Field of the Invention

The present invention relates to an optical sensor. Background of the Invention

Different types of optical temperature sensors are known.

However, the known optical sensors are usually unable to measure the temperature with a high spatial resolution. For example, the known optical sensors are unable to measure the temperature at a point location that may have an extension of only a few micrometres.

The ability to measure temperature with such a high spatial resolution would be of advantage for a number of applications and there is a need for advancement.

Summary of the Invention

In accordance with a first aspect of the present invention, there is provided an optical sensor comprising an optical waveguide, the optical waveguide having: a first portion comprising an optical material that is arranged such that, when the optical material is exposed to suitable electromagnetic radiation (EMR) , the optical material absorbs at least a portion of the suitable EMR and emits emission EMR wherein a property of the emission EMR is dependent on a temperature of the optical material; and a second portion for guiding the emission EMR from the first portion; wherein the optical sensor is arranged such that a temperature of only the first portion affects the property of the emission EMR.

The first portion may have an extension that is less than 20μπι, 15μπι, ΙΟμπι, 5μπι or Ιμπι along a length of the optical sensor and a diameter that is 400, 200, 100, 50, 20, 10, 5, or even 2 μπι or less. The total volume of a temperature-sensitive region of the optical material may be in the range of 1-2 ^χ 10 ^-11 m ³ , 2-5xl0 ^-11 m ³ , 5-lOxlO ^"11 m ³ , 10-20 ^χ 10 ^-11 m ³ or 20-40 ^χ 10 ^-11 m ³ . The first portion may be positioned at an end of the optical waveguide, or alternatively at a location along the length of the waveguide between end portions of the waveguide.

In one embodiment the optical waveguide has a tapered tip at which the first portion is located. The optical waveguide may be an optical fibre having a tapered core portion at which the first portion is located.

In one variation the optical fibre is a suspended core fibre and the first portion is positioned at the suspended core. Alternatively, the optical fibre may be a solid core optical fibre.

The optical property of the emission EMR may for example be a spectral property of the emission EMR. For example, the optical property may be a wavelength specific intensity or intensity distribution. The optical material may comprise a first material and at least one second material, the first material being doped with the at least one second material, the first and second materials being selected such that the optical material emits emission EMR when exposed to the suitable EMR.

The first and second materials may be selected such that the emission EMR emitted by the optical material is any one of the group comprising: stokes shifted emission, upconversion emission, a combination of stokes shifted emission and upconversion emission, and any other appropriate emission.

In one embodiment, the first material is any appropriate material that can be doped with suitable rare-earth ions and/or fluorophores, and may have a melting point lower than the softening point of silica glass. The first material may be any one of the group comprising: tellurite glass, silica, lead silicate glass, germanate glass, ZBLAN (Fluoride) glass, and polymers such as PMMA or PDMS . The at least one second material may be a rare-earth material or may comprise fluorophores . In one embodiment, the at least one second material is any one of the group comprising: erbium ions, ytterbium ions, europium ions, neodymium ions, and organic fluorophores . It will be appreciated that the optical material may comprise a plurality of second materials. In one example, the plurality of second materials comprises erbium and ytterbium. The plurality of second materials may comprise a still further second material, such as europium.

Using a plurality of second materials may offer an advantage in that the plurality of second materials assist in increasing an upconversion emission efficiency of the optical material compared to a case in which the first material is doped with only one second material. As such, an amount of energy required to cause excitation of the optical material can be decreased, in turn reducing any local heating that can affect temperature measurements, and may minimise the risk of cell damage if the optical sensor is used to determine a

temperature of a biological sample or similar.

In accordance with a second aspect of the present invention, there is provided an optical sensor system comprising a plurality of optical sensors arranged in an array, each optical sensor being in accordance with the optical sensor of the first aspect.

In accordance with a third aspect of the present invention, there is provided a method of fabricating an optical sensor comprising the steps of:

providing an optical waveguide; and

forming a first portion of the optical sensor by coating a portion of the optical waveguide with a material that is arranged such that, when exposed to suitable EMR, the material emits emission EMR, wherein a property of the emission EMR is dependent on a temperature of the first portion;

wherein the first portion is temperature sensitive and a portion of the optical waveguide forms a second portion for guiding the emission EMR from the first portion.

