WO2012058716A1

WO2012058716A1 - An optical sensor for measuring a property of a fluid

Info

Publication number: WO2012058716A1
Application number: PCT/AU2011/001403
Authority: WO
Inventors: Kamal Alameh; Linh Nguyen
Original assignee: Edith Cowan University
Priority date: 2010-11-02
Filing date: 2011-11-01
Publication date: 2012-05-10

Abstract

The present disclosure provides an optical sensor for sensing a property of a fluid. The optical sensor comprises an optical waveguide that has a core region. The optical waveguide has a region for guiding light to and from the fluid. A sensing region is arranged to provide an optical response to the guided light. The sensing region has a recess extending into at least a part of the core region. The recess has an opening for receiving the fluid and first and second surfaces for reflecting at least a portion of the light. The first and second surfaces are spaced apart from each other by a first distance along the core region. The sensing region further comprises at least one further surface for reflecting at least a portion of the light. The further surface is spaced apart along the core region from the second and first surfaces by a second and third distance, respectively.

Description

AN OPTICAL SENSOR FOR MEASURING A PROPERTY OF A FLUID

Field of the Invention

The present invention relates to an optical sensor for measuring a property of a fluid. In particular, although not exclusively, the present invention relates to a fibre-optic sensor for measuring a property of a liquid.

Background of the Invention

Optical fibres are of growing interest for sensor

applications and optical fibre sensors are now being

developed for measuring properties of liquids. Such sensors have the advantage that they usually are very narrow and consequently it is possibly to measure the properties in narrow spaces. Further the sensors usually are inert, which is of further advantage for various applications.

An example of an optical fibre sensor is an optical tip sensor, which may include an enclosed air cavity near an end of the optical fibre. The end of the optical fibre is exposed to the liquid and a property of the liquid can be derived from an optical response of the optical fibre sensor. An optical response of the optical tip sensor depends on a refractive index of the liquid, which in turn depends on the density of the substance dissolved in the liquid. The liquid may be water and properties of water, such as temperature or salinity, can be determined using such a sensor. However, known optical tip sensors have drawbacks. For example, it is not possible to determine two or more

properties of the liquid simultaneously. Summary of the Invention

In accordance with a first aspect of the present invention, there is provided an optical sensor for sensing a property of a fluid, the optical sensor comprising an optical waveguide, the optical waveguide having a core region and comprising: a sensing region arranged to provide an optical response to guided light, the sensing region comprising a recess extending into at least a part of the core region, the recess having an opening for receiving the fluid and first and second surfaces for reflecting at least a portion of the light, the first and second surfaces being spaced apart from each other by a first distance along the core region, the sensing region further comprising at least one further surface for reflecting at least a portion of the light, the further surface being spaced apart along the core region from the second and first surfaces by a second and third distance, respectively; and

a region for guiding light to and from the sensing region . The first and second surfaces may be opposing surfaces of a cavity. The further surface may be at an end-face of the optical waveguide. The further surface may also be a surface of a further optical cavity. Embodiments of the present invention have significant

practical advantages. The first, the second and the third surface have associated optical path lengths and allow the optical sensor to function in a manner similar to that of a three-cavity interferometer. At least two of the surfaces are in use typically in contact with the fluid, thereby allowing the simultaneous measurement of at least two different properties of the fluid, such as properties relating to a composition of the fluid and temperature.

The optical sensor may be used for a wide range of

applications in many areas of technology ranging from medical applications to applications in the oil and gas industry and applications relating to testing of water qualities.

