DE8701592U1 - Laser interferometer refractometer - Google Patents

Description

Dlpl.-lng. Sigurd Leins · Dlpl.-Phys. br, fiorbort KSnIg BurcfchardtstraBs 1 telephone (0511) 62 30 D-MOO Hanover 1 Our sign date

Physical Optics 47*77 ** * --«■*

and Optoelectronics ⁷⁸⁵ / ¹ 30 January 1987

Development-Manufacturing-Sales

Laser interferometer refractometer

The invention relates to a laser interferometer refractometer according to the preamble of claim 1.

When using laser interferometers, e.g. to determine length, precise knowledge of the refractive index of the gas, usually air, in which the measuring laser beam propagates is required, since the wavelength, the multiple of which gives the length to be determined, depends on the refractive index η of the gas in the following way:

L = L'/n,

where L ¹ is the laser wavelength in vacuum, which is assumed to be known, and L is the laser wavelength in gas. Laser interferometers are preferably used in the accuracy range ± 1 .E.-6. Therefore, a correspondingly precise knowledge of the refractive index is required. In practice, such accuracy can only be achieved interferometrically. DE-PS 35 27 579 describes a laser interferometer refractometer that has two parallel chambers, through which the two partial beams of an interferometer arrangement are passed, in order to be able to produce as identical conditions as possible in the two chambers during the initial state of a refractive index determination, and in order to be able to enter the measuring chamber as precisely as possible during the measurement itself.

Dr.K./H. -2-

■ * t ·

to allow ambient air to enter without obstruction,
The measuring chamber is open on all sides and has a
The side cover can be sealed vacuum-tight and evacuated. By evacuating both
Chambers can produce optically identical conditions in both. By subsequently opening the measuring chamber
can communicate freely with the ambient air, so
that the air in the measuring chamber is optically largely
is identical with the air in which a simultaneous and

nearby interferential length measurement is carried out .

CH-PS 609 456 shows a refractometer for measuring f the refractive index of air, in which the geometric J length of the measuring section is determined by a body whose !

Dimensions should be practically independent of the air parameters that affect the refractive index, such as temperature, pressure and humidity. The body is an arrangement made of a glass ceramic tube and two deflection prisms at the ends of the tube for multiple deflection of a partial beam within the tube. The dimensions of the refractometer should be kept small by the multiple deflection. The arrangement is located in a vacuum chamber .

chamber. j

In these two well-known refractometers, one

of two fundamentally used methods. ; A measuring cell of known length is alternately evacuated < or filled with gas or air. The number of
Interference fringes occurring during the exchange of vacuum with gas are counted and, if necessary, interpolated in order to determine the

To determine the refractive index of the gas. In this method
the use of a vacuum pump is required during the measurement, which is disadvantageous. Vacuum pumps are expensive, prone to failure and require regular maintenance. If not particularly good and therefore expensive vacuum pumps are used,

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The residual gas remaining in the vacuum does not have the same composition as the gas to be measured, but consists essentially of pump oil and possibly easily condensable gases. This results in a fundamental uncertainty for the refractive index of the gas to be measured.

The other method used uses two measuring cells of identical design, in particular identical geometric length, which are alternately introduced into the interferometer, with one cell being filled with the gas to be measured while the other is evacuated. Since in this method a vacuum pump is only used to create the vacuum in one cell, the disadvantages described in the first method are not so serious here. On the other hand, this method has the disadvantage that only interpolation between the interference fringes is possible, since during the cell change the interference disappears completely due to beam blocking and therefore the determination of the integer part of the interference order is not possible. The integer part must be determined approximately by measuring the temperature and pressure of the gas to be measured and possibly its composition. Therefore, such refractometers are in principle only devices for increasing the accuracy of so-called commercially available "air parameter compensators". {

Such a procedure was introduced at the conference "Laser interferometry in length measurement technology" at the PTB in Braunschweig on March 12th/13th, 1985, organized by the VDI/VDE Society for Measurement and Control Technology.

The object of the present invention is to provide a laser interferometer refractometer of the initially mentioned &ldquor;

The aim of the invention is to design the device in such a way that a higher measurement accuracy can be achieved and the disadvantages of the known arrangements are avoided. This object is achieved by the design according to the characterizing part of claim 1.

