SE522291C2 - Arrangement and method for sending a longitudinal displacement pulse along an optical fiber - Google Patents

Abstract

The present invention relates to an actuator arrangement for exciting longitudinal displacement pulses, or waves, in an optical fibre. The arrangement comprises a bracket, a washer element, a piezo member arranged between the bracket and the washer element, and an optical fibre that is engaged with an extending through a passage in the washer element. The piezo member is operative, responsive to an applied voltage, to displace the washer element and the fibre engaged therewith, in the longitudinal direction of the fibre, with respect to the bracket. Thereby, a longitudinal displacement pulse may be excited in the optical fibre. The actuator arrangement may also operate as a sensor for sensing longitudinal displacements in optical fibres. A set-up and a method for repeatedly sending longitudinal displacement pulses through a length of optical fibre is also disclosed.

Description

However, the optical fiber must terminate at the transducer. Accordingly, the above arrangement does not allow any light to propagate from one side of the sensor to the other side.

In many cases, you want to send acoustic waves into an optical fiber without interrupting the light's propagation path.

However, this is not allowed by the arrangement described above. Thus, there is a need for new and improved arrangements for sending longitudinal, acoustic waves, or longitudinal displacement waves or pulses, into an optical fiber.

SUMMARY OF THE INVENTION An object of the present invention is to provide a new and improved arrangement for sending longitudinal displacement waves, or pulses, into an optical fiber or the like, with which arrangement at least the above-identified problem of the prior art is eliminated. This object is achieved by an arrangement according to the appended claims.

The present invention has several further advantageous features, which will become apparent from the detailed description of some preferred embodiments below.

It has surprisingly been found that a longitudinal displacement pulse, or wave, can be excited in an optical fiber by rapidly moving the fiber at a point, even when the fiber is not preloaded. The inertia of the fiber itself is sufficient for an displacement pulse to be excited by said rapid movement of the fibers.

By longitudinal displacement wave is meant a compression / elongation that propagates along the fiber. Sometimes such a wave or pulse is called a longitudinal acoustic pulse.

In one aspect, the present invention provides an actuator arrangement for exciting a longitudinal displacement wave, or pulse, in an optical fiber. - ber. The arrangement comprises a washer element, or collar, with which the optical fiber is engaged. The arrangement further comprises a piezo part, which responds to an applied voltage by displacing said tray element in relation to a (relatively heavy) holder. Due to the fact that the fiber is in engagement with said washer element, said displacement is transmitted to the portion of the fiber which is in engagement, and propagates along the fiber.

Seen from another aspect, the present invention provides a sensor arrangement for sensing longitudinal displacements in an optical fiber.

In some embodiments of the above-mentioned actuator or sensor arrangement, the washer element is not included.

Instead of using a tray element, the fiber rests directly against the piezo part. In such a case, it is preferred to have the fiber prestressed in the direction of the piezo part by a tensile force.

Seen from another aspect, the present invention provides an arrangement, or arrangement, for transmitting a longitudinal displacement pulse through a stretch of an optical fiber. In said stretch of optical fiber, the longitudinal displacement pulse is repeatedly reflected back and forth between two reflection points. The arrangement includes an actuator arrangement and a sensor arrangement. The actuator arrangement is arranged to activate and amplify the longitudinal displacement pulse, and the sensor arrangement is arranged to directly or indirectly measure the time setting (synchronization) of the pulse with the aim of effecting the actuator arrangement in a way that is resonant. with the pulse reflected back and forth between said reflection points. An advantage of the arrangement according to the invention is that a longitudinal displacement pulse can be repeatedly sent along a stretch of optical fiber without the need for the propagation path of the light in the fiber having to be terminated. 10 15 20 25 30 35 522 291 n A Qxs! -, .- v v »n I | n ~ u v »4 In a preferred mode of operation of the above arrangement, the sensor arrangement is arranged to provide an output signal indicating the amplitude of the displacement pulse extending between the two reflection points. Feedback is provided from the sensor arrangement to the actuator arrangement, and activation of said actuator is adjusted so that the amplitude of the longitudinal displacement pulse, as detected by the sensor arrangement, is maximized. Maximum amplitude for the displacement pulse is obtained when the actuator is actuated resonantly with the displacement pulse, i.e. when the actuator arrangement is activated at the same time as the longitudinal pulse passes said actuator. Maximizing the amplitude of the pulse is therefore sufficient for the actuator to resonate with the longitudinal displacement pulse.