For example, the fluid may comprise an aqueous solution or suspension, such as water from lakes, rivers, oceans and water systems in which minerals are suspended and/or

chemicals (such as salt or other chemicals including toxic chemicals) are dissolved. The optical sensor may be arranged to measure properties of the water, such as salinity and temperature. The optical sensor typically is arranged such that these properties can be measured simultaneously. Further, the optical sensor may be arranged for in-vivo measurements, such as in-vivo measurements of the human body. For example, the optical sensor may be inserted into a body lumen, such as a blood vessel, and may be arranged to measure properties in the body lumen. In this example the fluid may be blood or another body fluid and the optical sensor may be arranged for simultaneous measurements of two or more properties of the body fluid. At least portions of the optical sensor are in this example typically formed from a bio-compatible material. Further, at least a portion of the sensor may be positioned in a catheter. The possible narrow design of the optical sensor is of particular advantage for in-vivo applications.

Further, the fluid may be naturally occurring oil or a product relating to oil, such petrol and the optical sensor may be arranged to measure properties of that fluid.

The fluid may also be a gaseous medium, such as natural gas, and the optical sensor may be arranged to measure properties of the gaseous environment. In one specific example the fluid comprises air and the optical sensor is arranged to measure properties of the air that may characterize an air guality.

The first and second distances typically are less than 200ym, less than lOOum or even less than 50um. In one specific example the first and second distances are in the order of tens of micrometers. The first and second distances typically are of the same order of magnitude. The at least one recess may have a stepped profile so as to provide at least one additional reflective surface for that recess . At least one of the surfaces, typically the further surface may be coated with layers of different materials so as to increase the reflectivity of that surface. In one specific embodiment the optical sensor is or comprises an optical fibre, such as an optical fibre composed of glass or a polymeric material having any suitable outer diameter, such as a diameter of 125μιη or less, ΙΟΟμπι or less or even less than 50μπι.

In accordance with a second aspect of the present invention there is provided a sensing system for sensing a property of a fluid, the sensing system comprising:

a sensor in accordance with the first aspect of the present invention;

a light source for directing light through the sensor; and

a detector arranged to receive light reflected from the sensor .

The sensing system may comprise a plurality of the sensors and an optical switch to which the plurality of sensors are coupled, wherein the optical switch is arranged for directing light to, and receiving light from, the sensors in sequence.

In accordance with a third aspect of the present invention, there is provided a method of sensing a property of a fluid, the method comprising the steps of:

providing a sensor, the sensor being in accordance with the first aspect of the present invention; charging the recess of the sensor with the fluid;

directing light through the sensor;

receiving light reflected from the sensor; and

determining a property of the fluid based on the received reflected light.

The method may comprise determining more than one properties of the fluid. The properties of the fluid may be determined sequentially or substantially simultaneously.

Brief Description of the Drawings

In order that the present invention may be more clearly ascertained, embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a schematic drawing of a sensor in accordance with an embodiment of the present invention; Figure 2 is a flow chart illustrating a method embodiment of the present invention;

Figure 3 is a schematic drawing of a sensing system

incorporating the sensor of Figure 1;

Figure 4a is a graph showing a measured reflection spectra of the sensor of Figure 1 immersed in water as the salinity of the water is varied from 0 to 6g in 300mL of pure water (approx. 0 to 20 ppt) at a fixed temperature (20°C ); Figure 4b is a graph showing a measured reflection spectra of the sensor of Figure 1 immersed in water as the temperature of the water is varied from 0°C to 50°C at a fixed salinity (pure water) ;

Figure 5a is a graph showing measured wavelength shifts of three nearby interference fringes measured by the sensor of Figure 1 immersed in water with respect to water salinity variation at a fixed (room) temperature, wherein an inset of the graph shows the reflection spectra at various water salinity levels; and

Figure 5b is a graph showing measured wavelength shifts of three interference fringes with respect to ambient

temperature variation at a fixed water salinity, wherein an inset of the graph shows the reflection spectra at various temperatures .

Detailed Description of the Embodiments

Figure 1 shows an optical sensor 10 for sensing a property of a fluid. The sensor 10 comprises an optical waveguide, in this example an optical fibre, having a core region 12, a cladding 14, and a recess 16 located near an end surface 18 of a first end of the sensor 10. The recess 16 defines first and second surfaces 20, 22 with the core region 12 and has an opening for receiving the fluid.