1.

&bull; » «» M «III ··

The invention therefore proposes that the part of the geometric path length formed by the vacuum cell be made variable with a constant overall geometric path length;-. This means that the proportion of the path in the vacuum and accordingly in the gas to be measured can be changed as desired, whereby a change in the optical path length can be achieved. The number of interference fringes caused by this change can be measured exactly. Since this number is proportional to the change in path in the vacuum or in the gas to be measured and to the difference between the vacuum and gas refractive index, the refractive index can be determined with very high accuracy if the change in path is known. The vacuum cell is only evacuated once and remains under vacuum at all times. This means that the disadvantages of one method explained at the beginning, caused by the vacuum pump, no longer occur. A messing of the parameters influencing the refractive index as in the other method explained above is also not necessary, since beam blockage does not occur during the measurement process.

Advantageous and expedient further developments of the solution to the problem according to the invention are characterized in the subclaims.

The invention will be explained in more detail below with reference to the accompanying drawing.
It shows

Fig. 1 schematically shows a first embodiment

of the laser interferometer refractometer according to the invention and

Fig. 2 a second embodiment of the 0 refractometer.

The laser interferometer refractometer shown in the drawing has a laser 1, whose light beam 17 is guided to a beam splitter 12, which splits the light beam into two partial beams 18 and 19. The partial beam 18 is deflected by a beam deflector 11 so that it

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runs essentially parallel to the other partial beam 19. The partial beams 18 and 19 then fall on a beam splitter 10, which splits the partial beam 18 into two further partial beams 20 and 20' and the partial beam 19 into two further partial beams 21 and 21'. In the path of the partial beam 20 there is a vacuum cell 25.

The vacuum cell 25 consists of a bellows 2 which is sealed vacuum-tight at both ends by end plates 4 and 5. In the end plates 4 and 5 there are windows 6 and 6' on which retroreflectors 8 and 7 are arranged. The retroreflector 8 is arranged on the outside of the end plate 4 and the retroreflector 7 on the inside of the window 6'. The retroreflector 8 can also be arranged on the inside of the end plate within the vacuum cell 25; in this case the end plate does not need to have a window or opening. The retroreflector 7 can alternatively be arranged on the outside of the window 6*. Its dimensions are smaller than those of the window 6' , so that an annular window area remains free for the partial beam 20.

In the end plate 4 there is a lockable evacuation support, for example in the form of a pipe extension. The evacuation nozzle can also be provided at any other suitable location in the vacuum cell.

A third retroreflector 9 is provided in the path of the partial beams 20' and 21', which reflects these partial beams parallel to itself.

The partial beams 20 and 20' or 21 and 21* caused to interfere fall on photodiodes 14, 15 or 13, 16, with which the interference fringes are measured. The output signals of the photodiodes are fed to an evaluation device (not shown) which calculates the refractive index from the signals of the photodetectors.

The beam splitter 10, the end plate 4 of the vacuum cell 25 and the third retroreflector 9 are arranged in a fixed position. The end plate 5 of the vacuum cell 25 is ^not

shown device in the direction of the double arrow 26,
ie in the direction of the paths of the partial beams 20 and 21
The laser interferometer-refractometer according to
Drawing works as follows:

The partial jet 20 generated in the jet splitter 10 enters
through the window 6' into the vacuum cell 25 and falls to
Passing through the same to the retroreflector 8, which reflects the partial beam parallel to itself. The reflected

I Partial beam hits after passing through the vacuum cell again \

and the window 6' onto the beam splitter 10. The other \

Partial beam 20' hits the third retroreflector 9, becomes I

there parallel to itself and meets at ent- j

appropriate structure of the optical arrangement the beam J

splitter 10 at the same position as the partial beam 20/ at the |

the partial beams 20 and 20' unite. There, \

the partial beams 20 and 20' with each other, and the resulting j

Interference beams 22 and 22' strike the photodetectors &xgr;