In another working method, the time distribution of the displacement pulses is measured, and the time for passage between the two reflection points is determined. Feedback is then given to the actuator arrangement for the purpose of activating said actuator arrangement at the time when the displacement pulse passes said actuator.

The present invention further provides a method of repeatedly transmitting longitudinal displacement waves, or pulses, along a stretch of optical fiber between a first reflection point and a second reflection point.

The present invention is intended to have its primary field of application in systems in which the reflection properties of fiber gratings are altered by means of longitudinal displacement waves, or pulses. Such a system is described in the Swedish patent application with number 0002415-8, which is incorporated herein by reference.

Brief Description of the Drawings In the detailed description below of some preferred embodiments of the present invention, reference is made to the accompanying drawings, in which: 10 15 20 25 30 35 o o un 'I va »ß; .ng 592 91: v: M ~~~ W W “.v ~ # H 2. ., _: en nNP-vj-nz »J .. n, 'nz". .. h ..... u. .I o .. ... en {,'. '“'.. 5 Fig Fig. 1 schematically shows a first embodiment of the present invention, Fig. 2 schematically shows a piezo part consisting of two semicircular parts, Fig. 3 schematically shows a second embodiment of the present invention, Fig. 4 schematically shows a third embodiment of the present invention, and Fig. 5 schematically shows an arrangement for sending longitudinal displacement pulses, or waves, in a stretch of optical fiber.

The drawings show the corresponding parts with the same reference numerals.

Detailed Description of Preferred Embodiments A first preferred embodiment of an actuator arrangement in accordance with the present invention is shown schematically in Figure 1.

Figure 1 shows a cross section of an actuator arrangement 10. As will be explained below, the same structural arrangement can also act as a sensor. The arrangement comprises a holder 11, or support, which is relatively heavy compared to other parts of the arrangement. The arrangement further comprises a tray element 12 and a piezo portion 13. The piezo portion 13 is arranged between the holder 11 and the tray element 12 and is in response to an applied voltage arranged to displace said tray element 12 relative to the holder 11, i.e. to change the distance between the tray and the holder.

A predefined portion of an optical fiber 14 engages the washer member 12 in such a manner that said portion of the optical fiber follows any movements of the washer member. In the embodiment shown in Figure 1, the fiber 14 is arranged in engagement with the washer element 12 by means of a belt 15, or bulb, on the fiber, said belt being clamped in a passage in the washer element. It should be understood, however, that the fiber may be 10 15 20 25 30 35. . 5 2 2 1 5 ': i' zf; = I -: - = .-- f --- = ... f.; -, ... I '..' '.§l -vN l "' 0 n S _ vn = ...:. ¿:; ~ -: - s ":: ~ - * v = f * n. =. I |, n .. '..; a4n o u. now. . , 6 attached to or arranged in engagement with the washer in any suitable manner, provided that the fixed portion of the fiber follows the washer.

The holder 11, the piezo part 13 and the tray element 12 each have a continuous passage which allows the insertion of a fiber 14 therein. The fiber 14 is in engagement with the washer element 12 and extends, in its longitudinal direction, through said passage. Consequently, each movement of the washer 12 will cause a longitudinal displacement of the portion of the fiber attached thereto.

