JPH05500419A - Device for measuring the refractive index of gaseous media - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] Device for measuring the refractive index of gaseous media The present invention relates to an optical measuring device, and furthermore More specifically, the present invention relates to a device for measuring the refractive index of gas.

British patent 2120383B discloses a single wavelength emitting device with a dual-pass optical interferometer. A laser light source and a gas cell in which the optical path length of one of the multiple paths is known. Concerning portable optical equipment for measuring the refractive index of air with high precision using There is a description. The measurement means is that the sample gas leaking into the cell is first exhausted. The counting of interference fringes requires a continuous gas cell with a corresponding change in optical path length. determined by From the knowledge of the cell length and the shrinkage change in the optical path length, the refraction of the gas The value of the rate can be easily obtained. However, all refractive index measurements are Repetition of this complete measurement method was generally required.

We developed an interferometer with similar refractive index measurement accuracy but with many important improvements. Thought.

1) It uses multi-wavelength laser technology which reduces the overall cost of equipment.

2) It uses two gas cell containers filled with a constant refractive index medium.

This entails 1) and the periodic evacuation of the cell that was required in the original refractometer operation. It can eliminate the requirement of Qi.

3) It can take two ways to measure the refractive index.

According to the invention, light source means for producing a beam consisting of polychromatic rays; beam splitting means for generating a pair of component beams from the beam of colored light; two cell container means having first and second containers; and said pair of components. passing a first component beam of the beams through the first container; deflecting means for passing the two component beams through the second container; , coupling means for coupling the second component beam after passing through the container; , difference the light beams of different wavelengths of the first and second component beams after combining. a dispersion means for deflecting light differently; and a dispersion means for receiving the light beam deflected by the dispersion means. an optical device for measuring a gas refractive index, comprising: a detection means arranged to Can be provided.

The invention will now be described in detail with reference to the accompanying drawings.

FIGS. 1a and 1s show an optical measuring device according to an embodiment of the present invention. , FIG. 2 shows an explanatory diagram of various signals related to the apparatus of FIG. FIG. 3 shows another embodiment of the invention.

Referring to the drawings below, Figure 1 shows an optical measurement device that uses multiple laser wavelengths. It shows. The laser beam from the light source is reflected by mirror M1, and Fused silica beam splitter with a transparent front surface and a total reflection back surface - .

The beam 1 reflected from the front surface is transmitted to the outer arm 3 of the double chamber 5, and the corner key reflected by the tube reflector 7 along a path parallel to the beam splitter. Ru. Beam 9 transmitted to the beam splitter is entirely made of aluminum. It is reflected from the rear surface and transmitted through the inner arm 11. and corner cube reflector 7 is reflected by the beam splitter-BS, where it is transmitted along the outer arm. combined beam. This superimposed interference beam is filtered by mirror M2. The light is reflected by the dispersing prism P and separated into separate wavelength components. at each wavelength The light intensity of the split photodetector (Fig. The electrical signal for processing is generated, and the processing The percentage of interference fringes (fringe fraction) is measured. This device is further One gas cell 13 is used, through which only the transmitted beam 9 passes.

This refractometer configuration uses measurements of the fringe fraction at each wavelength used. is used to measure the change in optical path length between two arms.

For example, if a dual wavelength laser is used to emit wavelengths λ1 and λ2, the device The change in optical path length (d'R) of either arm is given by ignoring dispersion due to air. Suppose, du = (mt+φ1)λ, -(m2+φ2)λ2 (1). In addition, m l and m2 are integers indicating the order of interference, and φ1 and φ2 are the fringe fins for each wavelength. It's a traction.

This sequence of fringe fractions passes through the optical path by λ1λ2/(λ1-λ2). Repeated each time the length changes. To remove this ambiguity, from another wavelength A third fringe fraction is used or measured by other means. An appropriate d1 value is used. However, any change in optical path length is It can be obtained in this way without using Ringe's count.

In the detailed case of a refractometer, a double-chamber gas cell is used, and the cell has exactly the length It has fl = 3L 64cm. This length is one fringe of optical path length change. is chosen to be approximately equal to a 1.times.10@-6 change in refractive index. cell The inner container has a known optical path length, while the outer container requires measurement of the refractive index. Contains air. Under these circumstances and if the refractometer has the same optical path length in both arms, From both measurements of the fringe fraction in the two cases, the gas [nl] at the wavelength λ (λ)] can be accurately determined from the following simple formula:

nz (λ) - nl (λ) + dΩ (λ) / N + (7 (2) However, nl (λ ) is the refractive index of the medium contained in the inner cell container at wavelength λ, and dfI (λ) is the refractive index of the medium contained in the inner cell container at wavelength λ. The change in optical path length, σ, is a correction value for other optical path length changes during measurement.

