DE29623263U1 - Device for spatially resolved substance detection - Google Patents

Description

Research &zgr; center Karlsruhe GmbH ANR 5661498

Karlsruhe, 28 April 1998 PLA 9774 Gü/he

Device for spatially resolved substance detection

- 4 Description:

The innovation concerns a device for spatially resolved substance detection according to the preamble of claim 1.

When locating damaged areas in optical fibers, e.g. in the case of cable breaks, an optical measuring technique is used that is based on the principle of time-resolved backscatter measurements (Optical Time Domain Reflectometry, OTDR).

OTDR systems for spatially resolved measurement of physical effects such as temperature or pressure changes in the fiber cladding are known. DE 40 19 980 Al describes such a "temperature sensor arrangement distributed over a fiber optic system". It is only used to determine the temperature distribution along the sensor fiber, not to detect the presence of chemical substances. It is therefore not possible to clearly distinguish whether a substance is present in the vicinity of the optical fiber.

In the publication H. Wada, E. Okuda, T. Yamasaki; "Optical Guided-Wave Oil Leak Sensor", OFS Conference, Tokyo 1986, p. 109, a chemical sensor with spatial distribution is presented, which is also based on OTDR technology. Here, however, the sensitive element is not the fiber itself, but rather complex, integrated optical sensors connected to the fiber. If oil is present on the surface of one of the point-like distributed chemical sensors, a change in the refractive index alone causes a reduction in the light reflection on the surface of the integrated optical waveguide, which can be detected as a loss of intensity. This means that only one substance can be detected in a small area of a maximum of 1 cm2 on the surface of the sensor element. This results in only a point-like distributed (guasi-continuous) measurement of chemical substances. The continuous, i.e. Continuous monitoring of a measuring section

- 5 is
However, this is not possible. Since only one evaluation wavelength is used, the method described cannot clearly distinguish whether the disturbance is caused by a chemical substance or another effect, such as a fiber break or a temperature change in the vicinity of the sensitive λΩ waveguide.

From H. Yoshikawa et al.; "Distributed Fluid Sensor Using Eccentrically Cladded Fibers", Electronics and Communications in Japan II, 71 (2), 1988, p. 89, a glass fiber itself is known as a sensitive element, but the analytes are not concentrated in the fiber. The fiber is only immersed in the analyte solution. In order to bring the evanescent field of the light transported through the fiber into contact with the analyte molecules in the solution, the glass fiber must be asymmetrical in cross-section (eccentric core). Pure refractive index effects on the evanescent field of the eccentric sensor fiber are described. This text only deals with the design of the fiber (e.g. degree of eccentricity). The measuring arrangement used in the investigations is not based on the OTDR principle, but rather measurements were taken "in transmission". The possibility of using the fiber with OTDR technology for continuously distributed measurements is mentioned but not implemented. Since only one evaluation wavelength is used, the method described cannot clearly distinguish whether the disturbance is caused by a chemical substance or another effect such as a fiber break or a temperature change in the vicinity of the sensor fiber.

EP 0 298 118 Bl describes the backscatter signal obtained during an OTDR measurement in detail. Although scattering effects are generally mentioned, this only refers to the scattering of light at mechanical inhomogeneities within the fiber, such as splices or couplings. Only the mechanical testing of glass fibers is addressed.

The aim of the innovation is to design a device for time-resolved backscattering measurement in such a way that it also enables specific substance detection.

This problem is solved by the characterising features of claim 1.

The subclaims describe advantageous embodiments of the innovation.

A large number of technical systems or objects in which environmentally hazardous chemical substances are transported, processed or even just safely contained require environmental monitoring in order to be able to detect the release of the substances as early as possible.

This minimizes the potential environmental hazard associated with these systems. Examples of relevant systems are pipelines, tank farms, reaction vessels in chemical plants, but also landfills in which environmentally hazardous substances (e.g. hydrocarbons) are stored. Due to the large spatial extent of such systems, it is necessary to have continuous, spatially extensive monitoring technology available that allows the release of chemical substances to be recorded as quickly and locally as possible. This is made possible in an advantageous way by the new method.

