WO2017197306A1 - High performance waveguide refractometer - Google Patents

Abstract

A refractometer has at least one light source, a least a first portion of a waveguide having a first refractive index positioned to receive light from the at least one light source and at least a second portion of a waveguide having a second refractive index positioned to receive light from the at least one light source, and at least one detector to measure light from the first and second portions.

Description

HIGH PERFORMANCE WAVEGUIDE REFRACTOMETER

GOVERNMENT FUNDING

[0001] This invention was made with Government support under grant number DBI- 1556385 awarded by the National Science Foundation. The Government has certain rights in this invention.

RELATED APPLICATIONS

[0002] This application claims priority to US Provisional Patent Application No.

62/335,920, filed May 13, 2016, and is related to US Provisional Patent Application Nos. 62/162,526, and 61/835,415, all of which are incorporated by reference here.

BACKGROUND

[0003] Refractive index (RI) relates to the density and composition of a material. Many industries use refractive index as a diagnostic parameter, including food and beverage, pharmaceuticals, and petroleum derivatives. In oceanography, the refractive index of seawater relates to its salinity, and has become one of the most widely measured parameter in the ocean. The main refractive index measurement approaches include using prisms that measure the refraction of light, interferometry (Fabry-Perot) and surface plasmon resonance, and the use of waveguides with at least a portion of the cladding removed for transduction. The principles of refractive index measurements of fluids for each of these types sensors have been known for decades. However, research continues, particularly in efforts to relate RI to the measurement of salinity for use by the oceanographic community.

[0004] An approach used to measure the refractive index of a fluid employs the standard prism-type approach, such as those inspired by the principle of an Abbe refractometer. Using a prism, the light beam is "bent" in a triangular shape and interacts with an interface that contains the fluidic sample, and a double surface forms an angle with a sample that is typically a glass material. For instance, one embodiment measures the deviation of light beams traveling across adjacent water cells, where one has seawater and the other has reference water. See for example Minato, H., Y. Kakui, A. Nishimoto, and M. Nanjo, "Remote Refractive Index Difference Meter for Salinity Sensor Instrumentation and

Measurement," IEEE Transactions, 1989, 38, 608-612, and Zhao, Y. and L. Yanbiao, "Novel Optical Fiber Sensor for Simultaneous Measurement of Temperature and Salinity," Sensors and Actuators B: Chemical, 2002, 86, 63-67.

[0005] . A charge-coupled device (CCD) is used for sensing. In one example, once the signal was processed, a linear relationship was established. More recently, a technique has been used in which the degree of light bending in a prism, which serves as a waveguide, is measured using a position- sensing detector (PSD). The major drawback of the technique is that it requires highly coherent (e.g. laser) light, which in turn makes it difficult to measure the refractive index over a broad wavelength range. The use of laser light also increases the cost of the instrument. The monochromatic light used in this prism approach and the formed glass pyramid make this approach more expensive than simple waveguides. In addition, the interface is relatively bulky compared to other approaches.

[0006] Many optical techniques and sensing schemes other than those using typical intrinsic fiber (waveguide) sensors have been postulated. Good sensitivity and detection limits can be obtained with interferometric techniques, including a Fabry-Perot interferometer. One design decoupled temperature from the measurement, because fringe contrasts are insensitive to temperature variations. The main disadvantage of the technique, however, is the relatively limited dynamic range.

[0007] Other optical approaches utilize measurements that are based on how certain values, or peaks, fall in the spectral range. Researchers using these methods claim such measurements could be more accurate because the wavelength dependence is an absolute measurement. However, standard uncertainties in the refractive index measured using these sensors may be too insensitive to be useful. Another technique that has been widely exploited uses a side-polished waveguide onto which a thin metallic layer has been deposited. The fundamental mode propagating through the fiber interacts differently with the modes transmitted through the thin metallic layer, surface plasmons, and depends on the refractive index of the medium surrounding the metallic layer. But the achievable refractive index sensitivity may not be accurate enough to be useful. Another technique uses intensity-based measurements as well as wavelength shifts (81,000 nm/ refractive index unit) and achieves good sensitivity. The main drawback of their approach is the use of more than one sensing element, causing issues with power and shift.

[0008] Recent improvements in sensitivity, simplicity, and versatility have also been reported for waveguide-based devices— also called intrinsic refractometers, mentioned above. Current approaches have good sensitivity, but one approach uses laser light. This is not desirable when the goals are simplicity and reduction of cost and power use. Good resolution has been obtained with single-mode fibers, but inexpensive multimode fibers have also been used in such sensors. Waveguide sensors have been used in various applications including quality-control monitoring and in wine production. See, for example, Noiseus, I., W. Long, A. Cournoyer, and M. Vernon, Simple Fiber-Optic-Based Sensors for Process Monitoring: An Application in Wine Quality Control Monitoring, Appl.