In the embodiment shown, the piezo part consists of two separate parts 13a, 13b, each of which has a substantially semicircular shape. It has been found that because the piezo part consists of two parts, internal stresses and other defensive circumstances can be avoided, whereby excitation of "pure" and well-defined, longitudinal displacement pulses in the fiber is facilitated. A section through the piezo part which consists of two parts 13a and 13b, with the fiber 14 in between, is shown schematically in Figure 2.

In the preferred embodiment, both the holder 11 (or the support) and the washer element 12 are made of brass.

The piezo part 13 is preferably made of PZT (lead zirconate titanate), which is a commercially available piezoelectric material. The optical fiber 14 attached to the washer member 12 has a typical outer diameter of 125 microns and the bulb 15 formed thereon has a typical outer diameter of about 160 microns. The outer diameter of the washer element 12 and of the piezo part 13 perpendicular to the longitudinal direction of the fiber 14 is about 1 mm. The washer element 12 has a thickness, in the longitudinal direction of the optical fiber 14, of about 0.3 mm, and the piezo part 13 (each 13b) thickness in the same direction of about 0.51 mm. and one of the semicircular discs 13a, has a In order to further provide stability to the actuator arrangement 10, a sleeve 16 could be mounted between the holder 11 and the piezo part 13, as schematically shown in Figure 3. 30 35: ou; 10:15. .o un.) Ü: san: - l .- 522 291 a - * šš._ï'-f: .- ë.š't- ° ía-š .- = '= "- *,;:' , 1 z luv.- I :: n J ... '.. f «n | o nu Q ... g u. U-nv-n ~ n .nu oc 7 Although it is preferable to have a tray element on the piezo part, and that the fiber is in engagement with the tray element, it is conceivable that the fiber itself rests directly on the piezo part. , which will be further explained with reference to Figure 4.

Having now described the basic structural features of the actuator arrangement of the present invention, its function will be described in more detail.

It is well known to those skilled in the art that a piezoelectric material can change its dimensions in response to an applied voltage. Similarly, it is well known that a change in the dimension of a piezoelectric material can create an output voltage if electrical connections are arranged in a suitable manner. The piezo part in the actuator arrangement is thus electrically connected to an external voltage unit. By applying voltage to the piezo part, its longitudinal dimension parallel to the fiber can be changed. Consequently, the distance between the holder and the washer element can be changed. It will be appreciated that, due to the relatively large inertia of the holder 11, a sudden change in the distance between the holder 11 and the washer element 12 caused by the piezo part 13, only causes a displacement of the washer element 12 and of the part of the fiber which is engaged hence. The holder 11 can also be permanently attached to an outer casing.

If the change in distance between the holder and the washer is fast enough, the inertia of the optical fiber will, at least to some extent, counteract the displacement. This leads to an excitation of a longitudinal displacement pulse in the fiber.

The structural arrangement described above can also act as a sensor for sensing longitudinal displacements in the optical fiber. A longitudinal displacement extending along the fiber 10 15 20 25 30 35 522 291 ~ | o ø a now and then reaches the washer element, will induce a change in the dimension of the piezo part in the direction parallel to the longitudinal direction of the fiber. Thanks to this, a tension is created by the piezo part. This voltage indicates the magnitude of the compression or elongation of the piezo moiety, and can serve as a sensor output signal corresponding to the amplitude of the longitudinal displacement pulse, or path, in the fiber.

The structural arrangement described above thus provides not only an actuator for exciting a longitudinal displacement pulse, but also a sensor for sensing longitudinal displacement pulses in a fiber. The arrangement is in fact a transmitter / receiver that can be operated in either of the mentioned operating modes.

Figure 4 schematically shows another embodiment of the present invention. In this case, the belt 15 (or bulb) of the optical fiber 14 engages the washer member 12 by resting against the same. In order to achieve good transmission of displacements between the optical fiber 14 and the washer element 12, the fiber is preferably biased by means of a pulling force F, which pulls the fibers 14 in the direction of the washer element 12. Otherwise, the one shown in figure 4 works. the embodiment in the same way as the embodiment shown in Figures 1 and 3. When the fiber is prestressed as in Figure 4, one can also omit the tray element and let the fiber rest directly on the piezo part.

