DE102004015946B3 - Raman spectrum generation and detection unit couples light into medium through optical fibre without focussing - Google Patents

Abstract

A Raman spectrum generation and detection unit couples light into a medium (9) through a low loss optical fibre (6) connected by an optical coupler (5) to a fibre (4) from the laser (1) source optical fibre without focussing and with minimal stripped length (7).

Description

The The invention relates to an apparatus and a method for producing and for detecting a Raman spectrum of a to be examined Medium having the features mentioned in the preambles of claims 1 and 15.

Raman spectroscopy has become an established method in both analytics and process monitoring through the introduction of lasers as high-intensity excitation sources. Raman spectroscopy, for example, is used for the analysis of organic compounds or oxidic solids. As part of process monitoring, this method can be used to characterize material flows and, if desired, to control them based on the measurement results. Here, probes are used in many applications, which are coupled by means of optical fibers both with a light source and with a detection unit for detecting the Raman spectrum, usually a spectrometer. Such Raman probes are used for example in the field of materials science, chemical engineering, pharmacy and environmental technology. The Raman probes are usually optimized so that high analytical requirements z. B. in terms of spectral resolution and detection limits are met. In this case, a high signal-to-noise ratio and the substantial elimination of background signals are desirable. For this purpose, inter alia, US 2003/0147593 and US 6,018,389 It is known to separate or largely separate the light paths for the excitation of the sample (from the laser to the probe) and for the detection of the scattered light signal (from the probe to the detection unit / detector). Due to the separation of excitation and Detektionslichtweg the backscatter of the optical fiber used, which usually consists of silicon dioxide, almost completely eliminated. Since the backscatter would produce a broad background signal, this approach can increase sensitivity and accuracy. Furthermore, for example US 5,999,255 It is known to increase the radiation intensity by arranging the sample in the focus of a suitable optical system. However, this can disadvantageously lead to an undesirably high thermal load on the sample, as a result of which an influencing of the process to be investigated can take place by the measurement itself. In order to prevent this, it is proposed, for example, in R. Salzer et al., Applied Spectroscopy 51 (1997) 1471, to rotate the sample in order to reduce the local energy input into the sample caused by the excitation light source. However, sample rotation is usually not possible during process monitoring.

Out WO 2004/051242 A1 discloses an instrument for Raman spectroscopy of a tissue, in the one optical fiber 120 via suitable optics ("coupling optics "110), for example via a lens is connected to the light source 100. As a result, the device a variety of costly components for a corresponding Function must be adjusted sufficiently.

Out US 2003/0049858 A1 is a device for determining the concentration of additives in a metallization bath using Raman spectroscopy known, in which a probe 120 must always have focusing elements, because the probe 120 is used to focus into the sample 124. This also makes this device with a variety of cost-intensive components accompanied, for an ent speaking function also be adjusted sufficiently have to.

Out WO 2004/031746 A1 is a spectral measuring device using acousto-optically adjustable Filter (acusto-optic tunable filters), in which for excitation of a sample always broadband light must be used, otherwise under utilization the acousto-optic filter no spectrum could be obtained.

Should the Raman probes for monitoring and control functions or for substance determination without any special analytical Requirements are used (use as low-cost sensors), so own the aforementioned devices and methods for the production and the Detection of a Raman spectrum some disadvantages: they are complicated, which is too relative high production and procurement costs. By default use Focusing optical components is the thermal load of the media to be examined greatly and often inadmissibly increased. That's why the devices are according to the prior art, in particular for process monitoring in the property as sensors with the typical application criteria (Accuracy requirements, costs) can not be used advantageously.

It is therefore an object of the present invention to provide a device (Raman probe) and a method for generating and detecting a Raman spectrum, which at given requirements in terms of material analysis (number of components, concentrations, detection limits required) a more cost-effective production and detection of a Raman spectrum as the known prior art devices. Another task of the inventor tion is to provide a device for generating and detecting a Raman spectrum, which is less susceptible to misalignment than the known devices of the prior art. Furthermore, too high a thermal load of the substance to be examined (medium) should be avoided.