Sepctosc, 2004, 58, 1010-1019.. More recently, one technique uses self-referenced intrinsic sensors to enable longer-term, continuous monitoring of the index of refraction in fluids. This technique is discussed in US Patent Nos. 7,024,060, and 7,543,572, commonly owned by the assignee of the instant application.

[0009] Most current waveguide refractometers use a waveguide with a refractive index higher than that of the fluid that is being measured. Some reports have recognized the use of waveguides with a refractive index lower than the liquid being measured. Recently, for instance, a NASA report presented by M. R. Callahan showed the use of a fluoropolymer sensing element that was wrapped around a cylinder to measure the refractive index and concentration of sodium chloride solutions. Some fluoropolymers have refractive indices lower than water so the possibility exists that this approach had a refractive index lower than water.

SUMMARY

[0010]

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 shows an example of measurements of salinity using a CTD and salinity obtained using a refractive index with a temperature sensor.

[0012] Figures 2 shows an embodiment of light input two straight waveguides.

[0013] Figure 3 shows a final measurement obtained by combining two sets of measurements from two different refractive index measurements.

[0014] Figure 4 shows an embodiment of a refractive measurement system, or refractometer.

[0015] Figures 5 and 6 show the results measured in this embodiment as function of salinity in the dimensionless PSS salinity scale.

[0016] Figure 7 shows a schematic of an embodiment of a refractive measurement system.

[0017] Figures 8 and 9 show calibration curves for glass and Teflon.

[0018] Figures 10-17 show calibrations for refractive index and salinity for a refractive measurement system.

[0019] Figure 18 shows another embodiment of a refractometer.

[0020] Figure 19 shows another embodiment of a refractometer.

[0021] Figures 20-25 show sets of results for the calibration of a refractometer.

[0022] Figure 26 shows an embodiment of a standalone sensor.

[0023] Figure 27 shows an embodiment of an antifouling screen.

[0024] Figure 28 shows an embodiment of an architecture of a refractometer. [0025] Figure 29 shows an embodiment of the mechanical-electrical schematic of a refractometer.

[0026] Figure 30 shows an embodiment of a mechanical housing.

[0027] Figure 31 shows an embodiment of a cap and body of a pressure proof container.

[0028] Figure 32 shows an embodiment of a liquid core waveguide.

[0029] Figure 33 shows an embodiment of a single waveguide.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] The below discussion will concentrate on measuring salinity, with the understanding that this implementation provides a means to assist with understanding the embodiments. It is not intended to limit the application of the embodiments to any particular fluid or characteristic of a fluid being measured.

[0031] Traditional aquatic measurements of salinity are typically performed using three independent measurements: conductivity, temperature, and depth, which are collected by oceanographic sensors called CTDs. The collected measurements are input into an equation resulting from the definition of the Practical Salinity Scale of 1978 (PSS-78) [24, 25]. The PSS- 78 implies that all waters with the same conductivity ratio have the same salinity, even if the composition is not the same. In other words, the PSS-78 is used under the general assumption that the relative concentration of ions in oceanic waters is constant, referred to as the Law of Constant Proportions. However, zones where important biological activities occur, such as hypersaline waters, estuaries, and isolated and subglacial lakes, do not generally follow the Law of Constant Proportions.

[0032] Although the salinity values in the PSS-78 are dimensionless, they are often followed by the practical salinity unit abbreviation (PSU) to indicate that the scale is in use. The PSS-78 equation is valid for: Temperature -2 °C < T < 35 °C for salinity 2 < S < 42, and Temperature 15 °C < T < 30 °C for salinity 42 < S < 50. During the development of the PSS-78 equation, oceanographers noted that an optical-based measurement of the water to gain salinity was desirable, but the refractive index instrumentation at the time lacked the required accuracy for an in situ or field sensor.

[0033] Other well-known instrumental issues associated with the in-situ determination of salinity using CTDs include the following. There is a mismatch in the acquisition frequency of temperature and conductivity measurements. The conductivity measurement is typically slower than the temperature measurement, producing spikes in the salinity data, especially when the sensor is exposed to a mass of water that moves relative to its own position so that thermal gradients are experienced. Strategies for synchronizing the data are available, but require additional processing. The conductivity sensor experiences a thermal lag due to heat transfer to or from the sensor body as the sample passes through the conductivity cell.

Significant biases can be generated and are appreciable in "yo-yo" vertical profiles that produce dissimilar results during the down-cast and up-cast or recovery stage. Strategies for solving for this thermal lag also exist, but like the strategies for synchronizing data, they require something more. In this case, the strategies typically require knowledge of both casts.