The multi-wavelength requirement was discussed in the previous chapter, but ideally changes in optical path length should be Requires a single light source that sequentially emits three separated wavelengths for accurate determination be done. However, this type of simple light source does not exist in reality, but the following This can be simulated by using the following.

1) Illuminating separated optical fibers with a common output end to illuminate the refractometer Using multiple different laser light sources for

2) A single laser source that emits two wavelengths and an atmospheric The pressure and gas temperature measurements are used to calculate the Nidoline equation [B, Nidoline “refractive index of gas” method]. Trorossia 2, 71 (1966)], together with the value of the refractive index of air, which can be calculated Using a sufficiently accurate value dΩ (λ) obtained from the known length g of .

3) Improved value dl (λ) obtained in the same manner as 2) above and emitting a single wavelength. More accurate measurement of atmospheric pressure and gas temperature using a single laser light source Use values. This means that there is no more than 0.5 fringe in the measured and calculated optical path difference. When all the combinations of the air components that do not produce the above change are known. It is only possible to

The second of them is preferred, since the two-wavelength laser is For the most accurate length measurements, an accurate air refractive index value is required - Readily available at the same common wavelength (633nm) used in laser measurement interferometers. Because there is. Furthermore, the suitable value of dg (λ) depends on the precision of pressure and temperature. This can be easily obtained by using a cheap and rough sensor with a temperature of ±500 Pa and ±1°C. can be done.

The above embodiments of the invention use a gas cell with an inner vessel of known constant optical path length. This is because the refractive index of air has to be adjusted to atmospheric pressure to achieve a measurement accuracy of 1 x 10 = 8. It must be maintained within about 3 nm over the entire range of power and gas temperature.

It is constantly evacuated and incorporates a getter pump to maintain and monitor the vacuum. or a central container filled with dry gas at atmospheric pressure. This is realized by

Both of these methods are followed by inserting the cell inside the refractometer. accuracy of about ±1 μm (lXl0− for refractive index measurements using a 31 cm long cell) It is necessary to measure the length ρ) of each cell with an error of Evacuate both cell vessels to determine the two wavelengths of the “zero” fringe fraction that should be is necessary, since both optical paths are exactly the same. This is a measurement error ensure a normal dispersion effect resulting in

For the first type of cell, the inner vessel is maintained and powered by a getter pump. The sample was then vacuum-sealed and the two-wavelength fringe fraction was measured as shown below. Gas leaks into the outer container. Atmospheric pressure and gas temperature are measured and their values calculates the appropriate refractive index of air from the Edlen formula using the measured cell length. and its refractive index should be calculated within a few fringes of the optical path length [dl (λ)] is allowed to change. Finally, the two-wavelength fringe due to this appropriate refractive index of air For difraction, the refractive index of the surrounding air is n2 (λ) = 1.000. Since it can be easily obtained by equation (2), an accurate optical path difference to be measured can be obtained. Tired The foldometer repeats the measurement of the “zero” fringe fraction every week or month. Due to this requirement, it is now easy to continuously measure the refractive index.

The second case involves evacuation of both cell vessels and a two-wavelength “zero” fringe flux. After the measurement, the inner container, shielded at atmospheric pressure, is Filled with Yea. Two wavelengths of light are generated along with the pressure and temperature of the air inside the cell. The rim fraction is measured similarly. Using the same technique as in the first case above, By using It can be determined accurately from the following equation by setting λ)=1-000.

n2 (λ) = n3 (λ) + dρ (λ) / f) + a (3) Surrounding gas is placed in the outer container and again the fringe fraction, atmospheric pressure and gas temperature The refractive index of air can be obtained from equation (2) of the method described above based on it. It will be done.

The refractometer can perform similar measurements for the “zero” fringe fraction as previously described. The constant conditions make it easy to continuously measure each refractive index of air. Refraction inside the inner container To maintain a constant rate, fused silica is used to build up the cell. This result is , meaningless refractive index correction due to changes in cell volume caused by changes in ambient temperature. It is positive.

In summary, in the first type of cell, the refractive index of the medium is exactly 1.000. point, therefore an additional 71111J determination step using equation (3) is required. The second cell, on the other hand, is more easily formed and maintained without the need for wires. Ru.

Air distribution must also be considered. This dispersion is due to the proposed two wavelengths (6 The refractive index at NPT must be corrected for (12 nm and 633 nm) This results in a difference of approximately 3×10−′. However, if the cell is filled with air If we have a central container with cormorant. In addition, Pl is the original pressure of the gas in the central container, and P2 is the large surrounding area. It is air pressure. Since P, , P2 have already been measured with sufficient accuracy by the conventional method, , this factor can be easily determined.