Using the specified method of OTDR measurement at two wavelengths (absorption or fluorescence wavelength and wavelength at which none of these effects occurs), it is possible on the one hand to locally resolve the return signal generated by a substance in a chemically sensitive optical fiber and on the other hand to clearly distinguish it from signals caused by other effects such as splices, fiber breaks or

- 7 Temperature effects
As the fibre used itself represents the sensitive element over its entire length, a seamless, spatially resolved and quantitative measurement of chemicals is possible. A local resolution of a chemically generated interference signal in the meter range is achieved, whereby the length of the measuring section can in principle be several hundred metres up to several kilometres (the attenuation value of the fibre used is approx. 10 dB/km in the visible range). All substances which accumulate in the fibre cladding can be measured, such as (chlorinated) hydrocarbons or other non-polar organic species. A further prerequisite is that the substances have an absorption band or fluorescence emission in the wavelength range which is transparent to the fibre optic material. Any commercially available quartz glass fibre with polymer cladding can be used as a sensor fibre. This does not have to be subjected to a complex procedure to make it chemically sensitive to relevant substances such as hydrocarbons. Any external mechanical nylon protective sheath which may be present can, for example, be used with a polymer cladding. The fibers can be stripped at 165 ^° C using propylene glycol to expose the silicone coating. In order to protect the mechanically more unstable fiber from breaking after the outer nylon coating has been removed, the fiber is pulled into an open-wound stainless steel flat wire coil before being removed. A perforated Viton tube can also be used as a mechanical protective cover. The measurements were shown in the visible wavelength range as an example, but this can of course also be applied to other wavelength ranges, depending on where absorption or fluorescence emission occurs in the substance in question. A preferred wavelength range for the absorption wavelengths is the near infrared (NIR) range, for example, where overtone and combination bands of organic substances occur.

The innovation is explained in more detail below using exemplary embodiments with the help of the figures.

Fig. 1 shows a schematic diagram of a device for carrying out the method.

Figures 2 to 5 show examples of time-resolved detector signals.

Fig. 6 shows a calibration curve, ₍

Fig. 7 shows an overview of different signals in the time spectrum and the

Fig. 8 Absorption and fluorescence spectra of hydrocarbons (crude oil).

The detection and localization of chemical substances along a light guide using the OTDR method requires that the optical fiber has a hydrophobic polymer fiber sheath (e.g. silicon). This polymer sheath acts as an optically thinner medium that transports the measuring light in the optical fiber. It also serves as mechanical protection for the quartz glass fiber core, without which the fiber is very brittle and difficult to handle in practice. Thirdly, it acts as a hydrophobic sensor layer in which relevant substances can accumulate. The light guide with its polymer cladding is therefore itself the sensitive element. By arranging such a fiber near a potential source of contamination, the organic substances that escape in the event of a leak can accumulate in the fiber sheath by diffusion. Due to the interaction of the diffused substances with the evanescent light field present in the fiber sheath, e.g. B. due to light scattering, absorption or emission, the contaminant accumulation point can be detected and localized using the OTDR technique.

- 9 The
The enriched substance in the fiber cladding represents a "disturbance point" for the short light pulses coupled into the fiber. From the time difference between the excitation pulse and a signal generated by the disturbance and transported back to the beginning of the fiber, the position of the enrichment point along the fiber can be determined via the speed of light in the fiber.

If an excitation pulse passing through the fiber in the evanescent field encounters such a defect in a substance enriched in the fiber cladding, different cases can be distinguished:

1. The absorption band of the enriched substance is not in the range of the wavelength of the pulsed laser light source:

a) The substance has a higher refractive index than the polymer fiber sheath:

- the light in the evanescent field is increasingly scattered due to the increase in the refractive index and part of the scattered light is absorbed by the fiber and transported back to the beginning of the fiber. An increased intensity of the backscattered light occurs at the enrichment point.

b) The substance has a lower refractive index than the polymer fiber sheath :

- at the enrichment point, the refractive index is reduced and the light is guided even better in the light guide. No additional scattered light occurs. At the enrichment point, the backscatter signal remains unchanged.

2. The absorption band of the enriched substance lies in the range of the wavelength of the pulsed light source:

• " · · · · ft ft

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the light in the evanescent field is weakened due to the substance's own absorption at the enrichment point. The scattered light intensity absorbed by the fiber and transported back to the beginning of the fiber is abruptly reduced by this own absorption at the enrichment point.

3. The absorption band of the enriched substance is in the range of the wavelength of the pulsed light source and, in addition, the substance is stimulated to emit fluorescence: the light in the evanescent field is weakened due to the substance's own absorption at the enrichment point, but at the same time it stimulates the substance to emit longer-wavelength fluorescent light. The scattered light intensity absorbed by the fiber and transported back to the beginning of the fiber is reduced by the self-absorption at the enrichment point, but the emission of the fluorescent light overrides this effect and overall the intensity of the backscattered signal at the enrichment point increases.