[0034] Several other techniques besides CTDs have been used to infer seawater salinity, including composition analysis of major components, evaporation to dryness, chlorinity measurement, densitometry, and measurement of sound speed. However, these techniques take more time and analysis, so the immediate relationship between salinity and other ecological components at the collection site is lost.

[0035] In optical instrumentation techniques for ocean analysis, the refractive index (RI) has been correlated with salinity and density. Researchers have investigated the difference between salinity measurements of seawater using RI and those made using conductivity by measuring the RI of seawater relative to the Copenhagen Standard Seawater. Rl-based measurements of salinity using a lab instrument have also been made on samples from hypersaline water bodies, where the PSS-78 validity is questionable. More recently, optical measurements have shown advantages over conductivity measurements of salinity. Figure 1 shows an example of measurements of salinity using a CTD (ocean explorer.noaa.gov) and salinity obtained using a refractive index with a temperature sensor. The top curve is for a CTD and the bottom is for refractive index, where blue lines are uncorrected data, while the red lines are corrected data.

[0036] Sensing RI can provide rapid measurements at the microsecond level because of instantaneous transduction. This frequency makes it easy to match the high acquisition frequency of the temperature sensor, eliminating spikes due to the short-term mismatch of the temperature and conductivity sensor, which affects the salinity calculation. The advantages of a fast sensor for salinity measurements is that it can then be used for environments with rapid dynamics in the environment, such as thermoclines, fronts, coastal areas and other areas with strong currents.

[0037] Because sensing RI is a light-based technique, it enables various methods of addressing fouling of the waveguide by biologic factors such as algae, etc., as will be discussed in more detail later. RI is less correlated with temperature (T) than is conductivity (C) (dn/dT =-0.048 °C^A vs. dC/dT=0.47°C ¹), while the sensitivity of conductivity to pressure and to RI are almost identical. RI's reduced temperature dependence minimizes the thermal lag effects that are observed during vertical profiling as shown in Figure 1. Finally, RI is unaffected by electromagnetic interference and includes the influence of nonelectrolytic compounds, which are not measured by conductivity. [0038] The embodiments here generally include at least two waveguides for improving the measurement characteristic of refractive index, or other variables relatable to this variable, such as concentrations of dissolved substances, salinity, density, and speed of sound. Figure 2 shows a schematic of rays of light that are input into two straight waveguides. In the first waveguide 10, referred to here as case A, the refractive index of the waveguide is higher than the refractive index of the solution being measured. In this case, the light input 12 at angles larger than the critical angle is lost and as the light propagates some light internally reflects within the waveguide. A fraction of the light that reaches the detector can be directly from the source in addition to that that is internally reflected.

[0039] In the second waveguide 14, referred to here as case B, the refractive index of the waveguide is lower than that of the fluid. The only light 16 that will reach the detector is that light that does not interact in the waveguide-fluid interface. If one is to calibrate the response of a detector with each waveguide as function of the refractive index of the solutions, two lines with slopes higher than one and less than will be found depending on the case under consideration.

[0040] By obtaining two different measurements using two different refractive indexes, one can achieve higher sensitivity than using only one refractive index. One can combine the measurements during analysis to achieve the higher sensitivity. In the embodiment of Figure 3, the two results are combined by dividing one set of results by the other. However, depending upon the relationships between the two refractive indices, the two measurements may be combined in other ways. For example, in the embodiment of A and B, one has a refractive index above that of the measured fluid and the other has one below. In

embodiments where the refractive indices are the same, the measures may be added. In other embodiments, the refractive indices may both be above the refractive index of the measured fluid, but different, or both below but different, different methods of combining the two sets of measurements by be used.

[0041] The embodiments here use a novel combination of the optical configurations described and materials selection. The embodiments here use multiple surfaces that interface with the fluid under measurement. This optical configuration and the materials selected for the waveguide effectively increase the resolving sensitivity in the refractive index.

[0042] Essentially the combined use of the two refractive indices measurements will improve the measurement characteristics with respect to each one of them. Increasing the sensitivity in the measurement resolution typical improves the limits of detection, and accuracy of the measurements. For the sensor embodiments, the Sensitivity is the slope of the optical signal vs the measured variable. This refers to attainable or measurable resolution. As used here, the term "measured sensitivity" of attainable sensitivity or resolution means the experimental standard deviation of several measured points divided by the sensitivity. For simplicity purposes, the embodiments here use two waveguides that produce a positive and a negative slope. In implementation, the calibrated data for two waveguides with any slope can potentially be manipulated to provide better sensitivities. For instance, the data points can be multiplied instead of divided.

