LINEAR CAVITY RING DOWN DEVICE

Abstract

Fibre linear cavity ring down device for decay time-based attenuation sensing, comprising a bi-directional fibre optic coupler having two fibre port coupling sides, a left-hand port side and a right-hand port side, and a port mirror; wherein signal input on one of said sides is coupled to signal output on the other of said sides; wherein the left-hand port side comprises a first fibre port (port 1) for coupling to an optical light source, the left-hand port side comprises a second fibre port (port 2) coupled to said port mirror (mirror 2), and the right-hand port side comprises a third fibre port (port 3) for coupling to a sensor fibre comprising one or more sensors and optically terminated by a sensor fibre mirror (mirror 1). The optical light source may be also an optical light receiver. The optical light source and receiver may preferably be an optical time domain reflectometer.

Claims

1-19. (canceled)

20. A fiber linear cavity ring down device for decay time-based attenuation sensing, comprising a bi-directional fiber optic coupler having two fiber port coupling sides, a first port side and a second port side, an optical time domain reflectometer and a port mirror; wherein the first port side comprises a first fiber port, and a second fiber port coupled to said port mirror, wherein the second port side comprises a third fiber port for coupling to a sensor fiber comprising one or more sensors and optically terminated by a mirror of said sensor fiber, and a fourth fiber port, and wherein either the first fiber port or the fourth fiber port is coupled to the optical time domain reflectometer as an optical light source and receiver.

21. The fiber linear cavity ring down device according to claim 20, wherein the bi-directional fiber optic coupler comprises two coupled optical fibers: a first optical fiber between the first fiber port and third fiber port, and a second optical fiber between the second fiber port and fourth fiber port.

22. The fiber linear cavity ring down device according to claim 20, wherein the fiber optic coupler has a coupling ratio between the second fiber port and the third fiber port of more than 50%.

23. The fiber linear cavity ring down device according to claim 20, wherein the coupler is a 12 bi-directional fiber optic coupler or a 22 bi-directional fiber optic coupler on which a fourth fiber port is unused.

24. The fiber linear cavity ring down device according to claim 20, wherein the sensor fiber mirror is also a sensor of the sensor fiber.

25. The fiber linear cavity ring down device according to claim 20, further comprising said sensor fiber.

26. The fiber linear cavity ring down device according to claim 20, further comprising a bidirectional amplifier placed in the optical path between the port mirror and the mirror of the sensor fiber.

27. The fiber linear cavity ring down device according to claim 26, wherein the amplifier is one of an erbium doped fiber amplifier, a semiconductor optical amplifier, a Raman amplifier, and a Brillouin amplifier.

28. The fiber linear cavity ring down device according to claim 20, wherein the respective mirrors are independently chosen from the group consisting of: reflective thin-films, fiber loop mirrors, external mirrors, faraday rotators, and fiber Bragg grating structures.

29. A method for using a fibre linear cavity ring down device as a decay time-based attenuation sensing device, wherein the fibre linear cavity ring down device comprises a bi-directional fibre optic coupler having two fibre port coupling sides, a first port side and a second port side, and a port mirror; wherein the first port side comprises a first fibre port, and a second fibre port coupled to said port mirror; and wherein the second port side comprises a third fibre port for coupling to a sensor fibre comprising one or more sensors and optically terminated by a mirror of said sensor fibre, and a fourth fibre port, the method comprising: coupling either the first fibre port or the fourth fibre port to an optical time domain reflectometer for use as an optical light source and receiver.

30. The method according to claim 29, wherein the bi-directional fiber optic coupler comprises two coupled optical fibers: a first optical fiber between the first fiber port and third fiber port, and a second optical fiber between the second fiber port and fourth fiber port.

31. The method according to claim 29, wherein the fiber optic coupler has a coupling ratio between the second fiber port and the third fiber port of more than 50%.

32. The method according to claim 29, wherein the coupler is a 12 bi-directional fiber optic coupler or a 22 bi-directional fiber optic coupler on which a fourth fiber port is unused.

