GB2039382A

GB2039382A - Delimining refractive index of gases

Info

Publication number: GB2039382A
Application number: GB7944203A
Authority: GB
Original assignee: Jenoptik Jena GmbH; Carl Zeiss Jena GmbH
Current assignee: Jenoptik AG
Priority date: 1979-01-05
Filing date: 1979-12-21
Publication date: 1980-08-06
Also published as: DD140791B1; GB2039382B; DE2946560A1; DD140791A1

Abstract

A method of measuring the wavelength of a frequency stabilised laser, involving the environmental influences of pressure, temperature and air moisture, comprises two beams brought to interference, with one of the two beams passing at least partially an evacuated tube 3 of known length. The evacuated tube 3 is ariated via a valve 12 and evacuated by a vacuum pump 10 in sequence until atmospheric conditions prevail in the tube 3. The wavelength varied under atmospheric conditions in relation to vacuum is measured by counting the interferences in an interferometer 1. <IMAGE>

Description

SPECIFICATION Method of measuring the wavelength of a laser beam This invention relates to a method of measuring the wavelength of a frequency stabilised laser beam in a gaseous medium, involving the ambient pressure, temperature and the gas composition in order to indirectly determine the refractive index of a gaseous optical medium. This method is particularly applicable in precision measuring devices and precision working device which employ the wavelength of a laser beam as a length normal.

The wavelength of a laser beam of a frequency stabilised laser depends on the refractive index of the optical medium passed by the beam. Precision measuring and working devices including laser length measuring systems require a screening of the environmental influences from the measuring normal in particular pressure and temperature, or the latter have to be detected to correct the measuring results. Defined conditions exist in the vacuum. In previous devices, such as in the Universal single Barrel repeater of the VEB Carl Zeiss Jena the measuring beams are directed into evacuated tubes as disclosed in the Jena Review 1977, copy 4, page 168.

Since the length of the evacuated tube has continuously to vary with the measuring path, construction problems will result with respect to sealing the evacuated tube, apart from producing the forces necessary to displace the evacuated telescope-like tube connected to the precision coordinate table.

Awidely known method is disclosed in the journal Feingeratetechnik 1976, copy 6, page 256, where a computer controls the correction of the measuring values of a laser length measuring system in dependence on the initial data for pressure and temperature which are conventionally obtained at the start of the measuring and working operation.

When the device is in continuous operation over a long period, pressure variations particularly due to air-conditioning systems or meterological events require a continuous correction according to the instantaneous data.

The expenditure for a continuous digital detection of the individual environmental factors such as pressure, temperature, moisture, CO2-content and their computation for measuring value correction is considerable.

Therefore, it has been suggested directly to measure the refractive index of the air by interferometrical means. The German Patent Specification 1 235 601 discloses in addition to the proper interferometer for monitoring the refractive index of the air.

The pressure in the completely sealed device is automatically corrected at any departures from a defined initial value. The initial values for pressure, temperature, etc. are measured and the desired refractive index is set by varying the initial pressure.

Apart from the above disadvantages the pressure tight wall involves a high expenditure and further disadvantages when such devices are used in industry (charging) as will become obvious from the journal Feingerätetechnik 1974, copy 6, page 274.

In the German Patent Specification 1 623 300 a compensation device is combined with a purely optical operation principle. A diffraction grating inserted into the laser path of rays deflects the measuring beam in dependence on the refractive index of the air.

When definite geometrical conditions of the entire interferometer system are kept to a compensation of the measuring error results.

The German Democratic Republic Patent 186777 and the journal Feingerätetechnik 1976, vol.7, p.307 also disclose the interfereometric monitoring of variations of the refractive index via a constant length normal.

Apart from the already mentioned methods of the input of initial data it was suggested to obtain the initial state by a laser length measuring system under use of a dynamic interfereometric measuring of a non-displaceable measuring means, for example, a line gauge.

Apart from the high expenditure in moved mechanical parts errors are feasible due to thermal instabilities of the non-displaceable measuring means by additional, for example, photoelectric scanning systems and by tilting errors in the course of the dynamic measuring operation.

The disadvantage of the previous methods lies in the fact that the refractive index variations as a function of time obtained through a standard interferometer are compared to the initial value either by subjective measurement of the individual factors or by expensive error involving comparison measuring.

It is an object of the invention to obviate the above disadvantages.

It is a further object of the invention to provide a simple, reliable automatic method for measuring the wavelength of a frequency stabilised laser at varying environmental influences to compensate for errors in a precision measuring and working device.

It is still a further object of the invention to provide a simple method which permits a continuous measurement of the initial state as well as variations of the refractive index of the air in the environment of the laser length measuring system itself.