[0043] The embodiments here consist of several different refractive index measurement systems that have been implemented with its main characteristics and components. In all of them, the application of at least an additional waveguide will enhance the overall measurement of refractive index.

[0044] Figure 4 shows a one embodiment of a system diagram with the various components. A light source, not shown, provides light piped into the waveguides by optical fibers 22 and 24. The light is collimated by collimators and focusing lenses contained in the capsules 26 and 28. The light enters the waveguides 30 and 32 that pass through a cylindrical closed container with fluid inputs such as 38 and 40 to host the solutions for measurement. The fluid flows into the system through the inflow tube 41 and after flowing into the waveguide 30 exits the system through the outflow tube 39. The detectors 34 and 36 detect the resultant light and generate the outputs and receive power from the wires 42. Different variations on this setup are of course possible.

[0045] In one embodiment, the system used off-the-shelf, readily available commercial components. A tungsten lamp was used as a light source, and the light detector consisted of a Newport light detector. The first waveguide 30 was a glass waveguide from Specialty Glass Products, and the waveguide 32 was a Teflon® waveguide from Random Technologies.

[0046] Figures 5 and 6 show the results measured in this embodiment as function of salinity in dimensionless PSS salinity scale. Deionized water was used to reference each

measurement. The water reference for each point is assumed to be at the same temperature of the solution being measured. For the glass waveguide, the measured sensitivity found was 0.302 in salinity and 6.94 x 10^~5 refractive index units. For the Teflon waveguide, the salinity measured sensitivity was 0.127 and that in refractive index units was 2.91xl0^"5 refractive index units. It should be noted that in the measurements shown in Figure 5, and to improve the individual waveguide measurements, the center of the Gaussian beam is blocked to increase the sensitivity individually as shown in Figure 7.

[0047] Figure 7 shows a schematic of a system such as that shown in Figure 4. The optics, in this case a collimating lens such as 50 and 54 and a focusing lens such as 52 and 56, direct the light into the waveguides 30 and 32. The light eventually reaches the detectors 34 and 36, which then output a signal that indicates the amount of light that is detected or sensed, such as a voltage. [0048] Figures 8 and 9 show the improved calibration curves taking the ratio of the glass measurements with those taken with the Teflon. Figure 8 shows the data for salinity calibration and Figure 9 for refractive index. The improved measured sensitivity in salinity is then 0.064 PSS units and the refractive index is 1.41x 10^"5.

[0049] In a second embodiment, aiming to show that measurements can be performed with low-cost light sources, such as LEDs and detectors photodiodes, the system integrated small boards into the sample cell. The cells were designed so that they would permit the necessary alignment of light source, optics, waveguide and detector to eliminate mechanical vibrations in the set-up. The detector side was connected to Labview National Instruments cards for the digitalization of the photodiode signal and temperature sensors (A/D and D/A conversion). Also, the cells enabled the measurement of the same solution with both waveguides.

Deionized water was again used before each measurement.

[0050] Figures 10 and 11 show the individual calibrations with the two waveguides in the above embodiment for refractive index and salinity. Figure 10 shows the calibration versus refractive index of the solution for the glass rod, and Figure 11 shows the same data for the Teflon rod. Figures 12 and 13 show the calibration performed against the salinity measurements for the glass rod and Teflon rod, respectively. The measured sensitivities in refractive index for the glass and Teflon waveguides found were 2.37xl0^~5 and 4.73xl0^~6, respectively. The measured sensitivities in salinity for the glass and Teflon waveguides were 0.1317 and 0.0263, respectively.

[0051] Figures 14 and 15 show the improved calibration curves taking the ratios of the glass measurements with those taken with the Teflon. Figure 14 shows the data for salinity calibration and Figure 15 for refractive index. The improved measured sensitivity in salinity is then 0.0113 PSS units and in refractive index is 1.974x 10^"6. [0052] One should note that two waveguides of the same material, and same refractive index, could be utilized to also improve the sensitivity of the measurements. In these cases, the points to obtain the calibration curves, which are assumed to be the same, are multiplied. Consider for instance, the values obtained for the glass and the Teflon shown in Figures 12 and 13, assuming no correlation between the measurements. The improvement on refractive index that would be obtained using two glass waveguides would be then:

Using two glass waveguides: The attainable resolution (or measured sensitivity) in refractive index 1.48 xlO ⁵

Using two Teflon waveguides: the improved measured sensitivity (or attainable resolution) in refractive index would be 2.46 xlO^"6

[0053] Figures 16 and 17 summarize the attained results that show the improvements in the measured resolution or attainable sensitivity that the sensor could obtain. Figure 16 shows the salinity resolution for several waveguides using a commercial lamp and detector, and Figure 17 shows the same data for the LED and photodiode boards of the second embodiment discussed above. The smallest resolution in these embodiments was obtained using a Teflon waveguide with an RI less than the measured fluid and a glass waveguide having a RI greater than the measured fluid.

