FR2598502A1 - Differential absorption spectroscopy and turbidity apparatus using optical fibres - Google Patents

Abstract

The principle for measuring the transmission of light between optical fibres is known but in order to make a differential measurement with such a system, allowing correction for source fluctuations and fibre contamination, the device forming the subject of the present invention uses the modification of the optical path between the two excitation and analysis fibres. This modification may be defined mechanically or using a computerized system. Two devices corresponding to the present invention are described.

Description

Differential absorption spectroscopy device
and turbidity using optical fibers
by Jean-Claude ANDRE, Research Director at CNRS
and Michel PAN
The gathering of information has not made as spectacular progress as its processing or transportation. However, the low efficiency of current methods of taking chemical or biochemical information can constitute an obstacle to the evolution of important methodologies or techniques.

 The present invention relates to absorption spectroscopy and turbidity measurements. Among the known applications of absorption spectrometry, the study of in-situ chemical reactions, the study of biological evolution in fermenters, the analysis of concentrations in contaminated areas, cited here as an example , are particularly delicate.

Progress in these areas naturally depends on the instrumental devices available to experimenters and in particular absorption spectroscopy and turbidity measurement devices.

 In most of their applications, devices of this type work in differential mode. This means that the measurements of the optical transmission factor are carried out simultaneously on a measurement sample and on a control sample. More generally, it is possible to carry out a differential measurement between two measurement samples subjected to different physical, temporal or chemical constraints. It is therefore known from the prior art that a differential spectrophotometer comprises a device emitting monochromatic photons, an optical distributor making it possible to apply to the two measurement and control samples the radiations coming from the emitter as well as at least one photodetector collecting the light transmitted through the two samples.

 According to a general characteristic of the present invention, the differential assembly does not use an optical distributor and uses a single sample. In addition, to allow in-situ measurement, the device which is the subject of the present invention uses at least one optical fiber connecting the measurement area and the other components of the device.

This type of assembly which will be described below makes it possible to solve the problems linked to industrial fermenters which require
- the absence of contaminating reagents, the possible sterilization of the sensors,
- the absence, if possible, of interference,
- rapid information gathering,
- signal processing as reduced as possible,
- etc ...

This type of assembly also has interest in the taking of information in chemical reactors, in the presence or not of radioactive products and in particular in conditions where the safety of the experimenter was limited.

 The principle of measuring the transmission of light between two optical fibers is part of general knowledge. However, if one wants to use such a now conventional system, the soiling and various contaminations which can intervene for example in the chemical or biochemical reactors which have already been mentioned mean that it is always difficult to obtain credible information from measurements. light transmission.

 With optical fibers, the principle of a measurement is based on the use of a source of monochromatic radiation or made monochromatic. This radiation penetrates into an optical fiber called excitation, and therefore is transmitted into the measurement area. Constituted by at least one optical fiber and if necessary by a deflection mirror placed at a given distance t from the secondary light source, corresponding to the end of the excitation optical fiber which is not connected to the primary source of radiation, the assembly makes it possible to transmit to the detector the quantity of light not absorbed or not diffused by the measurement medium. The present invention relies on the use of a distance t which is variable over time, allowing in-situ measurement s' eliminating problems linked to the time-varying transmission of optical fibers, because of their soiling or their partial degradation.

ou la densité optique pour 1 cm de trajet optique correspondant à
D =
L'utilisation d'une modification de la distance inter-fibres permet d'éliminer les variations de Io(X) qui peuvent être consécutives à l'évolution de l'intensité de la source primaire mais surtout à l'évolution dans le temps des transmissions des fibres d'analyse et d'excitation liées en particulier aux variations de l'état de surface des extrémités des fibres placées dans le milieu de mesure.Assuming an in-line assembly where the excitation fiber and the analysis fiber are spaced apart by a distance t and are located on the same axis and considering a homogeneous medium, for example, containing only an absorbent substance a the wavelength A, the calculations presented below will make it possible to appreciate the advantage of the present invention. Indeed, if and c represent respectively the molecular absorption coefficient and the concentration of the substance, and if Io (X) is the light flux leaving the excitation optical fiber, then, we measure, at a multiplicative factor K (t), function of Q, the flux I (R,) transmitted at the distance l when the transmissions excitation and analysis fibers are expressed respectively by Te (X) and T a I (#, l) = Te (#) Ta (#) K (l) .I0 (#). exp [- # (#). cl]
If we modify by means which are also the subject of the present invention and which will be described later in detail, the distance l which we will now call we then measure I ') = Te (X) Ta ( X) K (l '). I0 (#). exp [- # (#). c.l '] which allows to calculate the concentration of the medium knowing that it correstsond to