Light is directed through the sensor 10 from a second end 24. The surfaces 20, 22 and the end surface 18 provide reflective surfaces for reflecting the light as indicated by double headed arrows of Figure 1.

The fluid in the recess 16 typically has a refractive index that is different to that of the core region 12 of the optical fibre and conseguently analysis of the optical response allows identifying these properties.

In one example the fluid is water and the sensor is arranged to measure the salinity and temperature of the water.

In an alternative example the fluid is a fluid of the human body, such as blood and the optical sensor is arranged for in-vivo measurements. In this example the optical sensor comprises a catheter (not shown) . The optical fibre has a diameter of only 125μιη or less, which makes the optical sensor 10 particularly suitable for in vivo-applications .

It will be appreciated that the fluid may alternatively be of any other suitable type, such as an oil or a gaseous medium and the optical sensor 10 may consequently be arranged for measuring a wide range of properties that affect a refractive index of the fluid. It will also be appreciated that, although only one recess is shown, the sensor 10 may comprise any number of recesses. Further, the recess 16 may extend through the entire core region 12, or the recess 16 may have a stepped profile such that a single recess forms more than two surfaces fort reflection within the core region 12. In the example shown in Figure 1 the first and second surfaces 20, 22 are spaced apart from each other by a first distance of 74μιη along the core region 12, and the second surface 22 is spaced apart from the end surface 18 by a second distance of 130μπι along the core region 12. A third distance is defined between the first surface 20 and the end surface 18, the third distance being the sum of the first and second distances. This provides three different optical path lengths for light being directed along the sensor 10 and reflected from the surfaces 18, 20, 22.

It will be appreciated that the first and second distances may have any appropriate length, such as less than 200μιη or in the order of tens of micrometers. Further, the first and second lengths may be similar.

The surfaces 18, 20, 22 are coated with a thin layer of a reflective material so as to increase the reflectivity of the surfaces. For example, the surfaces 18, 20, 22 may be coated with various layers of different materials, such as S1O₂ and T1O₂, that can be tailored to give optimized reflective properties while the at the same being semi-transmissive for light . Figure 2 shows is a flow chart illustrating a method 25 of sensing a property of a fluid. The method 25 comprises step 26 of providing the sensor 10. Further, the method comprises step 27 of charging the recess 16 of the sensor 10 with the fluid and step 28 of directing light through the sensor 10. The method 10 also comprises step 29 receiving light reflected from the sensor 10 and determining a property of the fluid based on the received reflected light.

Figure 3 shows a sensing system 30 for sensing a property of a fluid, in this example water 32 in a container 34. The sensing system 30 is arranged for sensing the salinity and temperature of the water 32. The sensing system 30 comprises a plurality of sensors 10 that are immersed in the water 32 such that the recess 16 of each sensor is immersed in the water 32.

The sensing system 30 is configured such that light produced by a light source, such as tunable laser 36, is directed through each sensor 10, and light reflected by the surfaces 18, 20, 22 is received by a photo-detector 38. The laser 36 and the photo-detector 38 are coupled via an optical

circulator 40 for routing the light reflected by each sensor 10 to the photo-detector 38. In this example, output from the laser 36 is routed through an optical switch 42 that is arranged to sequentially direct the output from the laser 36 to each sensor 10. The optical switch 42 also functions to sequentially receive reflected light from each sensor 10 and to direct it, via the optical circulator 40, to the photo-detector 38.

Although not shown in Figure 3, the laser 36, photo-detector 38 and optical switch 42 are in communication with a data processing unit such as a computer running appropriate software so as to control the laser 36, collect the reflected signals received by the photo-detector 38 and control the optical switch 42. The data processing unit may also be used to process and analyse the received reflected signals so as to determine the spectral response of each sensor 10.

In use, the laser 36 is controlled such that its output wavelength is continuously swept over an appropriate

bandwidth such as the bandwidth of a typical optical

amplifier, for example an erbium doped optical amplifier, to which it is operatively coupled. The output of the laser 36 is then directed to each sensor 10 so that a property of the water 32 at each sensor location may be determined.