[0054] In another embodiment, shown in Figure 18, the collimating and a focusing lens were removed and a photodiode 66 was held at a 23 -degree angle 70 with respect to a plane orthogonal to the glass fiber axis 68. Furthermore, to minimize the straight light that reaches the detector, the center of the LED was moved about 5 millimeters from the center of the fiber axis. In addition, this embodiment also included working thermistors 62 to monitor temperature changes in the LED and the photodiode, which are known to affect the light output and the measured signals. A photodiode that measured the intensity of the source was also included as a reference signal. A mirror 64 was also included in the optics.

[0055] Another improvement in this set up is the addition of the thermistor to measure the temperature of the fluid that was being measured with the refractometer, which consists of the two waveguides, light sources and detectors. In this embodiment, the measured fluid was the water the sensor in which the refractometer was immersed. The thermistor was potted in a stainless-steel tube, and a custom circuit was designed to obtain a rapid stabilization time with good precision, (0.001°C). Figure 18 shows a schematic and a picture of the

refractometer embodiment. The exposed length of the fiber in this embodiment was 3.4" (8.64 cm). The thermistor signals and the photodiodes were all read using National

Instruments analog to digital card and a Lab View program. Figure 19 shows an embodiment of the sensor that can be deployed as water can freely fill around the waveguide without damaging any of the components.

[0056] Figure 20 shows the results of the calibration in this embodiment at a temperature of 19 °C. The attainable resolution (or measured sensitivity) in salinity obtained with this one glass waveguide was 0.10 in PSS units. If this new set of results with the glass waveguide was to be used with a sensor with a Teflon waveguide and sensor like that described in the photodiode embodiment, the improved salinity attainable resolution would be 0.02 in PSS units.

[0057] Another embodiment uses a different material for a waveguide with a refractive index higher than water. In this case, the waveguide consisted of an acrylic fiber. This embodiment used the same length of the fiber and the angle of light injection created by offsetting and titling the LED position. [0058] In this embodiment, the fiber refractive index was higher than that of glass, so the input angle, which is the tilting of the board with respect to the fiber axis, was varied. And it was found that the best results were obtained at an angle of 53 °. Figure 21 shows the results of this calibration at a constant temperature of 20 °C. The measured sensitivity found for salinity is 0.06 ppt units.

[0059] This experiment demonstrates how the use of two waveguides, even if they both have a refractive index higher than that of the measured solutions, can improve measurement statistics. If one were to use the data points that resulted in this case using an acrylic waveguide with the calibration found for the glass waveguide described above, and the points are multiplied, then the improved attainable resolution (or measured sensitivity) is 0.01 ppt.

[0060] As it can be imagined, any improvements to each one of the measurement characteristics provided by one fiber will improve the combination of them. Using this premise, an acrylic fiber 80 was again utilized, but in this embodiment, it was wrapped around a cylindrical "puck" 82 of 45 cm in diameter. This was the objective of using this acrylic fiber, since it is relatively easy to bend. The LED was now orthogonal to the body of the instrument, and again no collimating or focusing lenses were utilized. The same set-up that holds the boards, the light source, the detectors and the thermistor as that utilized before was employed. By increasing the number of interactions between the light and the waveguide-fluid interfaces by bending the fiber, the sensitivity would be increased. Figure 22 shows a picture of the puck and how the fiber was wrapped around it. Similar results, not shown here, were obtained for other sizes of pucks.

[0061] The calibration results are shown in Figure 23 and 24. Figure 23 shows the results for a calibration of the resulting signal verse refractive index, while Figure 24 shows the calibration of the resulting signal verses salinity. The measured sensitivity for this case was 0.026 ppt in salinity. The results for these sensors can also be calibrated to other variables as shown in Figure 25, showing density and sound velocity.

[0062] In the first two embodiments, the attachment of the fibers to the containers was done by gluing the fibers into the acrylic containers. In the later embodiments, a couple of plastic Upchurch connectors together with a small O-ring held the fiber in place. More recently, embodiments have held the fibers by passing them through a metal tubing with an internal diameter larger than the diameter of the fiber. The fiber is glued to the metal via a vacuum resin, and the metal tubes are held in place using Swagelok connectors. As noted above regarding Figure 19, the sensor may be packaged to be deployable as a standalone sensor.