The step of forming the first portion of the optical sensor may comprise immersing an end portion of the optical waveguide in the material when the material is molten. For example, the material may be an optical material comprising a suitable glass .

Alternatively, the step of forming the first portion of the optical sensor may comprise depositing a thin layer of the material on an end-portion of the optical waveguide for example via an evaporative process or another suitable film growth process.

In one example the optical waveguide is an optical fibre such as a suspended core fibre. The optical sensor may be arranged such that, when the first portion of the suspended core fibre is exposed to a suitable liquid, the liquid may penetrate into holes of the suspended core fibre, which allows

functionalising the surface areas using the suitable liquid and performing measurements using the functionalised surface area . Alternatively, the optical fibre may have a solid core. A first portion of the core of the optical fibre may be exposed to a suitable liquid, which allows functionalising the surface areas using the suitable liquid and performing measurements using the functionalised surface area. The optical material may comprise a first material and at least one second material, the first and second materials having been selected such that the optical material emits emission EMR when exposed to suitable EMR from an EMR source, the method comprising the step of doping the first material with the at least one second material.

The first and second materials may be selected such that the emission EMR emitted by the optical material is one of: Stokes shifted emission, upconversion emission, a combination of Stokes shifted emission and upconversion emission, and any other appropriate emission.

In one embodiment, the method comprises selecting the first material from any appropriate material that can be doped with suitable rare-earth ions and/or fluorophores, and that has a melting point lower than the softening point of silica glass. The first material may be selected from any one of the group comprising: tellurite glass, lead silicate glass, germanate glass, ZBLAN (Fluoride) glass, or another suitable low-meltin point glasses and polymers such as PMMA and PDMS .

The method may comprise selecting the at least one second material such that the at least one second material is a rare earth material or may comprise fluorophores . In one

embodiment, the at least one second material is selected from any one of the group comprising: erbium ions, ytterbium ions, europium ions, neodymium ions, and organic fluorophores .

Forming a plurality of optical sensors may comprise:

providing a plurality of optical waveguides; and forming first portions the optical sensors by coating portions of each optical waveguide with the optical material .

In accordance with a fourth aspect of the present invention, there is provided an optical sensing system, comprising:

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

an EMR source for generating suitable EMR to which the optical material is in use exposed; and an EMR detector for detecting the emission EMR of the optical material when the optical material is exposed to the suitable EMR;

wherein the optical sensing system is arranged such that the suitable EMR generated by the EMR source is directed towards the optical material, and such that the emission EMR of the optical material is directed towards the EMR detector.

Brief Description of the Figures

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying figures, in which:

Figure 1 is diagrammatic representation of an optical sensor in accordance with an embodiment of the present invention;

Figure 2 is a schematic drawing of a sensing system that incorporates the optical sensor of Figure 1;

Figure 3 is a microscope image of a suspended core fibre that can be used in the optical sensor of Figure 1;

Figure 4 is a diagrammatic representation of a method of fabricating the optical sensor of Figure 1;

Figures 5a to 5f are microscope images of the optical sensor that can be used in the optical sensor of Figure 1; and

Figures 6a to 6f are graphs showing examples of

upconversion emission spectra and temperature response data of the optical sensor of Figure 1. Detailed Description

Referring to Figure 1, there is shown an optical sensor 100 comprising an optical waveguide 102. The optical waveguide 102 has a first portion 104 that is temperature sensitive and a second portion 106 for guiding electromagnetic radiation (EMR) to and from the first portion 104. The first portion 104 comprises an optical material 108 that is arranged such that, when exposed to suitable EMR from an EMR source, the optical material 108 emits EMR, the emission being dependent on a temperature of the first portion 104. The optical sensor 100 is arranged such that a temperature of only the first portion 104 affects the emission.