or the optical density for 1 cm of optical path corresponding to
D =
The use of a modification of the inter-fiber distance makes it possible to eliminate the variations of Io (X) which can be consecutive to the evolution of the intensity of the primary source but especially to the evolution in time of transmissions of the analysis and excitation fibers linked in particular to variations in the surface condition of the ends of the fibers placed in the measurement medium.

qui dépend de l'évolution dans le temps de la concentration mais également des transmissions Te(X) et
Remarquons enfin que le calcul présente ici n'est fait que pour comprendre le principe de l'invention qui a de l'intérêt pour l'analyse de solutions complexes contenant au moins un produit absorbant et des substances diffusant la lumière à la longueur d'onde x d'excitation.Recall that using a system not using the method which is the subject of the present invention, an apparent value is measured

which depends on the evolution over time of the concentration but also on the Te (X) and
Finally, note that the calculation presented here is only made to understand the principle of the invention which is of interest for the analysis of complex solutions containing at least one absorbent product and substances diffusing light over the length of x excitation wave.

Other characteristics and advantages of the present invention will appear on examining the detailed description below, as well as the appended drawings, in which
FIG. 1 is a block diagram of an apparatus according to the invention,
FIG. 2 is a block diagram of an apparatus according to the invention in which a device diffusing or reflecting light is used,
- Figure 3 is a possible representation of the time course of the interfiber distance t.

 In FIG. 1, a light source which comes from a photon emitter 1 which can advantageously be either a tunable laser, or a more conventional source adapted to a disperser 2, is applied to one end of an optical conductor 3 thanks to an appropriate lens. The other end of the optical fiber enters the measurement medium 5. Co-axially, another fiber 6 is placed in this same medium, at a variable distance 7 from the end of the excitation fiber which is not not connected to the primary source of radiation. The other end of the analysis fiber is connected to a photodetector which advantageously can be a photomultiplier 8.

 The electrical signal 9 can advantageously be transmitted after amplification in the memory of a microcomputer 10. The optical fiber for analysis is made mobile in the measurement medium thanks to a servo 11 with a motor 12 allowing the displacement of the fiber. For sealing reasons, the fiber can advantageously slide in a cylinder which does not allow the measurement medium to flow; it can preferably be fixed to a hollow metal spring 13, already known in the prior art.

This spring can be produced in materials having good mechanical properties, chemically inert or with respect to biological species. The microcomputer 10 can advantageously be connected to the motor 12 for moving the analysis fiber 14 and to the device for scrolling the wavelengths of the primary source 15 to allow control and knowledge of t and of A. By way of For example, the motor 12 can be very simple like an electromagnet or more complex like a stepping motor driving a worm 11.

 In this simplified description, the optical analysis fiber 6 is assumed to be mobile, it could just as easily be possible to move the excitation fiber 3 and even both. These light conductors can be monofibers or multifibers; they can be used with optical devices, known in the prior art, making the emission of light almost parallel in the medium to be analyzed, which can be advantageous in certain cases.

 Fioure 2 represents another representation of an assembly according to the invention. Only in this figure are represented the elements defining the information sensor. The optical excitation 3 and analysis 6 fibers are coaxial. It is either two fibers joined together, or a double coaxial fiber already known in the prior art. The light emitted by the end of the excitation fiber reaches, when the medium to be analyzed is transparent, a device reflecting or diffusing the light 16.

This device is placed at a known distance 17 from the two fibers 3 and 6. Part of the light is reflected or scattered in the direction of the analysis fiber 6 and enters it. The light return device is fixed on a support 18 itself connected to a system 19 allowing its movement as has been described above. The displacement of the return device is carried out in the same way as has been described above.