15 and 14. j

The partial beam 21 generated by the beam splitter 10 [

is also passed through the window 6' of the vacuum cell 25 f

I and falls on the second retroreflector 1, which reflects the partial beam f

21 parallel to itself. The reflected partial beam 21 hits the window 6 |

j onto the beam splitter 10. The other partial beam 21' generated by the beam splitter 10 is | reflected by the third retro- |

reflector 9 parallel to itself and hits I

I with appropriate design of the optical arrangement the beam |

splitter 10 at the same place as the beam 21, at the |

the two partial beams 21 and 21' are combined. At the ■

The two partial beams 21 and 21' interfere, and \

the resulting interference beams 23 and 23' hit \

the photodetectors 16 and 13. \

The beam splitter 10, the end plate 4 with retrore- \

reflector 8 and the retroreflector 9 are relative to each other j

arranged in a fixed position. This means that the geometric path length |

of the partial beam 20 from the beam splitter 10 via the vacuum cell and the retroreflector 8 back to the beam splitter 10 is constant and independent of the position of the end plate 5 with window 6' and retroreflector 7. The optical path length of the partial beam 20, however, depends on the position of the end plate 5 with window 6 ¹ and reflector 7/ since, depending on the position of the end plate, different parts of the path are lost in the vacuum and accordingly in the gas to be measured.8 =

The optical path length of the other partial beam 20', on the other hand, is constant. Therefore, when the end plate 5 moves with the window 6' and the retroreflector 7, interference phenomena occur in the interference beams 22 and 22' (interference fringes), which are measured by the photodetectors and 14. The number of interference fringes is proportional to the change in position of the end plate 5 and to the difference between the vacuum and gas refraction index Ji.

As with the partial beam 20, the optical path length of the partial beam 21 is also dependent on the position of the end plate 5, but not because of the different refractive indices of gas and vacuum, but because of the dependence of the geometric path length on the position of the end plate 5. As with the beam 20', however, the optical path length of the partial beam 21' is constant. Therefore, the optical system consisting of the partial beams 21 and 21' as well as the retroreflectors 7 and 9 and the beam splitter 10 forms an interferometer for the precise measurement of the displacement of the end plate 5. The evaluation of the measured data is carried out as follows:

If the end plate 5 is displaced by a distance ζ, the number of interference fringes detected by the photodetectors 13 and 16 is

NO = 2 · z· n/L',
while the number of interference fringes detected by the photodetectors 14 and 15

N1 = 2 · z· (nD/L ¹

a · 9 I ■ · · _Q i_ * *

By eliminating ζ we get η = NO/(N0-'N1) or n-1 = N1 /(N0-N1).

This means that NO is larger than Ni by a factor of n/(n-1). The refractive index for air is 1.0003. Therefore, NO would be larger than N1 by a factor of about 3000.

Of particular importance for a distance-measuring laser interferometer is the size
1/n = 1-N1/NO.

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1/n as follows:

h (1/n) = (Ni/NO) · (SN1/N1+ δ NO/NO) =0 ,0003 · ( S N1 /N1 + £>N0 /NO)

Under the reasonable assumptions that NO and N1 can be measured with the same relative accuracy ( 6 N1/N1= ν>ν0/ν0) since both quantities are determined by the same path ζ, and that NO can be determined to δ N0 = 4-1, one obtains δ (1/n) « 2.E-7 with N1=1 (corresponding to NO Ri 3000). With this accuracy, the refractive index η could therefore be measured if the movable unit is displaced by approx. 1 mm at a laser wavelength of approx. 600 nm (He-Ne laser). This presupposes, however, that the condition N1=1 (ie* exactly one interference fringe has been measured on the photodetectors 14 and 15) can be determined with an accuracy of 0.0003; in other words: a strip interpolation by a factor of 3000 is possible. Such accuracy can be safely handled in the laboratory; for an instrument for practical use a more cautious approach is necessary. In practice a strip interpolation by a factor of 100 should be possible (high measuring speed is not required). If one wants to achieve &C\/n)< 1 .E-6, one gets: N1 s; 4 or ze 4 mnu Fig. 2 shows another embodiment of a laser interferometer refractometer. The basic structure corresponds to the structure of the refractometer according to Fig. 1, so that for simplicity components which are identical to those of the refractometer according to Fig. 1 have the same reference numerals as in Fig. 1. With regard to these components reference is made to the description of Fig. 1.