Although the following describes the analysis of the salinity and temperature of the water 32 at a single sensor location, it will be appreciated that the sensors 10 may be used to determine any appropriate property of the water 32 (or another fluid) , such as flow-rate and tube pressure, and at any number of locations.

The interference fringes, formed in the spectral domain as a result of the light reflections from the surfaces 18, 20, 22 of the sensor 10, exhibit three distinct groups of response sensitivities with respect to the variations in both the salinity and temperature of the water 32. Therefore, by using a sensitivity matrix calculation method (as described later) , the sensor 10 and sensing system 30 can determine water salinity and ambient temperature simultaneously. As mentioned, the recess 16 is immersed in the water 32 and is therefore filled with the water 32. Light directed through the sensor 10 is therefore subject to properties of the water 32 (and changes thereof) , which influences the reflection coefficients of the surfaces 18, 20, 22 and optical path lengths (i.e. the first, second and third distances) and allows the sensor 10 to function as a three-wave

interferometer. This approach results in three different sensitivity levels that enable, through the measurement of a sensitivity matrix, the discrimination between the water salinity and water temperature variations.

A specific experimental example will now be described. In the example that follows, the sensor 10 comprises a commercial single-mode optical-fibre and the recess 16 of the sensor 10 was formed near the cleaved end surface 18 using focused ion beam (FIB) milling. An FEI Dual-Beam XL820 system with a Ga liquid-metal ion source was used for micro-machining the recess 16. The accelerating voltage and the ion-beam current used to form the recess 16 were 30kV and 11.5 nA

respectively. The length of the recess 16, i.e. the first distance between the boundaries 20, 22, is 74 μιη and the length of an in-fibre cavity 17 (see Figure 1) i.e. the second distance between the surface 22 and the end surface 18, is 130 μπι.

The water salinity was varied by gradually dissolving sodium chloride into the surrounding water and the water temperature was changed using a heating plate (not shown) . Assuming a lossless medium for both the recess 16 as well as the in-fibre cavity 17, an approximate value of the

normalized total reflection power for the sensor 10 can be derived from:

P_R = R {l + T² + T⁴ -IT cos <¾ + 2T² cos(<¾ +δ₂)-2T³ cosδ₂} ( 1 ) where

and δ₂ = πη_∞ΓεΣ₈1 λ are twice the propagation- induced phase delay due to the recess 16 and the in-fibre cavity 17, respectively. Here λ is the signal wavelength and L_c and L_s are the lengths of the recess 16 and the in-fibre cavity 17, respectively, ni and n_core are the refractive indices of the water 32 and the core region 12, respectively. R and T are the power reflectance and transmittance

coefficients at the silica-liquid interfaces 18, 20, 22 and can be calculated using Fresnel equations for the case of normal incident light:

R ^{( n}core - ⁿi

ncore + ⁿi J

(2)

²ⁿcoreⁿi

From the structure of the sensor 10 and equation (1), it can be seen that the interference reflection spectra are affected by three cavities formed by different planes which we can denote as recess 16, in-fibre cavity 17, and combined cavity 19 (see Figure 1) which is a combination of recess 16 and in- fibre cavity 17. The optical path lengths of these three cavities 16, 17, 19 vary differently with respect to the environmental changes. In the case of an ascending

temperature, the refractive index of the water 32 within the recess 16 decreases while that of the silicon of the in-fibre cavity 17 increases. Combined cavity 19 has a different level of optical path change compared with those of its constituent components .

The same argument can be made for the case of water salinity variation as the levels of optical path change for all three cavities 16, 17, 19 will be different. The compound

interference pattern, on the other hand, is shaped by the reflected optical waves propagating in all three cavities. Therefore, several groups of fringes are present with different sensitivities with respect to variations in both the water salinity and temperature.