Such an arrangement facilitates use of the optical sensor 100 in performing temperature sensing of point locations, such as point locations that are of a comparable size to that of the first portion 104. For example, the first portion 104 may have a size that is less than any one of 20μπι, 15μπι, ΙΟμπι, 5μπι, or Ιμπι along a length of the optical sensor 100. In some embodiments, the first portion 104 has a size that is between ΙΟμπι to Ιμπι along a length of the optical sensor 100. The first portion can be dimensioned so as to minimise the dimensions of the active temperature sensitive region of the sensor .

In the particular example shown in Figure 1, the first portion 104 is at an end of the optical waveguide 102, however it will be appreciated that the optical sensor 100 may be arranged such that the first portion 104 is not located at an end of the optical sensor 100, and may be located at any appropriate position along the length of the optical sensor 100. The optical material 108 comprises a first material 110 and at least one second material 112, 114, the first material 110 being doped with the at least one second material 112, 114. The first and second materials 110, 112, 114 are selected such that the optical material 108 emits EMR when exposed to suitable EMR from an EMR source.

The first and second materials 110, 112, 114 can be selected such that the optical material 108 emits EMR by stokes shifted emission, upconversion emission, a combination of stokes shifted emission and upconversion emission, or any other appropriate emission.

The first material 110 is typically any appropriate material that can be doped with rare-earth ions and/or fluorophores , and may have a melting point lower than the softening point of silica glass. For example, the first material 110 may be any one of the group comprising: tellurite glass, lead silicate glass, germanate glass, ZBLAN (Fluoride) glass, or other suitable low-melting point glasses, and polymers such as PMMA or PDMS . In the example shown in Figure 1, the first material 110 is tellurite glass.

The at least one second material 112, 114 is typically a rare- earth ion and/or a fluorophore. For example, the at least one second material 112, 114 is typically any one of the group comprising: erbium ions, ytterbium ions, europium ions, neodymium ions, and organic fluorophores . The optical material 108 may comprise a plurality of second materials 112, 114. For example, the plurality of second materials 112, 114 may be erbium and ytterbium. The plurality of second materials 112, 114 may comprise a still further second material, such as europium. In the example shown in Figure 1, the optical material 108 comprises two different second materials 112, 114, namely erbium ions 112 and ytterbium ions 114.

Using a plurality of second materials may offer an advantage in that the plurality of second materials can assist in increasing an upconversion emission efficiency of the optical material 108. As such, an amount of energy required to cause excitation of the optical material 108 can be decreased, in turn reducing any local heating that can affect temperature measurements. Reducing local heating may also minimise the risk of cell damage if the optical sensor 100 is used to determine a temperature of a biological sample, for example.

Figure 2 shows an optical sensing system 200 incorporating the optical sensor 100. The optical sensing system 200 comprises the optical sensor 100, an EMR source 202 for generating the suitable EMR to which the optical material 108 is exposed, and an EMR detector 204 for detecting the emission of the optical material 108 when the optical material 108 is exposed to the suitable EMR. The optical sensing system 200 is generally arranged such that the suitable EMR generated by the EMR source 202 is directed towards the optical material 108, and such that the emission of the optical material 108 is directed towards the EMR detector 204.

In this particular example, the EMR source 202 is a 980nm laser diode, and the EMR detector 204 comprises an Ocean Optics QE65 Pro detector and an appropriate spectrometer.

The EMR generated by the laser diode 202 is directed towards the optical material 108. The generated EMR is reflected off a mirror 206 towards a dichroic mirror 208. The dichroic mirror 208 is arranged to reflect the EMR such that the EMR is directed to a lens 210, and then coupled into the second portion 106 of the optical sensor 100. The EMR is then guided, via the optical waveguide 102, towards the optical medium 108 so as to excite the optical medium 108.

In response to excitation of the optical medium 108 by the EMR, the optical medium 108 generates an emission having a spectrum that is dependent on temperature. The temperature dependent emission of the optical material 108 is then directed towards the detector/spectrometer 204. In particular, the emission exits the second portion 106 of the optical sensor 100, passes through the lens 210 and then through the dichroic mirror 208, which is arranged to allow the emission to pass through. The emission is then directed, via mirrors 212a, 212b to the detector/spectrometer 204.

Specific examples of the above described optical sensor 100 have been fabricated using three different fibre types:

Corning SMF28E, Corning multi-mode (having a core diameter of 62.5μπι) , and a micro-structured optical fibre (MOF) , although it will be appreciated that any appropriate fibre type can be used to fabricate the optical sensor 100.