33. The method according to claim 29, wherein the sensor fiber mirror is also a sensor of the sensor fiber.

34. The method according to claim 29, wherein the fiber linear cavity ring down device comprises said sensor fiber.

35. The method according to claim 29, wherein the fiber linear cavity ring down device comprises a bidirectional amplifier placed in the optical path between the port mirror and the mirror of the sensor fiber.

36. The method according to claim 29, wherein the amplifier is one of an erbium doped fiber amplifier, a semiconductor optical amplifier, a Raman amplifier, and a Brillouin amplifier.

37. The method according to claim 29, wherein the respective mirrors are independently chosen from the group consisting of reflective thin-films, fiber loop mirrors, external mirrors, faraday rotators, and fiber Bragg grating structures.

38. The method according to claim 29, wherein the fiber linear cavity ring down device comprises said optical time domain reflectometer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0057] The following figures provide preferred embodiments for illustrating the description and should not be seen as limiting the scope of invention.

[0058] FIG. 1: Schematic representation of embodiments of a linear CRD configuration, in particular (a) wherein both input and output are in port 1, (b) wherein both input and output are in port 4, (c) wherein input is in port 1 and output is in port 4, and (d) wherein input is in port 4 and output is in port 1.

[0059] FIG. 2: Output signal of the linear CRD obtained with a)-c) thin-film mirrors and d)-f) fibre loop mirrors. Note that FIG. 2(g) pertains to the embodiments where input and output are not in the same port.

[0060] FIG. 3: Output signal of a linear CRD for remote sensing, using a fibre length with a).sup.800 m, b).sup.5.5 km, and c).sup.11 km. Traces in (I) present linear response while in (II) correspond to the exponential behaviour.

[0061] FIG. 4: Decay time (ns) versus fibre length (km) used in port 3 of the linear CRD.

[0062] FIG. 5: Output signal of the linear CRD for no strain and strain applied (800 .sub.) to the chirped-FBG. a) Trace directly acquired by the OTDR, b) trace after signal processing and c) the corresponding exponential response.

DETAILED DESCRIPTION

[0063] The schematic of an embodiment of the disclosed fibre linear CRD configuration is presented in FIG. 1. Such configuration includes a single fibre coupler, with high splitting ratio, and two thin-film mirrors located at the end of ports 2 and 3. An OTDR is placed in port 1 and serves as both input and output of the system.

[0064] The basis of such configuration relies on the following operation principle: the OTDR (centred at 1550 nm) send pulses into the linear cavity, which is formed by an optical fibre coupler with a splitting ratio of, for example, 99:1. Each pulse enters in the cavity by means of 1% arm of the fibre coupler (port 3) and is back-reflected in mirror 1; then, passes through 99% arm of the fibre coupler (port 2) where is back-reflected again in mirror 2. Bidirectional amplification is also required for remote sensing for example in port 2 due to the low amount of light that travels in this fibre arm. In this case, an amplifier, for example an Erbium Doped Fibre amplifier (EDFA), was inserted in the linear cavity to provide an observable signal at the output. In this manner, the amplitude of the pulses will slowly decay as it travels inside the linear cavity, similar to the behaviour of the conventional CRD using a cavity ring. In order to use the proposed configuration for remote sensing, a roll of fibre can be placed in port 3 before the sensor head and the mirror.

[0065] One of the advantages of this CRD configuration is that a single fibre coupler with highly reflective mirrors at the end of the fibre arms forms the linear cavity. In addition, the use of an OTDR located in port 1 serves as both input/output of the transmitted/reflected signal. Therefore, the oscilloscope (and photodetector) at the output (port 4) is no longer needed to interrogate the sensing head as the OTDR serves that purpose (as already demonstrated in [11]). This is one of the major advantages of the proposed CRD configuration, combining the fact that the output signal acquired by the OTDR provides measurements in dB, which allows attaining the decay time (s) with a linear response (rather than an exponential behaviour obtained by the oscilloscope). In addition, an increase of the sensitivity is achieved because the light passes twice by the sensor [13]. Another advantage is that the mirrors can be based on various technologies, such as on highly reflective thin-films, fibre loop mirrors or even FBGs.