A specific example wherein the salinity and temperature of water was altered will now be discussed with reference to Figures 4a and 4b.

Figure 4a shows measured reflection spectra of the sensor 10 as the water salinity was varied from 0 to 6g in 300mL of pure water (approx. 0 to 20 ppt) at a fixed temperature (20°C) . Figure 3b shows measured reflection spectra of the sensor 10 as the water temperature was varied from 0°C to 50°C at a fixed salinity (pure water) .

Figures 4a and 4b show three distinct groups of response sensitivity with respect to both the water salinity and temperature variations. The fringe visibilities also varied because the refractive index variations affected the

reflected optical powers. The first three fringes (in the measured wavelength range) were located at 1532.65nm,

1536.76nm and 1540.20 nm, respectively. Owing to this distinct three-wave feature of the sensor 10, simultaneous measurements of water salinity and water temperature were realised using a sensitivity matrix approach, as discussed below .

The capability of the sensor 10 to simultaneously measure water salinity and temperature will now be discussed with reference to Figures 5a and 5b.

The wavelength shifts of all fringes were measured with respect to temperature and water salinity variation. Figure 5a shows the wavelength shifts of interference fringes at 1532.65nm, 1536.76nm and 1540.20nm, respectively, as the salinity was varied from 0 to 60 ppt at (a salinity range that covers most types of saline water) at a fixed

temperature. It can be seen that all the fringes shifted to the longer wavelength region as the salinity increased. The fringe at 1540.20nm exhibited the largest salinity- sensitivity while that at 1532.65nm had the smallest

salinity-sensitivity.

In a similar manner, Figure 5b shows the wavelength shifts of all fringes with respect to temperature varying from 0°C to 50°C at a fixed salinity. In this case, all the fringes exhibited blue-shift and the fringe at 1536.76 nm had the largest temperature-sensitivity while, interestingly, the one at 1540.20 nm had the smallest temperature sensitivity.

Due to the fact that the interference fringes have different spectral responses to changes in temperature and water- salinity, as shown in Figures 5a and 5b, simultaneous measurements of both changes in the ambient temperature and the water salinity are feasible. In the event of simultaneous variations in both temperature and salinity, the total wavelength shifts of a pair of two interference fringes within the wavelength range of interest can be expressed in a matrix form as:

(3)

where AS and ΔΤ are the water salinity variation and the ambient temperature variation, respectively. A is a

sensitivity matrix which takes into account the cross- sensitivity between the water salinity variation and the ambient temperature variation, with the sensitivity

coefficients K_s_i, K_T-i (where i = {1, 2}) being obtained from the linear fitting coefficients of the fringes' minima spectral shifts as shown in Figures 5a and 5b. From equation (2) , it is clear that if the sensitivity matrix A is

invertible (e.g. there exists the inverse matrix of A), the water-salinity variation and the ambient temperature

variation can be expressed as: AS K

(4)

AT -K_S_₂K_T__X K

Due to the fact that the fringes have different responses to both the water salinity and its temperature variations, the sensitivity matrix A is fairly invertible as its determinant is different from zero for an arbitrary pair of fringes.

Using the fitting coefficients obtained experimentally and shown in Figure 5, the water salinity and ambient temperature variations can be calculated from the response of the sensor 10 as follows:

(6)

(7)

Using either equation (5), (6) or (7) once the wavelength shifts in [nm] of the fringes were measured experimentally, the water-salinity variation and the ambient temperature variation were obtained separately in [ppt] and [°C], respectively. The normalized root mean square errors of the calculated water salinity and ambient temperature variations were 8.3% and 12% for equation (5), 3.8% and 3% for equation (6), 21% and 18% for equation (7) . Therefore, equation provides the most accurate results for this example.

The measurement error can further be minimised by optimising the fringe sensitivity (water salinity and ambient

temperature) for better linear responses by adjusting the length of the recess 16 and the length of the in-fibre cavity 17 as well as by improving the resolution of an analyser used to analyse the reflected light directed to the photo-detector 38 and by improving calibration data for temperature and salinity .