A cross section 300 of an example suspended-core fibre 102 is shown in Figure 3, which shows the relative sizes of the core 302, hole 304, and outer diameter 306. Inset A of Figure 3 shows a region of the cross section 300, including the core 302, at a higher magnification. The suspended-core fibres 102 were stripped, cleaned and tapered using standard methods, such as by using a Vytran GPX fibre processing system. Taper parameters of the suspended- core fibres 102 were optimised to give both long and short taper durations to test the efficiency of both approaches. The approximate taper diameter at the fibre tip of both types of tapered suspended-core fibres 102 was 2μπι.

In example embodiments used for temperature sensing

measurements, the first material 110 comprised tellurite glass and was doped with varied concentrations of the at least one second material 112, 114 which, in these examples, were rare earth ions. Two samples were prepared using the standard ZNT composition: one with 1% erbium doping and the other with 1% erbium, 9% Ytterbium doping.

In fabricating each optical sensor 100, a coating was first removed from a respective suspended-core fibre 102 using mechanical stripping, and the suspended-core fibre 102 was cleaved using a diamond cleaver. A group of cleaved fibres 102 were then mounted with their respective tips level, and immersed for 5-10 seconds in molten optical material 108, which in this example was rare earth doped tellurite glass 108, to a depth of 5 to 8 mm using a mechanical stage to control the dipping process. At the conclusion of the dip, the fibres 102 were removed and allowed to cool. By varying the temperature of the molten rare earth doped tellurite glass 108, the dip duration, and the immersion time, both the thickness of the optical material 108 and the coated length of the fibre 102 (i.e., the length of the first portion 104), can be adjusted to suit desired parameters. An example of a method 400 incorporating the above described steps is shown in Figure 4. In a first step 402, a tip of the suspended-core fibre 102 is immersed in the molten doped tellurite glass 108. In a second step 404, the fibre 102 is retained in the doped tellurite glass 108 for a predetermined length of time, in this example 10 seconds. In a third step 406, the fibre 102 is removed from the molten tellurite glass 108 and doped tellurite glass 108 that has coated the fibre 102 is allowed to cool and solidify.

The method 400 can be used to fabricate multiple optical sensors 100 simultaneously, without the requirements of individual post-processing that are typical to optical fibre based sensors. In one example, ten optical sensors 100 were fabricated at a time by immersing respective tips of the fibres 102 into the molten doped tellurite glass 108.

As a result of the method 400, the suspended-core fibre 102 is coated not only on its external surface, but due to capillary action the doped tellurite glass 108 also fills a short length up the void within the suspended-core fibre 102. The filled length is dependent on the depth and duration of the

immersion, as the doped tellurite glass 108 rapidly cools as it travels up the suspended-core fibre 102, preventing capillary filling from occurring as the doped tellurite glass 108 begins to solidify. Images of coated fibres 102 are shown in Figures 5a to 5f. Figures 5a to 5c show an example of a suspended-core fibre 102. The doped tellurite glass 108 has filled approximately 8mm up the length of the suspended-core fibre 102, as shown in Figure 5c. The external surface of the suspended-core fibre 102 has also been coated in the doped tellurite glass 108 in Figures 5a to 5c, with a coating thickness of approximately 2]im. Figure 5d shows an example of a coated multi-mode fibre 102, wherein the viscosity of the doped tellurite glass 108 was higher, resulting in a thicker coating and a pronounced bubble on the tip of the multi-mode fibre 102. In addition to the standard fibres, tapers were also coated as shown in Figures 5e and 5f. The thickness of the doped tellurite glass 108 coating in each of these examples is approximately 2 μπι.

Temperature measurements were performed using the optical sensing system 200 shown in Figure 2. The 980nm laser diode 202 was used for excitation of the optical material 108 of the optical sensor 100, with power levels varying between lOO W to ImW depending on the observed fluorescence from the optical material 108. For detection of the emissions, the Ocean Optics QE65 Pro detector was used, with the emission being coupled to the spectrometer 204 using a 400μπι diameter multi-mode fibre.