The refractometer can operate in two modes to generate refractive index data. first mode According to the author, in order to determine the fringe fraction, as shown in Figure 1, The optical path in the side arm is changed by several fringes. This change in optical path length is caused by the second As shown in the figure, the pressure within a single cell varies irregularly and also as a function of time. is generated by monitoring the changes that occur in the optical path.

Figure 2 shows the two magnified refractometer signals of the refractometer; generated by multiple photoelectric detectors placed against the output beam of the refractometer, each A photoelectric detector monitors interference from each wavelength as the optical path length is varied as shown. do. Before the adjustment at time 1 = 0, the clock that generates equally spaced pulses is reset. will be played. The number of pulses is calculated for each flip assuming that the optical path length changes in a given direction. The zero volt line is counted each time the voltage signal crosses the zero volt line, and they are The times t=a, b, c, and d at Furthermore, the number of pulses equal to the number of fringes is λ1. Between time t==a and C for each of λ2, and is measured in the period between times tb and d. The fringe fraction at each wavelength is , λ2. Simple fractions ΔNl/N1 and ΔN2 for each of λ2 /N, is easily determined. In this way, the fringe fraction data , obtained from the gas refractive index, which can be determined using the techniques already described.

A second embodiment of the invention is shown in FIG. 3, which shows a configuration for dual wavelength operation. The changes required above are illustrated.

The plane of polarization of the light emitted by the laser must be at 45 degrees with respect to the drawing. required. A quarter-wave plate 15, designed for dual-wavelength use, is one side of the refractometer. It is inserted into the arm of the gives a phase delay of The output beams of ordinary interference 17 and non-interference 19 are It is used to separate wavelength components by wavelength. Non-interference output beam beam 19 is incident on the photoelectric detectors PI, P2, while the interference output is transmitted through the polarization beam splitter. is separated into an S-polarized light component and a P-polarized light component. The intensity of these polarized components is Monitored by photoelectric detectors P3-P6. This configuration uses quadrature fringe detection. and PLM Heide of “Applied Optics 20.3382 (1981)” using the technique described by Mann. The technique is similar to the one mentioned above. Measure the fringe-detected electrons by varying the optical path length within the refractometer using the method It depends on that.

If a third wavelength were available, it could be used to measure the intensities of the non-interfering and interfering components. Additionally, three charge detectors are required.

In the first embodiment, the optical path length is adjusted each time fringe fraction data is needed. The drift of the detected electrons is removed in this way, while the second embodiment Now, if the laser intensity and photoelectric detector system are stable enough, the first part of the interferometer signal Correction provides fringe fractions in hours without re-correction .

international search report LLIwmmAse1wlle Na PCT/GB 9Q/Q1393 International inspection report G[l　9001393 S^　40035

Claims (6)

[Claims] 1. Apparatus for measuring the refractive index of a gaseous medium, the apparatus comprising: a device for producing a beam of light; light source means (L) for producing a pair of component beams from said beam; Dual vessel cell having beam splitter (BS) means and first and second vessels means (5) and a first component of said pair of component beams; passing the beam through the first container and passing the second component beam through the second container. deflection means for passing the first and second component beams through the container; into an optical device comprising a coupling means (BS) for coupling after passing through each of the containers. wherein said light source means generates a beam of polychromatic radiation which, after combining, said first and second beams; A component that differentially deflects light rays of different wavelengths included in two component beams. A scattering means (P) is provided for receiving the light beam deflected by said scattering means. An optical device characterized in that detection means (P1, P2) are arranged. 2. An optical device according to claim 1 for measuring the refractive index of a gaseous medium, A container (13) through which only one of the pair of component beams passes is further provided. An optical device characterized by being provided. 3. An optical device according to claim 1 for measuring the refractive index of a gaseous medium, The recording light source means (L) comprises a laser that simultaneously emits at least three different light beams. An optical device characterized by: 4. An optical device according to claim 1 for measuring the refractive index of a gaseous medium, The light source means (L) simultaneously emits light into a plurality of optical fibers having a common output. An optical device characterized by being equipped with a plurality of lasers. 5. An optical device according to claim 1 for measuring the refractive index of a gaseous medium, Furthermore, it is equipped with phase delay means (15) as well as quadrature phase detection means (M3, P3-P6). An optical device characterized by: 6. In a method for measuring the refractive index of gaseous media, a polychromatic beam of light is connected to a pair of components. one of the pair of component beams is placed in a double container set. A predetermined The optical path is transmitted, and the other of the pair of component beams is sent to the double container cell. pass through the other container, combine the component beams, and remove the combined beam. an interference film when the gaseous medium is placed in the second container. A method characterized by measuring a change in a ring.