The measurement setup of an OTDR system suitable for the spatially resolved localization and quantitative determination of chemical substances is shown in Fig. 1. Two short-pulsed light sources (e.g. laser diodes or an NdYAG laser) with different emission wavelengths are used (laser light source 1 or 2). The pulse widths should be in the range of a few nanoseconds in order to obtain a spatial resolution in the meter range. The light from these short-pulsed light sources is focused onto a branch of a 3:1 fiber splitter via a gradient index lens (9) (GRIN lens). The light pulses are guided into the sensor fiber for chemical substances mentioned above via the output branch of the fiber splitter. The excitation pulses pass through the sensor fiber and leave it at the other end, where part of the light intensity is reflected (Fresnel reflection) and onto

- 11 these
way back to the beginning of the fiber. However, if there is an impurity in the fiber caused by chemicals enriched in the fiber cladding, part of the light intensity of the excitation pulses is transported back to the beginning of the fiber at this impurity due to the interactions between enriched substances and evanescent field described above. The returned signal has a shorter delay time than the Fresnel reflection from the end of the fiber. The light transported back to the beginning of the fiber is picked up via the third branch of the fiber branching device. The light is then passed through bandpass filters (7) to select the desired wavelength. Two bandpass filters are used, which are placed alternately in the beam path. One filter selects light in the range of the emission wavelength of the first laser light source 1, while the second bandpass filter is tuned to the emission wavelength of the second laser light source 2. The measuring light selected by the respective filter is focused and detected by a GRIN lens on a fast-registering, sensitive detector 5 (e.g. a silicon avalanche photodiode). After appropriate amplification, the detector signals are transmitted to a fast evaluation unit 6 such as a storage oscilloscope and, after A/D conversion, the temporal progression of the return signals arriving there is recorded. The oscilloscope should have a bandwidth of around 1 GHz in order to be able to process the high pulse speeds. The position of the fault can be deduced from the time delay of the registered backscatter signals using the speed of light. The detector signals stored in the evaluation unit can also be fed to a computer for more precise evaluation. ''

The effect of chemical substances on the polymer fiber sheath of an optical fiber leads to the above-mentioned

- 12 -

Effects that can be used to locate and quantify the substance along the sensor fiber as follows:

Fig. 2 shows the OTDR measurement result with the measurement setup mentioned above when a substance accumulates in the fiber cladding which absorbs in the range of the wavelength of one of the pulsed laser light sources (eg at laser wavelength 1, above case 2). The substance 8 enriched in the fiber cladding interacts with the incident light at the enrichment point by absorption, and in this case an "absorption level ^1" (clear sudden drop in the backscattered light intensity) is shown in the backscatter signal/time diagram at the enrichment point. Here, light components that are backscattered at every point on the fiber are "abruptly swallowed up" by the absorption. The level depends on the concentration of the absorbing substance in the solution and on its absorption capacity (molar extinction coefficient) at the incident wavelength. In addition to this absorption level, the reflex of the start pulse, the backscatter signal from the 3:1 fiber splitter (4) and the backscatter signal from the fiber end (Fresnel reflex at approx. 680 ns, length of the sensor fiber approx. 70 m) can also be seen. The latter can serve as a reference signal.

Fig. 3 shows the OTDR measurement result for a substance that does not absorb at laser wavelength 2 and has a higher refractive index than the polymer fiber cladding (case l.a). Here, an increase (peak) in the backscattered light intensity occurs at the enrichment point (at approx. 540 ns). This backscatter peak is caused solely by the increase in the refractive index in the fiber cladding, which is generated when the substance penetrates. The increase in the refractive index represents a disturbance in the homogeneous fiber cladding structure, which causes increased light scattering. The reflection from the fiber end (Fresnel reflection at 680 ns) is significantly weakened or no longer present, depending on the refractive index of the enriched substance, because

an increasing part of the guided light is already scattered and coupled out at the enrichment point. The scattering signal can be recognized after just a few seconds of enrichment, whereby the level of the signal depends on the refractive index of the enriched substance, its concentration and its sorption speed.

Fig. 4 shows the OTDR measurement result for a substance that does not absorb at laser wavelength 2 and has a lower refractive index than the polymer fiber cladding (case l.b). Here, too, there is no absorption level, since the light of the excitation pulse is not weakened by the substance enriched in the fiber cladding. However, since the enriched substance has a lower refractive index than the fiber cladding, no backscatter signal occurs at the enrichment point with a delay time of approx. 540 ns. Only the reflections of the start pulse and the backscatter signal from the fiber end can be seen.

By recording the OTDR backscatter signals of a chemically sensitive optical fiber in a time-resolved manner at a measurement wavelength that is in the absorption range of the substance and at a measurement wavelength that is outside this absorption range, not only can the location of the substance enrichment be clearly resolved, but the different signal shapes of the two measurements also make it possible to clearly distinguish the chemical defect from other defects. At the enrichment location of a substance, an absorption step occurs in the backscatter signal/time diagram of the sensor fiber in the range of the absorption wavelength, while at the non-absorbing wavelength either a backscatter peak (case la) or no signal (case 1 b) occurs.