These Tasks are performed according to the invention a device and by a method for generating and for detecting a Raman spectrum of a substance to be examined (medium) with the features of claim 1 (device claim) and the claim 15 (method claim) in conjunction with the features in the preamble solved.

The inventive device for generating and detecting a Raman spectrum of a to be investigated Substance comprises a first optical fiber, wherein a first end the first optical fiber over a means for coupling light with a light source and via Means for coupling out light with a detection unit for detection connected to a Raman spectrum, the second end of the optical Fiber no additional Comprising components for focusing the light and the means for coupling the excitation light into the first optical fiber has an optical coupler, which via a second optical fiber is connected to the light source, wherein the light source is a laser is.

The The first optical fiber preferably has, except in the region of its kopplerabgewandten, second Finally an opaque Serving. Preferably the optical fibers have a low attenuation, especially in the infrared and visible range of the electromagnetic spectrum. In a particularly preferred embodiment The invention provides for the light coupling and the Extraction of the backscattered light via a and to realize the same optical fiber. Alternatively, it is possible, for light coupling and coupling different optical fibers to use, thereby however, the achievable cost advantage would be reduced. The Probe tip (second end of the first optical fiber) has none additional optical components for focusing the light. This means, that the light coupling and the light extraction directly over the exposed, second end of the first optical fiber (in particular over the Face) will be realized. The optical fibers can each with the help of conventional optical connectors are extended without the fundamental operability impaired becomes. For generating and detecting a Raman spectrum of a the substance to be examined or a process medium becomes the second End of the first optical fiber (probe tip) preferably immediately in the substance to be examined, that is to say in the volume of the substance to be examined Materials, arranged. In principle, however, is also an arrangement only possible in the area of the medium to be examined, if the medium is sufficient is lit and the backscatter light sufficiently coupled into the sensor fiber. As to be examined Substances are especially powdered solids, but also other solids as well as liquids and gases. It is particularly easy to mix fabrics with sharp ones Raman bands like some organic compounds or metal oxides (Titanium oxide or molybdenum oxide) with the device according to the invention detect.

The Training the sensor tip can be done differently, it must but at least guaranteed be that face the first optical fiber is exposed. Preferably over the end face of first optical fiber, the excitation light is diffusely decoupled. It enters thereby with the substance to be examined (medium) Interaction without causing significant warming.

The The backscattered light from the substance to be examined (in turn) is transmitted via the open end of the first optical fiber (sensor tip) coupled and over the optical coupler to a detection unit for detecting the Raman spectrum directed.

Basically, one would expect that is a decoupling of the excitation light in the manner described and a coupling of the backscattered Light from the substance to be examined on the exposed end of the optical fiber without optical focusing have too low a luminous efficacy would, to be suitable for a detection of the Raman spectrum to be used of the medium to be examined. Finally, it would continue to expect that only at a high light intensity of the excitation light in the substance to be examined sufficient backscattering yield could be achieved being a high light intensity to a high level of thermal energy in the investigated Lead substance would, which are usually only suppressed by moving or rotating the sample could. This However, it is not possible, especially in process monitoring.

• – Einkoppeln von mittels eines Lasers erzeugter elektromagnetischer Strahlung über eine erste optische Faser in das zu untersuchende Medium ohne Verwendung zusätzlicher Komponenten zur Fokussierung der elektromagnetischen Strahlung, wobei die elektromagnetische Strahlung vom Laser über eine zweite optische Faser und einen optischen Koppler in die erste optische Faser eingekoppelt wird,
• – Einkoppeln der vom zu untersuchenden Medium rückgestreuten elektromagnetischen Strahlung in die erste optische Faser und
• – Detektion des Spektrums der vom zu untersuchenden Medium rückgestreuten, in die erste Faser eingekoppelten, elektromagnetischen Strahlung.