Sometimes such an arrangement may be preferred due to its simple design.

In the following, an arrangement (an arrangement) is described in which both the actuator arrangement and the sensor arrangement described above are used. The arrangement is shown schematically in Figure 5. A typical situation where an arrangement according to Figure 5 is used is when longitudinal displacement waves, or pulses, are used in order to change the reflectivity of a Bragg grating in a fiber. A Bragg grating in a fiber, which 10 15 20 25 30 35. _. .

: -,; -, Q z :: '::. :. .: ';: fff: _-.: ...: _. ..... p IQ 0 ° 'Qv ¥ II | I, 0-c' ui U "0I '_ ~ _.... .. u» - ~.-.- __; t- f- - »---.. -. .... -; - ~; -, _, ._. .... _ »_ _¶ .._...

- I o o o u. Reflects light in an undisturbed state, can be made to let light through, temporarily and only within a narrow wavelength range, when a localized and temporary compression and / or elongation is present in the grating.

The arrangement shown in Figure 5 comprises a holder 11, an actuator arrangement 50 and a sensor arrangement 51, the holder 11 acting as a common support for said actuator 50 and said sensor 51.

In addition, the array comprises a piece of optical fiber 52 having a Bragg grating inscribed in its core. The Bragg grating is usually a chirped grating which, in its undisturbed state, constitutes a continuous, broadband reflector.

Said piece of fiber 52 engages with a washer element in the actuator arrangement 50 and with a washer element in the sensor arrangement 51, for the purpose of exciting longitudinal displacement waves in the fiber and to sense longitudinal displacement waves in the fiber. The bit fiber 52 comprising the Bragg grating is arranged between said actuator 50 and said sensor 51.

An input end of said piece of fiber is connected by means of a first fiber connection 61 to a first piece of standard fiber 53. Said first piece of standard fiber 53 is in turn provided with an incoming fiber connection 54 to which an external input fiber is to be connected. Similarly, an output end of said piece of fiber is connected by a second fiber connection 62 to a second piece of standard fiber 55. Said second piece of standard fiber is in turn provided with an output fiber connection 56 to which an external output fiber is to be connected. It should be noted that there is a continuous light path between said input fiber connection 54 and said output fiber connection 56.

The entire arrangement is enclosed in a protective casing, in which the fiber is arranged in recesses formed in the material of the casing. Preferably, the housing is filled with a protective gas, e.g. nitrogen. With the aim of achieving- 10 15 20 25 30 35 un u. 0 ~ sve-u 'l' "'' '' _ '. ._' '_. N .._'_ .glrn fl v ... nu u t ..., ........-.-..-.-, - '~ -; f -. ._ ... ... --- ---.. ~ ..- .- - 3 ,; g -, - g-.., I ng ii L <of. U 'Q.' 'Ü V i II p.. »N. _." ¿U -z-I- o ø c o v on 10 additional protection for the fiber inside the casing, especially during transport or in the event of mechanical shock, cushions 57 can be provided which protect the fiber. A preferred material for such cushions is so-called air glass (“airglas”). An important purpose of the protective cover is to provide temperature stability, although general mechanical protection is definitely important.

Inside the housing, bit fiber 52 between the actuator arrangement 50 and the sensor arrangement 51 is preferably bent in a semicircle. The radius of curvature of said bend must be large enough so that light in the optical fiber does not leak out. Such a bend can, however, have beneficial effects on the longitudinal displacement wave which propagates along the fiber, which will be further elucidated below.

A preferred working method for the above-mentioned arrangement will now be described.