The invention starts from the basic idea that a length normal monitored interferentially in known manner with respect to variations of the refractive index is temporarily evacuated to detect the initial values of the length normal. Since the refractive index of the vacuum is known, a continuous measurement of the variations in the course of evacuation and aeration, respectively, and, above that, at any desired moment indicates the absolute refractive index of the air and the wavelength of the laser beam, respectively, so that the measuring values of the laser length measuring system of the precision measurement or working device can continuously be corrected by a control computer.

Accordingly, the present invention consists in a method of measuring the wavelength of a frequency stabilised laser involving the environmental influences of pressure, temperature and air moisture, comprising two beams brought to interference, one of said two beams passing at least partially an evacuated cavity of known length, characterised in that the evacuated caavity is ariated and evacuated in sequence, respectively evacuated and ariated until atmospheric conditions prevail, and in that the wavelength varied under atmospheric conditions in relation to vacuum is measured by counting the interferences.

The method may be carried out in a device wherein the portions of the path lengths of the two beams propagating in the air and in the glass, respectively, are of equal length, the one beam propagating in the evacuated cavity, the other beam propagating outside of said cavity between an interferometer and a measuring and a reference reflector, respectively.

Depending on the path lengths of the two beams two kinds of operation are feasible. The cavity is either evacuated before the device is operated and ariated during operation, or is in the ariated state before operation, and is evacuated before starting operation. The measurement of the variations during operation is then effected through the other partial beam.

In order that the invention may be more readily understood, reference is made to the accompanying drawings which illustrate diagrammatically and by way of example two embodiments thereof, and in which Figure 1 shows one embodiment of a measuring arrangement in accordance with the invention, and Figure 2 is an alternative of Figure 1.

In Figure 1 a measuring beam 2 emitted from an interferometer 1 passes the reference path of known length arranged in an evacuated tube 3, is reflected at an angular reflector 4 and interferes with a reference beam 5 reflected at an angular reflector 6 in the interferometer 1.

The angular reflector 6 is so arranged that the path portions in the air are equally long for both beams so that environmental influences do not affect the interference when the tube 1 is evacuated.

To compensate for the glass path 7 a respective glass-plate 8 is arranged in the reference beam 5.

With this arrangement it is important that the angular reflectors 4 and 6 are maintained at a constant distance relative to each other which is obtained by members 9 which do not vary at temperature variations. Length tolerances of the evacuated tube 1 are substantially non-critical.

The evacuation is effected through a vacuum pump 10 via an adjustable valve 11 before starting operation of the device.

After switching ON the interference counter (not shown), ariation takes place via a valve 12 until a complete balance of pressure is obtained.

The interference counter is in operation during the whole operational phase and continuously detects the difference between the wave numbers and the actual state of the refractive index of the air.

A slight exhausting of the tube 1 to have a complete balance of temperature is reasonable unless an interfering pressure loss results in the former.

In Figure 2 like reference numerals refer to like components in analogy to Figure 1. Therefore, only the differences in operation are described In contrast to the embodiment of Figure 1 both interference beams are of the same length.

The glass paths 7 and the path portion 2 of the beams outside of the evacuated tube 3 compensate for their optical effects with corresponding path portions 5 and 8 in the reference beam. Only the path portion in the evacuated tube 3 and an equally long portion of the reference beam in the air remain optically effective . The tube 3 is gradually evacuated after switching ON the interference counter. During operation of the measuring or working device the tube is maintained evacuated. Variations of the refractive index of the air are detected by the respective path portion of the reference beam and measured by the interferometer.

The paths between the interferometer 1 and the angular reflectors 4 and 6 are equally long and insensitive towards thermal length variations of the connecting element, when the interferometer 1 and the two angular reflectors 4 and 6 are arranged on a common base plate 9.

This embodiment has the disadvantage of continuous evacuation during the operation of the measuring the working device.

Claims

1. Method of measuring the wavelength of a frequency stabilised laser involving the environmental influences of pressure, temperature and air moisture, comprising two beams brought to interference, one of said two beams passing at least partially an evacuated cavity of known length, characterised in that the evacuated cavity is ariated and evacuated in sequence, respectively evacuated and ariated until atmospheric conditions prevail, and in that the wavelength varied under atmospheric conditions in relation to vacuum is measured by counting the interferences.

2. Method of measuring the wavelength of a frequency stabilised laser, substantially as herein described with reference to and as shown in the accompanying drawings.

3. A device for carrying out the method claimed in claim 1 or 2, wherein the portions of the path lengths of the two beams propagating in the air and in the glass, respectively, are of equal length, the one beam propagating in the evacuated cavity, the other beam propagating outside of said cavity between an interferometer and a measuring and a reference reflector, respectively.

4. A device as claimed in claim 3, wherein the entire path lengths of both beams between the interferometer and the measuring and reference reflector, respectively, and the beam portions propagating in the glas are of equal length.

5. A device for measuring the wavelength of a frequency stabilised laser, substantially as herein described with reference to and as shown in Figure 1 or Figure 2 of the accompanying drawings.