[0063] As shown in Figure 26, for example, the fiber waveguides such as 92 are mounted in a round casing and the measurement volume, which is area around the fibers where the fibers encounter the fluid to create the interfaces against which the light will reflect, could be inside the casing. The casing or housing 90 may deploy vertically as shown. The housing may have adaptations to adjust to the environment in which the sensor is deployed. One consideration is turbulence from the motion of the water in which the sensor may deploy. To reduce the turbulence, such as eddies around the fibers, the casing may have some adaptations.

[0064] As shown in Figure 26, the housing 90 may have cutouts such as 94 to allow the fluid to smoothly move in and out of the measurement volume. Similarly, the bottom of the housing may have channels such as 96. This allows the fluids to flush out of the

measurement volume and reduce skin drag. These adaptations may also alleviate bubbles that may occur in the early deployment stages.

[0065] Another adaptation that addresses both turbulence and fouling consists of a screen that at least partially surrounds the housing. Figure 27 shows an example of such a screen 100. The screen may alleviate fouling. In some environments, the waveguides or their protective coverings may become fouled with biologic matter, such as algae, seaweed, etc. that either forms on the waveguides or contacts them from the water and leaves a residue. The screen will help eliminate this fouling.

[0066] Other antifouling mechanisms include application of a pH adjustment tool, such as application of a seawater electrolyzer through a nozzle such as 102. Other alternatives include brushes such as 104 that can be driven by a power screen to run up and down the lengths of the waveguides to clean them. Since the waveguides receive light another light path could be set up that shines UV light through the waveguide fibers.

[0067] So far, the different implementations of the refractometer system required the use of external devices and to be attached to a computer for operation. They may also require external power. Another embodiment 110, shown in Figure 28, consists of a completely standalone sensor design, containing two waveguides and several peripherals to help with the measurements. Figure 28 shows an electronics schematic-level design of the refractometer system and all its subcomponents, and are described below. Several commercially available components are available for each component

[0068] The system includes a temperature sensor to measure the external water. One embodiment includes a sensor that has 0.05 °C resolution, and subsecond response time. A pressure sensor 114 determines the pressure of the water at the depth of the sensor as deployed. One embodiment is capable of measuring at least 1 Hz measurements.

[0069] Each waveguide 116 and 126 has its own light source and photodiode, which may be on the same board. Each waveguide may have its own temperature sensor 120 and 124 to monitor the temperature of the light sources and detectors 118 and 122. In this embodiment, the waveguides 116 and 126 are external to the housing or canister 128, with the remainder of the components inside the canister, which may be a pressure-proof, water tight container to hold all the measurement and other components.

[0070] The main system board 130 may contain several elements including a GPS 132 that allows location of the sensor to be tracked. A data card 144 may enable the acquisition and storage of raw data. Each waveguide may have a circuit 134 and 136 to convert the voltage output of the detectors into optical measurements and/or refractive index calculations.

Similarly, the external temperature sensor and pressure sensors may have a circuit for performing measurements and correlations of the pressure and temperature readings. These boards communicate with the controller 142, under the timing of a clock 140.

[0071] The data from the card, or directly from the controller board may be downloaded via Blue Tooth or USB, as well as transmitted by Blue Tooth or other near-field communications using the port 146 to a PC. The sensor receives power from batteries 148 under control of the power controller 150.

[0072] Figure 29 shows a schematic of the mechanical-electrical implementation that contains all the components mentioned above. Figure 29 shows how each waveguide such as 114 will be interfaced to one LED as a source board 118 and two photodiodes on the second board 122. One will measure the light as it has interacted with the measured fluid through the waveguide and the other will monitor the LED source. The LED driver provides constant light output (either with constant current or via feedback loop on from the reference photodiode (including or not including temperature compensation), and it can be used with different color LEDS such as red and cyan. The LEDs may be driven at different power levels set by the user.

[0073] Figure 30 shows a mechanical housing that enables the implementation of the components that are combined to complete this example of a completed sensor system. Waveguide 116 is housed in the cap on the pressure proof contained (rated to 500 m). A pressure relief valve 160 is included in this embodiment. Figure 30 also shows the approximate location of the second waveguide 126. The second waveguide is a wrapped (bent) waveguide that will be attached to the walls of the cylindrical pressure proof container and penetrating the walls at each end of the fiber. The fiber could be glued or held in with a fixture. Figure 31 shows the cap 170 and the body 172 of the pressure proof container.

[0074] Up until now, the discussion has focused on solid waveguides and measured fluid surrounding the waveguide. However, the configuration of this set-up can be changed and a tubing of the materials can be used. This is illustrated in Figure 32. To take advantage of the enhancement using two waveguides, as shown so far, one of the tubing 190 can be made with Teflon with a refractive index lower than water while the other can be higher. Optics and means to control the input light into the waveguide can be used as discussed in other embodiments.