However, if the sensor fiber breaks cleanly, a peak in the backscatter intensity is observed at both wavelengths.

- 14 These
Peaks are similar to the Fresnel reflection occurring at the fiber end (see Fig. 2, 3, 4).

With the measuring method specified, other disturbances (e.g. fiber breaks, splices or temperature changes in the fiber cladding) always have the same effect at both wavelengths, so that the signals obtained are similar in their "pattern". This is shown in Figure 7a: breaks and splices lead to the same signals at both wavelengths. However, disturbances caused by substances penetrating the fiber cladding result in clear differences in the signal at the absorption wavelength and at a wavelength at which no absorption occurs (Figure 7b).

Fig. 5 shows that the measurement method specified can in principle also be used in the case that a substance accumulates in the fiber cladding that is excited to emit fluorescence with an incident laser wavelength of 1. Here too, part of the fluorescent light is transported back to the beginning of the fiber and can be detected by selecting a suitable bandpass filter. At the accumulation point (here at approx. 125 ns) an increased signal intensity occurs and the fluorescence decay behavior typical for the respective substance results in an asymmetrical peak shape. In addition to this peak, the reflection of the start pulse, the backscatter signal from the 3:1 fiber splitter and the backscatter signal from the end of the fiber can also be seen (the sensor fiber in this case was only approx. 23 m long). This makes the return signal clearly different from the other return signals discussed so far. The signal level of the return signal caused by the fluorescence emission depends, among other things, on the concentration of the substance, as shown in Fig. 6. This also makes it possible to quantitatively determine the substance concentration.

The enrichment of a fluorescent substance also leads at absorbing and non-absorbing laser wavelengths

to different signals (Figure 7c). At the absorption wavelength, the characteristic fluorescence signal is obtained, while at the non-absorbing wavelength, depending on the refractive index of the enriched substance, either no signal or a backscatter signal is obtained. In this way, such an impurity can be clearly distinguished from another impurity, where the same return signal is obtained at both wavelengths (Figure 7a).

However, a fluorescent substance can also be clearly identified if only one laser wavelength is used, because the characteristic, exponentially falling peak flank differs from the other signals. By using an edge filter, the excitation wavelength can also be blocked out so that only the longer-wave fluorescent light is allowed through. In this way, the accumulating fluorescent substance is specifically detected. If there is a mechanical disturbance in the fiber, no signal is visible because the shorter excitation wavelength is intercepted by the edge filter.

Figure 8 shows absorption and fluorescence spectra of crude oil. For leak monitoring in a crude oil pipeline, an excitation wavelength of between and 350 nm is recommended. When excited in this wavelength range, sufficient fluorescence intensity is obtained, which can still be detected over longer fiber lengths despite the increased attenuation of the optical fiber in the UV range.

Claims (6)

Device for spatially resolved substance detection consisting of at least one pulsable light source, an optical fiber into which the light pulses can be coupled, a fiber optic beam splitter, a photodetector and an evaluation unit for detecting the backscatter signals of the pulsable light source coming from the optical fiber in the time domain, characterized by: two lasers (1, 2) as pulsed light sources, the wavelength of one laser (1) being in the range of an absorption band of a substance (8) to be detected, and the wavelength of the second laser (2) being in a range in which the substance (8) to be detected does not absorb, a medium surrounding the optical fiber (3), which is permeable to the substance to be detected (8) and whose refractive index is smaller than that of the optical fiber. 2. Device for spatially resolved substance detection consisting of at least one pulsed light source, an optical fiber into which the light pulses can be coupled, a fiber optic beam splitter, a photodetector and an evaluation unit for detecting the backscatter signals of the pulsed light source coming from the optical fiber in the time domain, characterized by: a laser (1) as a pulsed light source, the wavelength being in the range of an absorption band of a substance to be detected (8), a medium surrounding the optical fiber (3), which is permeable to the substance to be detected (8) and whose refractive index is smaller than that of the optical fiber and an optical filter (7) which is in front of the photodetector (5) 3. Device according to claim 1, characterized by a Bandpass filter (7). 4. Device according to one of claims 1 to 3, characterized in that the medium surrounding the optical fiber (3) is silicone or Teflon. 5. Device according to one of claims 1 to 4, characterized by a mechanically stabilizing protective cover for the optical fiber (3) that is permeable to the substance (8). 6. Device according to one of claims 1 to 5, characterized in that the stabilizing protective cover is a flat wire spiral made of stainless steel.