Surprisingly, it has been found that the coupling-out of excitation light and the coupling-in of backscattered light via an exposed optical fiber suffice to produce an evaluable Raman spectrum for a large number of processes to be monitored, for example catalytic processes on metal oxides and thus possibly accompanying changes in the catalyst structure to generate and detect. One cause of the unexpectedly high backscatter yield may be the fact that the excitation region is magnified by the reflections within the sample (the substance of interest). A reflection of the backscattered light in powdered samples can also lead to an amplification of the detectable backscatter yield. Advantageously, the diffuse light entry into the substance to be examined by coupling the excitation light from the light source via the preferably circular end face of the sensor tip leads to a low thermal load and therefore practically does not influence the process to be investigated. This represents a decisive advantage, since, in particular during process monitoring, alternative methods for reducing the thermal load are virtually unavailable. As a result, the scope of the inventive device for generating and detecting a Raman spectrum over the known devices is significantly expanded. Further advantages are the robust construction and the low production costs of the device according to the invention. It is furthermore advantageous that the fact that the excitation light path coincides with the backscatter light path in the region of the sensor fiber, the device is relatively insensitive to a misalignment. For evaluating the Raman spectrum of the substance to be examined, the background spectrum occurring due to the backscattering in the optical fiber must advantageously be eliminated, which can be achieved very precisely by subtracting a reference spectrum (without substance to be investigated). The process according to the invention is accordingly characterized by the following process steps:

• - Coupling of generated by a laser electromagnetic radiation via a first optical fiber in the medium to be examined without using additional components for focusing the electromagnetic radiation, wherein the electromagnetic radiation from the laser via a second optical fiber and an optical coupler coupled into the first optical fiber becomes,
• - Coupling the backscattered from the medium to be examined electromagnetic radiation in the first optical fiber and
• - Detection of the spectrum of backscattered from the medium to be examined, coupled into the first fiber, electromagnetic radiation.

In a preferred embodiment of the inventive method the detection unit is connected to an evaluation unit.

In a further, preferred embodiment the method according to the invention will be added a Raman spectrum determined, the sensor tip of the optical Sensor fiber (the first optical fiber) in a region without medium with measurable Raman spectrum is arranged (reference measurement). The addition measured Raman spectrum is then subtracted from those spectra which in the arrangement of the sensor tip of the optical sensor fiber in the area of the substance to be examined (actual measurement) were detected (background correction).

This additional Step for determining a reference spectrum, which leads to the elimination of the especially due to the inherent dispersion of the optical fiber Underground is necessary, although in many devices after The prior art omitted, but this step means in a process monitoring only a single measuring procedure to be performed before the process control, while in process monitoring only a permanent one Measurement of the Raman spectra must be made of the medium to be examined. In that sense, this is the reason required additional expenditure in relation to the scope of the total measurements negligible low. Due to the low manufacturing cost of the device according to the invention and the resulting cost advantage is a use as a low-cost sensor despite the slight more complex Evaluation procedure (consideration the reference measurement) Systems of the prior art are usually advantageous.

In a preferred embodiment exists the serving the optical fibers of an opaque material. The optical Fiber is preferably a glass fiber, typically a polyimide-coated one Quartz fiber with a diameter of 50 to 500 μm. The uncovered end of the Sensor fiber (sensor tip) preferably has a length of about 1 mm.

In a further, preferred embodiment are by means of optical coupling plug, the light source or the Detection unit with the optical coupler and the optical coupler connected to the sensor fiber. The optical coupler is preferably a bidirectional Y-coupler.

When Detection units for detecting the Raman spectrum are preferably a dispersive or a Fourier transform Raman spectrometer with integrated Filter used to reduce the intensity of the Rayleigh line.

In addition to the material analysis and quantification, in particular as a low-cost sensor in process applications, the device according to the invention or the method according to the invention can, inter alia, also for determining the temperature of the be used for investigating substance or the medium to be examined. For example, the ratio of anti-Stokes and Stokes Raman bands can be used for this purpose.

In a further, preferred embodiment it is provided on the exposed end of the optical fiber (Sensor tip), preferably on the end face of the optical fiber, metal cluster applied. Their size and shape are to be chosen that the intensity the Raman spectrum of the surrounding, preferably liquid, Substance significantly increased (Surface Enhanced Raman Spectroscopy - Surface Enhanced Raman Spectroscopy [SERS]). In a preferred embodiment The metal clusters are made of silver. This modification of the sensor tip it also allows the detected substances in terms of their Characterize interaction with the metal clusters and from it additional Derive information for the determination of the substances.

The Invention will be explained in more detail with reference to an embodiment.