[0075] The above discussion has focused on using two waveguides to obtain measurements using two refractive indices. However, one can also obtain two refractive indices using a single waveguide with portions that have different refractive indices. Figure 33 shows an embodiment of such a waveguide. One should note that while the embodiment shown consists of a waveguide having multiple curves, any waveguide having portions with different refractive indices and an ability to differentiate the output of each portion.

[0076] The embodiment of Figure 33 shows a single waveguide 202 that receives light from a light source 200. A first portion 204 of the waveguide has a first refractive index. A detector 208 receives the light that passes through that portion. A second portion of the waveguide 202 has a different refractive index than 204, although it is possible that the second portion has the same refractive index as the first portion, but different from the refractive index of the waveguide 202. More detectors such as 210, 216 and 214, may be deployed around the waveguide. One of these detectors 212 may be a camera. Therefore, while the system uses two refractive indices, it may obtain measurements from one waveguide with different portions, rather than requiring two waveguides.

[0077] The measurement of the refractive index in the embodiments here in each waveguide relates the relative light signal as it interacts along the waveguide-measured fluid interface with respect to the one that is input into the measured section of the waveguide. The current calibration procedure includes taking the analog signal of each waveguide, taking the ratio including a reference measurement. A calibration against this relative light measure is a method to obtain measurements of refractive index, salinity, speed of sound or composition in the measured fluid.

[0078] The first set of result figures, Figures 5-15, were generated using a measurement using deionized water as the reference, while the rest of the figures, use the light measured at the end of the fiber after the waveguide- fluid measurements divided by that the light measured by a photodiode close to the light source, as shown in Figure 29.

[0079] Because the light sources output intensity and the light detector characteristics may depend on its temperature, the system includes the measurement of the temperature of the boards to allow for correction in the analytic process to derive the refractive index of the measured fluid under an environment where the temperature of the boards may be changing. The process would then include a step of determining the temperature and applying a correction to the measurements taken as a function of temperature.

[0080] In this manner, the embodiments have shown that the refractometer can relate to salinity, density and speed of sound. The implication of this fast refractometer is for its use in dynamically changing marine environments to measure the mentioned variables, providing an advantage in accuracy over the CTDs traditionally used. A measure of the water salinity is needed by oceanographers and environmental scientists for a variety of investigations, as well as the military. It is well-known that salinity is a parameter that is needed in other sensor systems. For instance, this variable is needed for using underwater instrumentation, to quantify volatile organic compounds and dissolved gases, e.g. mass spectrometers. This sensor, since it is geo-referenced can also be used as an animal tag. For military applications, salinity and speed of sound are important in underwater communications and submarine warfare.

[0081] A rapid sensor could also measure dynamically changing environment such as coastal areas, with thin temperature layers in the water column, and mixing, density variations influenced by water movement. Refractometers can however be used in many other applications. The improved sensitivity in refractive index can potentially improve limits of detection if the concentration of other chemicals can be related to the refractive index.

Examples of applications of the wide market use is the potential for using this refractometer as an in-line instrument for process control, or a quality control tool for industries such as drugs, food, e.g. orange juice, beer and wine, as well as other chemicals.

[0082] The described type of optical refractometers that measured transmitted power through waveguides have also been used as chemical sensors, by coating the sensing region of the fiber with a material that changes its optical properties in the presence of a target analyte measured. Optical refractometers have been used to indicate the extent of reaction. As the fiber or waveguide is exposed to a material that reacts, the refractive index changes and the transmission power though the fiber is measured, quantifying the changes that are taking place. One should also note that the waveguides discussed up to this point have no cladding at all for the examples chosen here. However, using a fiber with partial cladding may enhance the signal to noise ratio of the measurements and allow the use of these coatings.

[0083] Using similar principles, the optical sensing of environment corrosiveness, and the relative capacity of an environment to cause corrosive changes in a metallic structure, have been proposed. Multi-wavelength light transmission measurements for similar corrosion studies have been performed.

[0084] Other applications include sensing for organic alcohols, moisture, ammonia, pH, ¾,

O2, etc. is performed by utilizing or modifying the uncladded region of the fiber. Sensors using embedded substances that change their optical properties due to the target analyte presence have been implemented by embedding the dyes or compounds in a polymer, sol-gel or ormosil matrix and coating on in the fiber tip. Because the sensing region can be made very long, attainable accuracies are higher. Many target analytes/species can be measured with waveguides. They can be placed in series, following regions in the same pipe-light, or in parallel in a multi-fiber approach. In sum, the approach for marine sensors and other applications could be used as a high precision refractive index meter to obtain salinity and/or density of seawater, but also as fiber optics chemical sensors (FOCS), if indicators specific to the analyte of interest or family of compounds/biologicals are used by coating these on a portion of the waveguide.

