the sum of the two powers will always be equal to the total power.

The total power Ρτοτ satisfies, therefore, the following mathematical relation:

i.e. the total power Ρτοτ is the sum of the powers of the two signal components x and y of the main axes of the fiber.

The total power Ρτοτ can also be expressed as:

Ρτοτ=αΡτοτ +βΡτοτ

where, anytime and anywhere along the fiber, considering as negligible the attenuation caused by propagation, i.e. the overall power loss is not determinant for the result, the total power PTOT is constant, i.e. the relation α+β=1 is true.

A stress of any kind acting upon the fiber (vibration, shock, strain, temperature variations, etc.) will result in changes in all the other relationships between the coefficients a and β. In particular, their sum will always be constant (α+β=1), but by observing, for example, the variation occurred in the individual a and β or the difference (α-β) or the ratio (α/β) or the product (α*β) thereof, it will be possible to discover that a phenomenon has affected the fiber, and also to evaluate the extent of the stress. Other relationships are possible as well.

If one is only interested in a qualitative evaluation of the stress (i.e. if anything has happened, e.g. the fiber has received a shock), it will be sufficient to detect the occurrence of a relative variation in the values of the coefficients a and β (difference, ratio or product) by processing the electric signal downstream of the photodiodes PDl and PD2. In fact, the electric signals outputted by the photodiodes PDl and PD2 are proportional to the respective powers Px and Py associated with the different components x and y. In particular, the output signal of the photodiode PDl will be proportional to the power Px, while the output signal of the photodiode PD2 will be proportional to the power Py.

If, vice versa, a quantitative evaluation is of interest, it will be first necessary to perform a calibration of the system to associate a value of the phenomenon with each pair of coefficients (α, β), e.g. in a lookup table stored in a digital system.

Other types of processing are also possible, depending on the information that one wishes to obtain.

For example, if one only wants to measure fast variations and filter out slow variations (e.g. due to temperature variations) from the signal, a high-pass filter, whether analogue or digital, can be inserted downstream of the photodiode (PDl and PD2). Vice versa, if one only wants to measure slow variations and filter out the outputs due to fast variations (e.g. when a temperature variation is to be measured, and one wants to avoid that a vibration or a shock might interfere with the result), a low-pass filter, whether analogue or digital, can be inserted downstream of the photodiode (PD1 and PD2).

One possible example of qualitative application is blastproof glass equipped with a fiber for stress monitoring. In this case, the stress extent is unimportant, since it will suffice to know if a stress has occurred.

Referring back to Figure 2a, said two outputs Ul and U2 may change over time as a consequence of the stresses undergone by the sensitive fiber that constitutes the optical path.

With this solution there is no need for a polarization controller on the transmission side; therefore, any laser source can be used on the transmission side without the need for knowing a priori the polarization of the generated signal.

Upon reception, the incoming signal x+y is split according to its two fundamental polarizations, parallel to the main axes of the fiber, resulting in two power components Such components x and y of the signal are received by the two photodiodes PD1 and PD2, which measure their respective powers Px and Py (see graphs in Figures 2b and 2c). In particular, the sum of the powers Px and Py of the two components x and y of the signal is always constant (except for negligible losses). Vice versa, the ratio (or difference or product) between the powers Px and Py of the two components x and y of the signal will change because of external stresses.

Therefore, in the expression Ρτοτ= *Ρτοτ+β*Ρτοτ the coefficients a and β change in time and space along the fiber. In particular, with reference to Figures 2b and 2c, it can be noticed that the sum of Px and Py is one at all instants, but, for example, at the instant 17 both coefficients a and β vary (see Figures 2b and 2c) and their ratio changes to indicate the effect of a stress. As a consequence, analyzing by way of example the graphs of Figures 2b and 2c, it is possible to detect that at the instant 17 a stress occurred, and that the power Px dropped from its constant value 0.7 to the new value 0.2, while the power Py grew from the constant value 0.3 to the peak value 0.8. As can be seen in the graphs shown in Figures 2b and 2c, the sum of the two coefficients a and β is always equal to 1 (see Figure 2d), but, for example, the difference between them varies over time (see Figure 2e). In particular, for example, at the instant 6 the difference is α-β= 0.7-0.3= 0.4, while at the instant 17 it is α-β= 0.2-0.8= -0.6).

The advantages of this solution over the prior-art solution described in the above- mentioned patent are the following:

- polarization control on the transmitter side is not necessary; the architecture of the optical source can thus be simplified, resulting in lower costs;

- two outputs are available on the receiver side, the sum of which has a constant power, and the variations in the coefficients a and β indicate that a stress has occurred.

This increases the possible measurement dynamics. In fact, in the prior-art solution that makes use of a polarizer, part of the input power was reflected, and therefore the output power was not maximum because a part was lost.

Unlike the polarizer solution, the solution proposed herein, which comprises the splitter device PBS and the two photodiodes PD1 and PD2, can be easily implemented as integrated optics (e.g. silicon photonics). This is because at present polarization control is hardly implementable as integrated optics, e.g. on platforms such as silicon.

On the contrary, the device PBS and the photodiodes PD1 and PD2 are classic discrete components that can be more easily implemented as an integrated optical circuit.

In particular, all three discrete components can be implemented on a single platform, i.e. on a single integrated device implemented as integrated optics. This will result in a reduction in the space required for implementation and in lower costs.

As an alternative, the three components may be implemented separately and then used for building the system.

According to an alternative implementation, the PBS and one photodiode are integrated on the same device, whereas the second photodiode is a separate component. The implementations described herein are based on the assumption that all the fiber in use is sensitive.

However, an implementation in which all the fiber is sensitive might be inefficient, depending on the targets of the system.

In an alternative embodiment, in order to limit the length of the section of sensitive fiber, a laser is used on the transmitter side which has a polarization-maintaining (PM) output, followed by a PM fiber section with polarization tuned to the laser output (it is just a matter of purchasing the proper fiber, since no polarizer is needed between the two). PM fiber is a fiber wherein symmetry is broken to such an extent (i.e. it has such an elliptical core) that one polarization along a main axis is maintained up to the exit. Since the second polarization is never excited, no scattering of the PMD polarization modes occurs.

The PM fiber will then be connected to the portion of sensitive fiber, followed by the splitter device PBS and the two photodiodes PD1 and PD2.

In an alternative embodiment, if it is necessary to keep the photodiodes PD1 and PD2 remote from the PBS, the connection between them can be effected by means of sections of optical fiber of any length. In this manner, the interrogator will be kept at a distance from the section of sensitive fiber. Thus, the photodiodes may be located at a distance of kilometres from the PBS. When implemented through discrete components as opposed to an integrated circuit, the PBS and the two photodiodes will still be connected by means of a fiber, usually a short one.

Although the present invention has been described herein with reference to particular embodiments thereof, it must be underlined that the present invention is not limited to the particular embodiments described herein and illustrated in the drawings, since the present invention comprises also all those variants and/or modifications of the embodiments described herein and illustrated in the drawings which fall within the scope of the claims.