The sensor arrangement 51 to which the fiber is connected not only has the function of being a sensor, but also functions as a reflection point for longitudinal displacement pulses in the fiber. Thus, when a longitudinal displacement pulse reaches the sensor 51, said pulse will, at least to some extent, be reflected back through the fiber in the direction of the actuator arrangement 50. Similarly, the actuator arrangement 50 acts as another reflection point. Consequently, a longitudinal displacement pulse (an acoustic pulse), once excited in the fiber, will be reflected back and forth between the sensor 51 and the actuator 50 through the bit of fiber 52 including the Bragg grating. In other words, multiple passages for the displacement pulse through the Bragg grating will be achieved.

At each reflection point (i.e. at the sensor 51 and at the actuator 50) a part of the amplitude of said pulse is lost. The pulse will therefore gradually fade away unless it is amplified somewhere. For this purpose, it is preferred that the longitudinal displacement 10 15 20 25 30 35 a. Now lwlqw-.v, 0 n o: .n -u. ,,., - '... u. eu ~ u | uv. | '.f, nu n .: nu. . <.,. , .. u. e: u z .- ,. »', - - .nu u .HI, ll ningspulsen is given some additional energy, i.e. increased amplitude, each time said pulse is reflected at the actuator 50. By sensing the pulse at the sensor 51, feedback can be given for the purpose of driving the actuator 50 in resonance with the displacement pulse which propagates.

Alternatively, the sensor can be replaced with a passive reflector. In this case, the operating mode of the actuator arrangement is switched intermittently from an activation mode to a sensing mode. When the actuator is in sensing mode, the time distribution of the displacement pulses is measured, whereby feedback and calibration are provided for the subsequent work in activation mode.

Two different ways of utilizing the output signal from the sensor arrangement will now be briefly described.

In a first operating mode, the sensor arrangement provides time information regarding the displacement pulses extending between the two reflection points. From the said time information, a passage time between the two reflection points can be determined. Feedback is then given to the actuator arrangement for the purpose of activating the actuator at the time when the longitudinal displacement pulse reaches the actuator after reflection.

In a second operating mode, which is considered to be the currently most preferred mode of operation, the sensor arrangement detects the amplitude of the longitudinal displacement pulse extending between the reflection points. The amplitude is monitored, and the time distribution of the pulses sent into the fiber from the actuator arrangement is adjusted in small steps until the measured amplitude is maximized. When maximum amplitude is obtained, pulses are excited by the actuator in unison with the displacement pulse propagating between the reflectors. Any other time setting for the actuator would give the displacement pulse a lower amplitude. By tuning the activation of the actuator arrangement so that the amplitude of the pulse, as detected by the sensor arrangement, is maximized, said activation will take place in resonance n sun .. 10 15 20 25 30 o o an. I n ao f., Vf '', "'' '' 'n nu .u ø |»,; va -'. 'L' j '' “;" "f * 'Qfv 0 o .ø-. ~ 1- = | 1 ~ -, -, ~, -I - - ~; = ~ v n .. ........... ..- | -. - _ g f- - ~ ng . - ... i »w-vw-f- .--," "'' 3 .I ~ v.., .. vu nn u. .... .n.. u, n -“ nun » Preferably, the amplitude of the longitudinal displacement pulse is continuously monitored during operation, and the output signal from the monitoring is used to adjust the actuator so that it is activated at a rate which gives maximum amplitude to the displacement pulse propagating in the bit fiber, i.e. with a rate that is in unison with the trip time of the offset pulse between the two reflectors.

As mentioned above, beneficial effects can be obtained by providing the fiber with a bend between them (i.e. the sensor). If the fiber is straight between the reflection points, the two reflection points between the actuator and there is a risk that standing waves will develop in it.

By providing the fiber with a bend between the reflection points, the risk of standing waves developing is reduced. However, the propagation of a longitudinal displacement pulse along the bit of fiber is substantially unaffected by said bending. Consequently, noise caused by the development of standing waves is reduced by providing the bit of optical fiber between the actuator 50 and the sensor 51 with a bend.