[0085] It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.

Claims

WHAT IS CLAIMED IS:

1. A refractometer, comprising:

at least one light source;

a least a first portion of a waveguide positioned to receive light from the at least one light source, the at least a first potion of a waveguide having a first refractive index and at least a second portion of a waveguide positioned to received light from the at least one light source, the at least a second portion having a second refractive index; and

at least one detector to measure light from the first and second portions.

2. The refractometer of claim 1, wherein the first refractive index is higher than a refractive index of the measured fluid and the second refractive index is lower than a refractive index of the measured fluid.

3. The refractometer of claim 1, wherein the first refractive index and the second refractive index are the same.

4. The refractometer of claim 1, wherein the first portion and the second portion are made of a same material.

5. The refractometer of claim 1, wherein the first portion and the second portion are made of different materials, each different material having a different refractive index.

6. The refractometer of claim 1, wherein the first and second portions are solid core waveguides.

7. The refractometer of claim 1, wherein the first and second portions are portions of a liquid core waveguide.

8. The refractometer of claim 1, further comprising optics arranged between at least one of the light source and at least one of the first and second portions, and between the first and second portions.

9. The refractometer of claim 8, wherein the optics includes at least one of a focusing lens, a collimating lens, and a mirror.

10. The refractometer of claim 8, wherein the optics comprise optics arranged between the light source and at least one of the first and second portions, the optics having a dot to block light from a center of the waveguides.

11. The refractometer of claim 1, wherein the at least one light source is positioned at an angle to an axis through a center of a portion.

12. The refractometer of claim 1, wherein at least one portion is curved.

13. The refractometer of claim 12, wherein at least one of the portions is wrapped around a circular puck.

14. The refractometer of claim 1, wherein at least one portion changes one of diameter or shape long a main axis of the waveguide.

15. The refractometer of claim 1, wherein none of the portions are cladded.

16. The refractometer of claim 1, wherein at least one of the portions is partially clad.

17. The refractometer of claim 16, wherein an unclad portion of the partially clad portion is coated with a material having light sensitivity to a target analyte.

18. The refractometer of claim 1, wherein the at least one light source comprises two light sources one for each of two portion, each portion having two detectors.

19. The refractometer of claim 1, wherein the detector comprises one of a photodiode, photomultiplier, charge couple device, and a camera.

20. The refractometer of claim 1, further comprising a pressure sensor to monitor depth of the refractometer in a fluid, a temperature sensor to monitor temperature of the refractometer, and a temperature sensor to monitor temperature of a measured fluid.

21. The refractometer of claim 1, the system further comprising circuits to receive optical measurements from the detector, and a data port to output the results of the optical measurements to a computer.

22. The refractometer of claim 1, the system further comprising a controller contained within the refractometer to receive the optical measurements and produce results.

23. The refractometer of claim 1, further comprising a pressure proof housing.

24. The refractometer of claim 1, further comprising a global positioning satellite component.

25. The refractometer of claim 1, further comprising an anti-fouling mechanism.

26. The refractometer of claim 25, wherein the anti-fouling mechanism comprises one of a copper screen, a pH adjustment tool, a UV light source, and at least one mechanized brush attached to a motorized screw.

27. The refractometer of claim 1, further comprising turbulence reduction components.

28. The refractometer of claim 27, wherein the turbulence reduction components comprise at least one of a metal screen, channels arranged adjacent a measurement volume to flush out fluid, cut outs for fluid to exit the measurement volume, and a housing having a hydrodynamic shape.

29. A method of measuring characteristics of a fluid, comprising:

detecting light from a first at least a portion of a waveguide having a first refractive index and from a second at least a portion of a waveguide having a second refractive index; converting the detected light into a first set of measurements corresponding to the first refractive index and a second set of measurements corresponding to the second refractive index; combining the first and second set of measurements to produce a final measurement having higher sensitivity than measurements obtained from one refractive index.

30. The method of claim 29, wherein converting the detected light further comprises: measuring a signal from each at least a portion of a waveguide; and

taking a ratio including a reference measurement to produce the first and second set of measurements.

31. The method of claim 29, wherein combining the first and second sets of

measurements comprises one of adding the first and second sets, multiplying the first and second sets, or dividing one of the first and second sets by the other set.

32. The method of claim 29, wherein converting the detecting light includes adjusting the first and second sets of measurements to account for temperature.