For calibration and testing purposes, in addition to the optical sensor 100, a thermocouple or resistance temperature detector (RTD) (not shown) was co-located at substantially the same position as the optical sensor 100 within an incubator 214. The incubator 214 was then temperature cycled to record the thermal response of the optical sensor 100.

Software was written to simultaneously record both the reference temperature from the thermocouple/RTD and the spectra from the optical sensor 100. This software then integrated the fluorescence spectra of the optical material 108 over a desired range, allowing for comparison of the reference temperature and fluorescence ratio in real time. Examples of the fluorescence spectra obtained from the optical sensors 100 are shown in Figures 6a and 6b , wherein Figure 6a corresponds to the spectra from a suspended core probe filled with erbium tellurite, and Figure 6b corresponds to a

suspended core probe filled with erbium ytterbium tellurite. Also shown are the regions over which the fluorescence was integrated (indicated in red and blue) . The blue region corresponds to the ² Hn _/2 ~^ ⁴ I is/2 Er ³⁺ transition while the red corresponds to the ⁴ S3/2 ⁴ Iis/2 transition. A third strong upconversion band in the red ( ⁴ F _9/2 ~^ ⁴ I i ₅ / ₂ ) is also observed from the fibres, however it was not used for temperature measurements in these examples .

The fluorescence ratio here is then defined as the sum of the ⁴ S ₃ /2 ~ ⁴ Ii5/2 band divided by the sum of the ² Hn/ ₂ ~ ⁴ I is/2 band. This ratio has been shown in literature to be well correlated with the temperature of the glass host.

The results shown in Figures 6c to 6f show a good correlation between the temperature and the fluorescence ratio of both the erbium doped (Figure 6b ) and erbium-ytterbium doped (Figure 6d to 6f ) samples.

Some separation between the observed fluorescence ratio and the true temperature is seen over the duration of the

measurement. This is most likely due to drift of the alignment into the spectrometer due to fluctuations in the ambient temperature at which the measurements were taken.

The fluorescence ratio changes with temperature appear to be relatively linear over the tested range, with an increase in the observed ratio of approximately 1% per degree. By optimising the power coupled into the fibre 102 and adjusting the power of the laser diode 202, the fluorescence signal is adjusted to 30,000 to 50,000 counts, which allows the

fluorescence ratio to be measured with good precision.

Experiments suggest that the temperature resolution of the optical sensor 100 is of the order of at least 0.1 to 0.3 °C.

The optical sensors 100 and the optical sensing system 200 can be used in various fields, such as embryology, medical research, and fields in which it is desirable to obtain a temperature reading at a specific point, for example when materials are not suitably homogeneous. Another field in which the optical sensor 100 and optical sensing system 200 can be used is in the semiconductor field, wherein point temperature measurements are desired at specific location, such as at semiconductor junctions, and/or to provide a heat map of a semiconductor .

Modifications and variations as would be apparent to a skilled addressee are determined to be within the scope of the present invention.

For example, an optical sensor array may be formed and each optical sensor of the array may be provided in accordance with the optical sensor 100 described above. Such an array can be used to perform simultaneous point temperature measurements at a number of point locations. For example, such an array may be used to measure a temperature distribution.

Further, the first portion of the optical sensor may not necessarily be formed by exposing an end-portion of the waveguide to molten glass. The first portion of the optical sensor may alternatively be formed by depositing a thin layer of a suitable material on the end-portion of the optical waveguide for example via an evaporative process or another suitable film growth process.

In one example a sensor that is based on a suspended core fibre is formed by exposed the suspended core fibre to a suitable liquid thattravels into holes of the suspended core fibre allowing secondary measurements to be performed by functionalising an area of the suspended core fibre. In an alternative example, the sensor is based on a conventional solid core optical fibre and is formed in a similar manner. The first portion of the core of the conventional optical fibre is exposed to (dipped into) a suitable liquid that allows secondary measurements to be performed by

functionalising an area of the core of the conventional core fibre .

In the description of the invention, except where the context requires otherwise due to express language or necessary implication, the words "comprise" or variations such as "comprises" or "comprising" are 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.