[0066] In order to validate the disclosure, three distinct types of mirrors were placed at the end of ports 2 and 3, namely, thin-films, fibre loop mirrors and FBGs. The FBG placed in port 3 was used as sensing element as well. For the proof-of-concept, the OTDR was set with an operation wavelength at 1550 nm and it was used to send pulses with 200 ns-width into the linear cavity. A fibre bobbin with .sup.800 m was also used in port 3 (see FIG. 1).

[0067] FIG. 2 shows the trace observed in the OTDR when different mirrors are used, namely, the thin films (FIGS. 2a-c) and the fibre loop mirrors (FIGS. 2d-f), for configurations corresponding to FIGS. 1(a) and (b), where input and output are in the same port. FIG. 2a) presents the trace directly observed on the OTDR, while FIGS. 2b) and c) present the trace after signal processing. The decay time of the proposed linear CRD is 21.47 ns and 30.81 ns for thin-film mirrors and fibre loop mirrors, respectively. Note that FIG. 2(g) pertains to the embodiments where input and output are not in the same port such as FIGS. 2(c) and (d). In FIG. 2(g), the relationship between peak 1 and peak 2 is a ratio of 99:1. However, this signal is only visible in the oscilloscope with amplification, as shown in FIG. 2(g), showing the viability of the configuration (i.e. the first signal peak is not amplified, whereas the other peaks are amplified).

[0068] The linear CRD was also demonstrated for remote sensing. In this case, thin-film mirrors were used at the end of ports 2 and 3. FIG. 3a) to c) shows the trace acquired by the OTDR, after signal processing, for fibre lengths with 800 m, 5.5 km and 11 km, respectively, for configurations corresponding to FIGS. 1(a) and (b), where input and output are in the same port. Traces are also depicted by the linear response (I) and the corresponding exponential behaviour (II). The insets of FIG. 3 are the original OTDR traces for each fibre length used in the experiment. The spacing between consecutive peaks increases with increasing fibre length. The decay time was determined for the three increasing fibre lengths and values of 21.47 ns, 88.61 ns and 165.21 ns, respectively, were attained. FIG. 4 presents the linear behaviour of the decay time with increasing fibre length. The fitting was performed and a slope with 14.09 ns/km was obtained.

[0069] For the purpose of demonstrating sensing features of the disclosed CRD, a FBG was used as mirror and simultaneously as a sensor in port 3 (see FIG. 1). In this case, a chirped-based FBG was used instead of the standard FBG due to its large bandwidth, acting as a band-rejection filter in reflection. The chirped-FBG placed inside the linear cavity is centred at 1554 nm and it has 4 nm width. The output signal is back-reflected by mirror 2 and interrogated by the OTDR.

[0070] To perform strain measurements, the chirped-FBG was fixed at two points that were 300 mm apart, and submitted to specific strain values by means of a translation stage (via sequential 20 m displacements). FIG. 5 shows the traces observed in the OTDR for the cases of no strain applied to the sensor and 800 of applied strain, for configurations corresponding to FIGS. 1(a) and (b), where input and output are in the same port. FIG. 5a) shows the trace directly acquired by the OTDR, FIG. 5b) shows the trace after signal processing and c) the corresponding exponential response. Performing the exponential fit of the results obtained in FIG. 5c) it was possible to determine the decay times of 169.12 ns and 133.8 ns for no strain applied to the FBG and 800 of strain applied to the sensor, respectively.

[0071] The term comprising whenever used in this document is intended to indicate the presence of stated features, integers, steps, components, but not to preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

[0072] It will be appreciated by those of ordinary skill in the art that unless otherwise indicated herein, the particular sequence of steps described is illustrative only and can be varied without departing from the disclosure. Thus, unless otherwise stated the steps described are so unordered meaning that, when possible, the steps can be performed in any convenient or desirable order.

[0073] The disclosure should not be seen in any way restricted to the embodiments described and a person with ordinary skill in the art will foresee many possibilities to modifications thereof.

[0074] The above described embodiments are combinable.

[0075] The following claims further set out particular embodiments of the disclosure.