 A preliminary calibration of the measurement system is carried out using light absorbing substances in the spectral zone of interest at known concentrations. This calibration makes it possible in particular to know the device function K (t) value which is kept in memory in the microcomputer. Depending on the programs used and the applications planned, the device can operate in the following ways - In industrial control, we often place a single wavelength well selected and we seek to measure the evolution of the signal transmitted over time (or its stability). For this type of measurement, it is not necessary to work with a microcomputer and the system for modifying the optical path can itself be simplified. We can, for example, use an electromagnet or a synchronous motor coupled by a crank system to the system. FIG. 3 illustrates the temporal evolution of the signals under these particular conditions of use. The frequency of displacement can be adapted to the foreseeable evolution of the concentration; however, it cannot be greater than a few hertzs.

 - The device for modifying the optical path can advantageously use a stepping motor and a cam defining discrete distances tos l1 ... tn distributed according to a logarithmic law. The operating principle of the device is then as follows: the microcomputer 10 controls the stepping motor 12 so that the optical path in the measurement medium is minimal, ie t0. A signal lo is then measured. The measurement carried out and stored in memory, the motor acts to reach tî. Let I1 be the signal for t = l1.

If 11 is between 0.16.Io and 0.44.I0 the measurement is carried out. We then keep in memory the value of the signal and of t1. If the signal is less than 0.16.Io but remains greater than 0.01.I0, the measurement is carried out while knowing that it is less precise than previously. If it is less than 0.01.Io, the computer signals the impossibility of making an accurate measurement. If the signal is greater than 0.44.Io, we adapt t to go to t2 and the same comparison is made. If t n represents the maximum length of the path, the measurement is not made if the signal In is greater than 0.9.Io. The values presented here are only given as an indication to define the operating principle of the device according to the invention. Knowing k (t), the optical density is then easily calculated at the wavelength considered and the next analysis wavelength is passed.

 These two presentations define two extreme operating modes. It goes without saying that a modification of the software makes it possible to obtain measurements under other conditions.

 However, for an optical path of 1 cm, conventional commercial devices can measure optical densities of between approximately 0.05 and 2. The use of t values. O to variables between 0.5 mm and 5 cm thanks to the device described above makes it possible to carry out, with the same sensor, optical density measurements included, for a cm of optical path, between 0.01 and 40 approximately.

Claims (10)

Claims 1 - Differential spectrophotometer comprising optical means suitable for  apply a monochromatic measurement light to at least one fiber  excitation optics, partially transmitted in at least one optical fiber  analysis which directs the light towards a photomultiplier as well as  electronic and micro-computer mechanical means allowing, in  adapting the optical path in the measuring medium to measure the density  optics of this medium. 2 - Differential spectrophotometer according to claim 1 characterized in  that the fibers are placed axially and separated from each other,  and at least one of which is mobile. 3 - differential spectrophotometer according to claim 1 characterized in  that the fibers are placed one against the other or co-axially, the  light transmitted through being by a reflecting or diffusing device  placed a variable but known distance from the end of the fibers. 4 - Differential spectrophotometer according to claim 1 characterized in that  that the ends of the optical fibers are provided with a focusing device. 5 - differential spectrophotometer according to claim 1 characterized in  that the excitation source is a tunable laser. 6 - differential spectrophotometer according to claim 1 characterized in  that the excitation source is a source, pulsed or continuous, emitting  light in a continuous spectrum, coupled to an optical scatterer  and a focusing device for attacking the fiber  of excitement. 7 - Differential spectrophotometer according to claim 1 characterized in that  that the optical path varies continuously in a substantially sinusoidal manner. 8 - Differential spectrophotometer according to claim 1 characterized in that  that the optical path varies by sudden jump.  9 - differential spectrophotometer according to claim 1 characterized in  that the mobile system consists of a piston sliding in a  hermetic cylinder. 10 - Differential spectrophotometer according to claim 1 characterized in  that the mobile system consists of a hollow metal spring  chemically inert.