The retroreflectors of the embodiment according to Fig. 1 (also called triple prisms) are replaced by mirrors 30, 32, 34 in the embodiment according to Fig _{. 42.} Since the partial beams 20, 20', 21 and 21' are reflected in themselves, λ/4 plates 36 _to 38 must be arranged in the path of these partial beams in order to avoid feedback into the laser, and the beam splitter 12 according to Fig. 1 must be designed as a polarizing beam splitter, or a polarizing beam splitter 40 must be arranged between the arrangement of beam splitter 12 and beam deflector 11 and the beam splitter 10, as shown in Fig. 2. The mirror 34 can be realized by a rear-side mirror coating 34' of the λ/4 plate 38.

The partial beams 20 and 20' or 21 and 21", which are intended to interfere with each other, are combined in the beam splitter 10 in a manner analogous to the embodiment according to Fig. 1. The two interference beams 22 and 23 fall on two photodiodes 44 and 46. The use of the additional polarizing beam splitter 40 has the advantage that the interference partial beams 22' and 23' from the beam splitter 10 can also be detected by photodiodes 50, 52. The output signals of the photodiodes are in turn fed to an evaluation circuit (not shown) to determine the refractive index in the gas to be measured, as has already been described above with reference to the figure.

In the embodiments according to Fig. 1 and 2, the change in position of the end plate 5 is measured by an interferometric measurement. To achieve sufficient accuracy, this change in position can also be determined by precision drive spindles and suitable displacement sensors 48 (see Fig. 2). In this case, the beam splitter 12 and the beam deflector 11 as well as the beam deflectors 7 and 32 can be omitted in the embodiments according to Fig. 1 and 2. In the embodiment according to Fig. 2, the photodiodes 46 and 52 can also be omitted. Accordingly,

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in the embodiment according to Fig. 1, the two photodiodes 13 and 16 can be dispensed with. Finally, in the embodiment according to Fig. 1, it is still possible to dispense with pairs of photodetectors for detecting the interference, so that in the simplest embodiment with interferometric measurement of the change in position of the end plate 5, only the detectors 15 and 16 or 13 and 14 are used, or if the interferometric measurement is dispensed with, only the detector 15 or 14 is used.

In order to achieve a change in the position of the end plate 5 of the vacuum cell in the embodiments according to Figs. 1 and 2, the casing of the vacuum cell 25 is designed as a bellows 2. Instead of a bellows, a spherical, flexible hollow body 51 (shown in dashed lines in Fig. 2), an ellipsoid-shaped, flexible hollow body (not shown) or a telescopic arrangement of tubes (not shown) can also be used, which are sealed against each other in a vacuum-tight manner using sliding seals.

Instead of the laser 1 and the optical system consisting of the beam splitter 12 and the beam deflector 11, two lasers can also be used to generate the two beams 18 and 19.

The λ/4 plates 36, 38 in the paths of the partial beams 20, 20', 21 and 21' can be replaced by a λ/4 plate (shown in dashed lines in Fig. 2) between the beam splitters 10 and 40 or between the beam splitters 10 and 12 if the beam plate 40 is missing.

The deflection means 8 can also be arranged outside the vacuum cell or the window 6, whereby, for example, the transmission of vibrations due to the movement of the end plate 5 is avoided.

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Claims (22)