The radius of curvature of the bend must, of course, be large enough so that light does not leak from the core of the fiber.

The invention has been described above by some preferred embodiments, which are schematically shown in the accompanying drawings. It is to be understood, however, that various changes and modifications are conceivable within the scope of the invention as defined in the appended claims.

Claims (13)

10 15 20 25 30 35 o u n 'nu. ,, _ ø -. .u -J fi. . ',, °' "fr" u n ... n. n ... , I v. -4. '.,, ,, p. V n n- v ... wn. n. .- ann .... ~ ....,: ß fnr fi '' a nu. n m: n u. du: .n-. , __ an ... n v e I o v. o o,., ... n ._ n '.. ° _: n u n e. 13 PATENT REQUIREMENTS Arrangement for sending a longitudinal displacement pulse along a piece of optical fiber (52) between (50) reflection point (51), said arrangement comprising a first reflection point and a second actuator (50) arranged to excite a longitudinal displacement pulse in said bit fiber (52), and a sensor (51) for sensing said longitudinal displacement in said bit fiber, said first reflection point and said second reflection point being arranged to reflect said pulse excited by the actuator, and wherein the actuator is further arranged to amplify said pulse as it passes the actuator, based on feedback provided by said sensor, which optical fiber engages and extends through a respective passage in said actuator and said sensor. An arrangement according to claim 1, wherein the actuator (50) constitutes the first reflection point and the sensor (51) constitutes the second reflection point. The arrangement of claim 2, further comprising a feedback loop from the sensor to the actuator, wherein an output signal from the sensor in said feedback loop causes the actuator to amplify the longitudinal displacement pulse at the time said actuator is reached by a previous displacement pulse reflected from the sensor. An arrangement according to any one of claims 1 to 3, wherein the optical fiber (52) engages the actuator (50) and the sensor (51) by a bulb (15) on said fiber. ...., ..,: _ - - ..: en: u-: fn-.zuh en .., "u .. .H - - --- ~ fl¿ J.“ n; .'- U. " 14 is clamped to a respective washer element (12) of the actuator and the sensor. Arrangement according to claim 4, wherein the actuator (50) comprises a piezo part (13) which is arranged to displace the washer element (12) and the optical fiber (52) which is in engagement therewith in response to an applied voltage. An arrangement according to claim 5, wherein the piezo part (13) comprises two separate parts (13a, 13b) each having the general shape of a semicircular disc. An arrangement according to claim 4, wherein the sensor (51) (13) on a longitudinal displacement pulse in the optical comprises a piezo part which is arranged to provide in response the fiber (52) a voltage indicating the amplitude of said displacement pulse. An arrangement according to claim 7, wherein the piezo part (13) comprises two separate parts (13a, 13b) each having the general shape of a semicircular disc. An arrangement according to any one of claims 1 to 8, wherein the optical fiber (52) is provided with a bend between the actuator (50) and the sensor (51). A method of repeatedly transmitting longitudinal displacement pulses through a piece of optical fiber (52) between a first reflection point and a second reflection point, said piece of fiber engaging and extending through a respective passage in the actuator (50) steps: and a sensor (51), which method comprises sending by means of said actuator a longitudinal displacement pulse into said piece of optical fiber, by means of said sensor detecting the amplitude of said displacement pulse, and ï-: IEQI-'ïjæ: ršgïiš-.Jšçtší fl šgví: fy- š-ss 15 to amplify by means of said actuator the displacement pulse in said piece of optical fiber at the time when it reaches said actuator after one or more reflections, the time setting being based on said sensor. 11. ll. The method of claim 10, further comprising the step of adjusting the timing of longitudinal pulses sent into the bit of optical fiber, based on the detected amplitude of said displacement pulse. The method of claim 11, wherein the step of adjusting the time setting is performed in small steps until the detected amplitude is maximized. The method of claim 12, wherein the step of adjusting the time setting is performed repeatedly.