GB1593733A

GB1593733A - Distance measuring instruments

Info

Publication number: GB1593733A
Application number: GB54140/76A
Authority: GB
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1976-12-24
Filing date: 1976-12-24
Publication date: 1981-07-22
Also published as: DE2709577A1; DE2709577B2

Description

(54) IMPROVEMENTS IN OR RELATING TO DISTANCE MEASURING INSTRUMENTS.

(71) We THE MARCONI COMPANY LIMITED, a British Company, of Marconi House, New Street, Chelmsford, Essex, CMl IPL. do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to distance measuring instruments which are capable of measuring fairly short distances to a high degree of accuracy. Requirements for instruments of this kind arise, for instance, in checking the profile of a dish shaped radio antenna, where the accuracy of the profile is very important and critical.It has been proposed to use an amplitude modulated laser beam for this purpose, and the accuracy of the measurement is dependent, amongst other factors, on the precision and stability of the modulation source, and on the depth of modulation produced. The present invention seeks to provide a laser beam measuring instrument in which the accuracy of the amplitude modulation is primarily dependent on the frequency stability and accuracy of the source of the laser beam itself.

According to this invention, a laser beam measuring instrument includes a laser source of light, so arranged and dimensioned as to be capable of naturally generating light at two separate frequencies within the amplitude characteristic of the laser source, means for detecting the amplitudes of the light at the two separate frequencies, and controlling the laser source in dependence thereon so that the two amplitudes are substantially equal, means for combining the two separate frequencies to generate an amplitude modulated beat frequency, and means for directing the light beam at the beat frequency to a point whose distance is to be measured.

By arranging that the amplitudes of the two frequencies are equal, a stable frequency difference is produced.

When, as will generally be the case, the two longitudinal modes generated at the two separate frequencies within the amplitude characteristic of the source are orthogonally polarized, it is necessary to bring them into the same plane of polarization prior to combining them to produce the amplitude modulated beat frequency.

The two separate frequencies represent two discrete longitudinal modes of oscillation of the laser.

Preferably, the laser source is of the kind containing heated gas in an elongate envelope.

Preferably, the amplitude characteristic of the laser source is adjusted by controlling the temperature of the laser gas so that the amplitudes of the two longitudinal modes are equal.

The invention is further described, by way of example, with reference to the drawings accompanying the Provisional specification, in which Figure 1 illustrates an amplitude characteristic of a laser source in a distance measuring instrument in accordance with the present invention, and Figure 2 shows diagrammatically such an instrument.

Referring to the drawings, a laser 1, generates within its amplitude envelope 2 two longitudinal modes 3 and 4 which are orthogonally polarized. In Figure 2, the optical paths are represented by broken lines, and electrical paths by solid lines, light 10 emitted from the rear of the laser is used to equalize the mode amplitudes.

The two longitudinal modes are received by a polarizing beam splitter 5, which separates out the two beams by virtue of their polarizations and sends each to an amplitude detector 6 and 7 respectively.

The detectors 6 and 7 generate signals representative of the amplitude of the light beam received by it, and these signals are compared by a comparator 8. The detectors 6 and 7 could conveniently take the form of conventional photodetectors, and the comparator could simply be a dif fcrcntial amplifier. If the amplitudes of the two spectral lincs are equal, no output signal is provided by the comparator, but if their amplitudes differ, the difference signal is passed to a control circuit 9 which shifts the amplitude envelope characteristic 2 of the laser 1 until equality of amplitude is achieved. Figure 2 illustrates the two longitudinal modes 3 and 4 having equal amplitudes. The control is most readily accomplished by altering the temperature of the laser gas by means not shown.

The laser 1 is of the kind containing hot gas within an envelope of particular length, and it is the internal structure of the envelope and its length which determines the number of longitudinal modes which are generated, and their polarization directions.

Light 11 emitted from the "front" of the laser consists of two longitudinal modes orthogonal polarization. A polarizer 12, orientated at 45" to the two directions of polarization, is placed in the output beam. The effect of this is to produce a single beam of light which is amplitude modulated, having a modulation frequency equal to the difference in frequencies of the two original spectral lines, i.e. a beat frequency is produced.

This beat frequency is projected onto a point or surface to be measured, and the phase of the modulation of the reflected light is compared with that of the transmitted light so as to accurately measure the distance of the point of reflection from the laser source. Clearly, ambiguities will arise if the reflection point is at a distance greater than the wavelength corresponding to the beat frequency, but for most applications such ambiguities can easily be resolved, since the approximate distance is known. Typical applications arise in the checking of profiles of large dish antennae, where the large number of extremely precise measurements needed, makes conventional measurement techniques cumbersome time consuming, and often insufficiently accurate.

WHAT WE CLAIM IS: 1. A laser beam measuring instrument including a laser source of light, so arranged and dimensioned as to be capable of naturally generating light at two separate frequencies within the amplitude characteristic of the laser source, means for detccting the amplitudes of the light at the two separate frequencies, and controlling the laser source in dependence thereon so that the two amplitudes are substantially equal, means for combining the two separate frequencies to generate an amplitude modulated beat frequency, and means for directing the light beam at the beat frequency to a point whose distance is to be measured.

2. An instrument as claimed in claim I and wherein the laser source is of the kind containing heated gas in an elongate envelope.

3. An instrument as claimed in claim 2 and wherein the amplitude characteristic of the laser source is adjusted by controlling the temperature of the laser gas so that the amplitudes of the two longitudinal modes are equal.

4. A laser beam measuring instrument substantially as illustrated in and described with reference to Figure 2 of the drawings accompanying the Provisional specification.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. fcrcntial amplifier. If the amplitudes of the two spectral lincs are equal, no output signal is provided by the comparator, but if their amplitudes differ, the difference signal is passed to a control circuit 9 which shifts the amplitude envelope characteristic 2 of the laser 1 until equality of amplitude is achieved. Figure 2 illustrates the two longitudinal modes 3 and 4 having equal amplitudes. The control is most readily accomplished by altering the temperature of the laser gas by means not shown. The laser 1 is of the kind containing hot gas within an envelope of particular length, and it is the internal structure of the envelope and its length which determines the number of longitudinal modes which are generated, and their polarization directions. Light 11 emitted from the "front" of the laser consists of two longitudinal modes orthogonal polarization. A polarizer 12, orientated at 45" to the two directions of polarization, is placed in the output beam. The effect of this is to produce a single beam of light which is amplitude modulated, having a modulation frequency equal to the difference in frequencies of the two original spectral lines, i.e. a beat frequency is produced. This beat frequency is projected onto a point or surface to be measured, and the phase of the modulation of the reflected light is compared with that of the transmitted light so as to accurately measure the distance of the point of reflection from the laser source. Clearly, ambiguities will arise if the reflection point is at a distance greater than the wavelength corresponding to the beat frequency, but for most applications such ambiguities can easily be resolved, since the approximate distance is known. Typical applications arise in the checking of profiles of large dish antennae, where the large number of extremely precise measurements needed, makes conventional measurement techniques cumbersome time consuming, and often insufficiently accurate. WHAT WE CLAIM IS:

1. A laser beam measuring instrument including a laser source of light, so arranged and dimensioned as to be capable of naturally generating light at two separate frequencies within the amplitude characteristic of the laser source, means for detccting the amplitudes of the light at the two separate frequencies, and controlling the laser source in dependence thereon so that the two amplitudes are substantially equal, means for combining the two separate frequencies to generate an amplitude modulated beat frequency, and means for directing the light beam at the beat frequency to a point whose distance is to be measured.