Expectations 1 "Laser interferometer refractometer for measuring the refractive index of a gas with a laser, with at least one beam splitter for splitting the laser light beam into at least one pair of partial beams, with an evacuatable cell which forms the vacuum path of a total path of a partial beam of the partial beam pair formed from a path in the vacuum and a path in the gas to be measured, and with at least two beam deflectors which affect the unification of the partial beams in order to generate interference between the partial beams, characterized in that the optical path length of the partial beam (20) is adjustable. 2. Refractometer according to claim 1, characterized in that the vacuum path and/or the path in the gas to be measured is (are) adjustable to adjust the optical path length. 3. Refractometer according to claim 1 or 2, characterized in that, in order to adjust the optical path length, a wall (5) lying in the path of the partial beam (20) and delimiting the vacuum space of the cell (25) is designed to be displaceable. 4. Refractometer according to claim 1 or 3, characterized in that the cell (25) is a Dr.K./H. ' -2 -2- · the length-adjustable casing part (2), one end of which is defined by the adjustable wall (5) and the ) other end face is vacuum-tight by another wall (4) 'i is locked. 5. Refractometer according to claim 4, characterized in that the walls (5, 4) are end plates, of which at least the end plate (5) dividing the total path has a window (6 ¹ ) for the passage of the partial beam (20) and the other end plate (4) with a part e 10 beam reflecting deflection means (8). 6. Refractometer according to claim 4, characterized in that &iacgr; indicates that the casing part is a bellows (2) j a telescopic tube arrangement, a spherical or ellipsoidal \ shaped flexible hollow body (51). &xgr; 15 7. Refractometer according to claim 5, characterized in that the movable end plate (5) is operatively connected to a mechanical drive device. 8. Refractometer according to claim 5, characterized in that the other end plate (4) has a window (6) with a deflection means (8) applied on the outside, or that the end plate (4) is followed by a deflection means arranged outside. 9 * Refractometer according to claim 5 or 8, characterized in that the deflection means is a retroreflector or a mirror* 10. Refractometer according to one of the preceding claims, characterized in that the other partial beam (20 ¹ ) is guided by a further (third) deflection device (9) for combining and interfering with the beam from the first -3- . 1 it at···· ·· The partial beam (20) reflected by the deflection means (8) into the beam splitter (10) is also reflected into the beam splitter (10) ^ 11. Refractometer according to claim 10, characterized in that the further deflection device (9) is a retroreflector or a mirror. 1 1. . KiraKtometer according to claim 1, characterized in that the interference beams (22, 22 ¹ ) are directed onto a light-sensitive device (15, 14). 13. Refractometer according to one of the preceding claims, characterized in that the adjustment path of the end plate (5) is measured with the aid of a precision drive spindle and a displacement sensor (48). 14. Refractometer according to one of the preceding claims, characterized in that for the interferometric measurement of the adjustment path of the adjustable end plate (5) a second laser light beam (19) is directed onto the beam splitter (10), which generates two partial beams (21, 21'), one (21) of which is directed onto a further (second) deflection device (7) mounted on the window (6') of the adjustable end plate (5) and the other (21) onto the third deflection device (9), the deflection devices {7, 9) reflecting the partial beams (21, 21') into the beam splitter (10) for combining and interfering. 15. Refractometer according to claim 14, characterized in that the second deflection device (7) is a retroreflector or a mirror. 16. Refractometer according to claim 14, characterized in that the interference beams are -A- a light-sensitive device (13, 16). 17. Refractometer according to claim 14, characterized in that an optical arrangement comprising a beam splitter (12) and a beam deflector (11) for splitting the laser light beam into two parallel partial beams (18, 19) is arranged between the beam splitter (10) and the laser (1) to generate the two laser light beams (18, 19). 18. Refractometer according to one of the preceding claims/ characterized in that when using mirrors as deflection devices (7, 8, 9) the beam splitter (12) is a polarizing beam splitter, and that in the paths of the partial beams (20, 20', 21 and 21') Λ/4 plates (36, 38) are arranged. 19. Refractometer according to one of the preceding claims, characterized in that when using mirrors as deflection devices (7, 8, 9) between the beam splitter (12) and the beam splitter (10) a polarizing beam splitter (40) is arranged. 20. Refractometer according to claim 18 or 19, characterized in that the third deflection device (9) is formed by a rear-side mirror coating of the A/4 plate (38) in the path of the partial beams (20', 21'). 21. Refractometer according to one of the preceding claims, characterized in that the interference partial beams are detected by pairs of photodetectors (14, 15; 13, 16; 44, 50; 46, 52) as light-sensitive devices. &diams;« ·<-<5&tgr;&mdash; 22. Refractometer according to claim _21f , characterized in that the output signals of the photo diodes are fed to an evaluation device in which the refractive index of the gas to be measured is calculated from the determined number of interference fringes, which is proportional to the adjustment path of the adjustable end plate (5) and to the difference between the vacuum and gas refractive index* 23* Refractometer according to claim 19^ , characterized in that instead of the &lgr; /4-plates (36, 38) in the paths of the partial beams (20, 20', 21 and 21·), a &Lgr;/4-plate (54) is arranged between the beam splitters (10 and or 10 and 12).