In one example, the length of the recess 16 and the length of the in-fibre cavity 17 can be designed so as to provide temperature independent salinity measurements since the respective decrease and increase of the refractive index of the water and silicon as the temperature increases will compensate for one another.

It will be appreciated that the sensor 10 can be extended to cover the measurements of one more parameter beside the water salinity and ambient temperature using a [3x3] sensitivity matrix . In the examples above, interrogation of the sensor 10 was carried out with a broadband light source, and the reflected light was analysed using an optical spectrum analyser, as mentioned above. Nevertheless, the broadband light source and the optical spectrum analyser can be replaced by a sweeping light source (for example a highly functional tunable laser) and a single photo-detector, as described previously

reference to the sensing system 30 of Figure 3.

Having described embodiments of the present invention with reference to the accompanying drawings, it is to be

understood that modifications and variations as would be apparent to a skilled addressee are determined to be within the scope of the present invention. In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention .

Claims

The Claims:

1. An optical sensor for sensing a property of a fluid, the optical sensor comprising an optical waveguide, the optical waveguide having a core region and comprising:

a sensing region arranged to provide an optical response to guided light, the sensing region comprising a recess extending into at least a part of the core region, the recess having an opening for receiving the fluid and first and second surfaces for reflecting at least a portion of the light, the first and second surfaces being spaced apart from each other by a first distance along the core region, the sensing region further comprising at least one further surface for reflecting at least a portion of the light, the further surface being spaced apart along the core region from the second and first surfaces by a second and third distance, respectively; and

a region for guiding light to and from the sensing region .

2. The optical sensor of claim 1 wherein the first and second surfaces are opposing surfaces of a cavity.

3. The optical sensor of claim 1 or claim 2 wherein the further surface is at an end-face of the optical waveguide.

4. The optical sensor of claim 1 or claim 2 wherein the further surface is a surface of a further cavity.

5. The optical sensor of anyone of the preceding claims wherein the sensing region comprises a plurality of recesses or cavities .

6. The optical sensor of any one of the preceding claims wherein the first and second distances are less than 200μπι.

7. The optical sensor of any one of the preceding claims wherein the first and second distances are less than ΙΟΟμπι.

8. The optical sensor of any one of the preceding claims wherein the first and second distances are in the order of tens of micrometers.

9. The optical sensor of any one of the preceding claims wherein the first and second distances are of the same order of magnitude.

10. The optical sensor of any one of the preceding claims wherein at least one of the first, second and further surfaces is coated with layers of different materials so as to increase the reflectivity of that surface.

11. The optical sensor of any one of the preceding claims being arranged to measure more than one fluid property at the same time.

12. The optical sensor of any one of the preceding claims wherein the fluid is a liquid.

13. The optical sensor of any one of claim 12 wherein the fluid is a body fluid and the optical sensor is arranged for in-vivo measurements.

14. The optical sensor of claim 12 wherein the fluid is water and the optical sensor is arranged to measure the salinity and the temperature of the water.

15. The optical sensor of any one of the preceding claims wherein the optical waveguide is an optical fibre.

16. A sensing system for sensing a property of a fluid, the sensing system comprising a sensor in accordance with any one of any one of the preceding claims, a light source for directing light through the sensor, and a detector arranged to receive light reflected from the sensor.

17. The sensing system of claim 16 comprising a plurality of the sensors and an optical switch to which the sensors are coupled, wherein the optical switch is arranged for directing light to, and receiving light from, the sensors in sequence.

18. A method of sensing a property of a fluid, the method comprising the steps of:

providing a sensor, the sensor being in accordance with any one of claims 1 to 15;

charging the recess of the sensor with the fluid;

directing light through the sensor;

receiving light reflected from the sensor; and determining a property of the fluid based on the received reflected light.

19. The method of claim 18 wherein the method comprises determining more than one properties of the fluid substantially simultaneously.