It demonstrate:

1 a device according to the invention for generating and detecting a Raman spectrum of a substance to be examined or a process medium in a schematic representation and

(a)

des Faser-Untergrundes sowie der ausgewählten Substanzen:

(b)

Titandioxid-Modifikation Rutil (TiO₂),

(c)

Titandioxid-Modifikation Anatas (TiO₂) und

(d)

p-Bis(o-Methylstyryl)-Benzen (bis-MSB, C₂₄H₂₂: organischer Feststoff).

2 according to the inventive arrangement accordingly 1 Raman spectra measured at room temperature

(A)

of the fiber substrate and the selected substances:

(B)

Titanium dioxide modification rutile (TiO ₂ ),

(C)

Titanium dioxide modification anatase (TiO ₂ ) and

(D)

p-bis (o-methylstyryl) -benzene (bis-MSB, C ₂₄ H _22: organic solid).

1 shows a device according to the invention for generating and detecting a Raman spectrum of a substance to be examined or a process medium in a schematic representation. For generating and detecting a Raman spectrum of a substance to be examined or of a process medium to be investigated by means of the device according to the invention (process probe), light is emitted from the light source 1 via an optical connector 3 in an optical fiber 4 coupled. Via an optical coupler 5 is this fiber with another optical fiber 6 (Sensor fiber) connected. The end not connected to the coupler 7 (Sensor tip) of the sensor fiber 6 is at least on his face 8th exposed. The sensor tip 7 becomes in the medium to be characterized 9 (Substance) or positioned directly in its vicinity. That from the substance 9 Backscattered light is released over the exposed face 8th at the sensor tip 7 back into the sensor fiber 6 coupled and via the optical coupler 5 , an optical fiber 4 and an optical connector 3 to the detection unit 2 directed.

In the embodiment, an Nd-YAG laser with an excitation wavelength of 1064 nm was used as the light source 1 used. The laser power was about 30 mW. As optical fibers 4 . 6 Conventional UV / Vis optical fibers with diameters between 50 and 500 μm were used. The sensor tip was positioned in various powdered and liquid samples. As optical coupler 5 a bidirectional Y-coupler was used. As a detection unit 2 with evaluation unit was an FT-Raman spectrometer RFS 100 from. Bruker used. The optical fibers 4 were connected to the spectrometer via two conventional optical connectors 3 connected. An extension of the optical fibers 4 By coupling several individual fibers was realized by means of optical connectors, wherein an air gap remained between the fiber ends.

To eliminate the background in the Raman spectra of the substances to be examined, which is caused by the backscattering in the optical fibers, a reference measurement was carried out with the sensor tip 7 in air. This reference spectrum was subtracted from the measured spectra after multiplication by an empirically determined weighting factor (ideally at the same optical ratios of 1.0).

In 2 are the spectra of the substances rutile (spectrum b), anatase (both TiO ₂ , spectrum c), of the organic solid p-bis (o-methylstyryl) benzene (bis-MSB) obtained with the device according to the invention (process probe) ( Spectrum d) and the background spectrum of the optical fibers (reference spectrum, essentially quartz, spectrum a).

The Detectable with the device according to the invention Raman spectra surprisingly show a very high signal-to-noise ratio. An analysis of Process media is therefore problem-free with the device according to the invention possible, if substances contained therein have sufficient Raman activity.

1

light source

2

detection unit

3

optical connector

4

optical fiber

5

optical coupler

6

optical Fiber (sensor fiber)

7

Exposed End of the sensor fiber (sensor tip)

8th

Face of the sensor tip

9

To characterizing medium

a

measured Raman spectrum of the optical fibers used (background spectrum or Reference spectrum)

b

measured Raman spectrum of rutile

c

measured Raman spectrum of Anatas

d

measured Raman spectrum from bis-MSB

Claims (19)

Device for generating and detecting a Raman spectrum of a medium to be investigated ( 9 ) with a first optical fiber ( 6 ), wherein a first end of the first optical fiber ( 6 ) via a means for coupling light with a light source ( 1 ) and a means for coupling out light with a detection unit ( 2 ) for detecting a Raman spectrum, wherein the second end of the optical fiber ( 6 ) has no additional components for focusing the light and the means for coupling the excitation light into the first optical fiber ( 6 ) an optical coupler ( 5 ), which via a second optical fiber ( 4 ) with the light source ( 1 ), the light source ( 1 ) is a laser. Device according to claim 1, characterized in that the first optical fiber ( 6 ) except in the area of its second end ( 7 ) has an enclosure. Device according to claim 1 or 2, characterized in that the first optical fiber ( 6 ) consists of a material with a low optical attenuation for the visible spectrum and the spectrum in the near infrared. Device according to one of the preceding claims, characterized in that the end ( 7 ) of the fiber ( 6 ) without wrapping has a length smaller than 10 mm and a diameter between 50 μm and 500 μm. Device according to one of the preceding claims, characterized in that the end face of the medium to be characterized ( 9 ) facing end ( 7 ) of the fiber ( 6 ) is exposed. Device according to one of the preceding claims, characterized in that the means for decoupling the backscattered light from the fiber ( 6 ) an optical coupler ( 5 ), which via an optical fiber ( 4 ) with the detection unit ( 2 ) is connected to the detection of the Raman spectrum. Device according to one of the preceding claims, characterized in that the light source ( 1 ) is an Nd-YAG laser, a He-Ne laser or a semiconductor laser. Device according to one of the preceding claims, characterized in that the detection unit ( 2 ) is a Raman spectrometer with integrated filter for reducing the intensity of the Rayleigh line. Device according to one of the preceding claims, characterized in that the detection unit ( 2 ) has at least one optical grating and at least one CCD line. Device according to one of the preceding claims, characterized in that the detection unit ( 2 ) has at least one optical grating and at least one photodiode. Device according to one of the preceding claims, characterized in that the detection unit ( 2 ) has at least one optical filter. Device according to one of the preceding claims, characterized in that the optical coupler ( 5 ) is a bidirectional Y-coupler, an optical prism, or a semitransparent mirror, or comprises two partially parallel fibers propagated in one fiber. Device according to one of the preceding claims, characterized in that the fiber ( 6 ) at its second end ( 7 ) and / or the end face ( 8th ) Has metal clusters. Device according to one of the preceding claims, characterized in that the device has an evaluation unit for the evaluation of the detection unit ( 2 ) has generated signals. Method for generating and detecting a Raman spectrum of a medium to be investigated ( 9 ) characterized by the following method steps: - Coupling of generated by a laser electromagnetic radiation via a first optical fiber ( 6 ) into the medium to be examined ( 9 ) without the use of additional components for focusing the electromagnetic radiation, wherein the electromagnetic radiation from the laser via a second optical fiber ( 4 ) and an optical coupler ( 5 ) in the first optical fiber ( 6 ), - coupling the medium to be examined ( 9 ) backscattered electromagnetic radiation into the first optical fiber ( 6 ) and - detection of the spectrum of the medium to be examined ( 9 ) back into the first optical fiber ( 6 ) coupled, electromagnetic radiation. A method according to claim 15, characterized in that the electromagnetic radiation over the end face ( 8th ) at the end ( 7 ) of the optical fiber ( 6 ) into the medium to be examined ( 9 ) is coupled. Method according to one of claims 15 or 16, characterized in that in addition a reference spectrum is determined by means of the same arrangement, wherein the electromagnetic radiation over the end ( 7 ) of the optical fiber ( 6 ) is conducted in a non-Raman active medium and the reference spectrum thus obtained from the Raman spectrum of the medium to be investigated ( 9 ) is subtracted, which in arrangement of the end ( 7 ) of the fiber ( 6 ) directly in the area of the medium to be investigated ( 9 ) is detected. Method according to one of claims 15 to 17, characterized in that from the detected Raman spectrum of the medium to be examined ( 9 ) Structural parameters, the chemical composition and / or the temperature of the medium to be investigated ( 9 ) be determined. Method according to one of claims 15 to 18, characterized in that by applying metal clusters on the end ( 7 ) and / or the end face ( 8th ) of the fiber ( 6 ) the intensity of the Raman spectrum is significantly increased and additional information about physical and chemical properties of constituents of the medium to be characterized ( 9 ) be won.