GB2060931A - Method and apparatus for determining the velocity of light under varied environmental conditions in combination with an interferometer - Google Patents

Abstract

An evacuable tube (30) is used to separate two optical elements (32, 34) spaced at a known predetermined length L. A laser interferometer (20) is arranged to measure the change in apparent distance DELTA between the elements (32, 34) when the tube is filled with ambient air and when the tube is evacuated. An interferometer calibration factor VA can then be determined from the equation VA=1- DELTA /L, where VA is the ratio of the wavelength of light in vacuum to the wavelength of the light in ambient air. This calibration factor VA is used to correct further interferometer measurements for existing ambient environmental conditions of temperature, relative humidity and atmospheric pressure. <IMAGE>

Description

SPECIFICATION Method and apparatus for determining the velocity of light under varied environmental conditions in combination with an interferometer This invention relates to the measurement of distance, and more particularly to method and apparatus for determining the velocity of light under varied environmental conditions In combination with an optical measuring system.

Interferometers capable of measuring distance are well known in the prior art. Although the present invention is applicable for use with optical measuring systems generally, one type of such a system is a laser interferometer representative of the current state of the art as is described in Bagley et al., U.S.

Patent No. 3,458,259, issued July 26th 1969, and this type of interferometer is shown in conlunction with the specific embodiments of the invention described and claimed hereinafter.

Interferometric length measurements are made by counting the number of wavelengths of light in the distance to be measured. The frequency of light in a laser interferometer, such as a Bagley et al, device is fixed; the wavelength of the light depends upon the velocity of the light in the ambient environmental situation. Therefore, the ambient velocity of the interferometer light varies with the environmental atmospheric pressure, temperature, and relative humidity. These factors must be taken Into consideration in order to arrive at a true ambient velocity of light in a laser interferometer in order to obtain a precise measurement evalution.

The Hewlett-Packard Model 5501A Laser Interferometer System Is an apparatus of the type described in the Bagley et al. patent noted above. This interferometric system, as will be described In more detail hereinafter, Involves the production of two light beams of different frequency which are mixed together, and the resulting beat frequency Is established. The apparatus which measures the beat frequency has a limited tracking speed as the environmental conditions change.

This measuring system of the prior art measures distance or length In terms of the units or length ior fractions of the wavelength of the laser light in ambient air.

The wavelength of the laser light n ambient air Is constantly changing with atmosoheric vaiiations and therefore must be continuaily measured. Its wavelength is always longer than the known wavelength in vacuum because of the density of the air and Its Index of refraction. This produces a decrease In wavelength of approxiametly 0.9997300 pius or minus 0.0001000. This number is termed herein as VA, the vacuum air number.

For convenience a multiplying VA counter is provided which displavs the last for digits of this number. The last digit Is one part In ten mini ion This decimal fraction. VA, is the ratio of the wavelength of the laser light in amn en @ .)ir to the wavelength of laser light in vacuum In the fraction the wavelength of laser light n vacuum s repre- sensed as unity The VA multiplying counter is used in the measuring system to correct the wavelength of the laser light in ambient air to correspond to the known wavelength of the laser light in vacuum so that measurements are made in ambient air consistent with those made in vacuum. This system requires for its operation that the environmental speed of light be given to the apparatus as a seven digit decimal number which equals the fraction: Wave Length in Air Wave Length in Vacuum; hereinafter this ratio is referred to as "VA" Under typical environmental conditions this number could be approximately 0.9997000, although it might range between 0.996000 to .999800. Previously there have been two methods for obtaining this ratio: 1. Using a corrected barometer, thermometer, and hygrometer, the atmospheric pressure, temperature and humidity are measured. Then using existing reference tables these values are used to find this path length ratio number for the atmospheric conditions; and 2.

The Hewlett-Packard 5510 Automatic Compensator apparatus provided with the above-mentioned Hewlett-Packard System, uses sensors to measure atmospheric pressure, temperature and relative humidity. This compensator apparatus automatically and periodically computes the required ratio number.

The number is then applied to the measuring system to correct the length used from the ambient air length to the vacuum length and uses it for the measurement.

Each of the above methods has potential errors.

Human reading and computation errors occur In the first method. Sensor errors and occasional false ratio errors can occur in the second method.

Finally, there has been developed an environmental laser compensator apparatus which is described In U.S. Patent Application Serial No. 946,465 filed September, 28th 1978 by C.L. Farrand et al.. for "Interferometric Apoaratus This compensator Is for use with such a laser Interferometer system also. and operates well In tracking the change of the wavelength ratio under changing environmental conditions. However. this apparatus too requires the determination of an initial VA ratio number. and this apparatus It Is obtained by making an accurate comparison of a length standard with a measur@ ag axis of the laser system. This requires the position Ing of the laser measuring axis which is not possible or desirable In all apolications.

In the present Invention, a portion of 3 oredeter- mined known ength of the optical oath 3 laser interferometer IS enclosed In a container The predetermined known length of tune nut a path in the container can be determined at its place of manufacture by measuring the path length at 3 known temperature.

The container s fliled wlth ambient air nd then evacuated. as the air 5 removed from the @nta@ at a rate within the tracking rate of the laser interferometer system the wavelength @@@e @aser light is increased and the number of wavelengths the fixed path is reduced. The apparent @@@@ge in length of the optical path due to the change on the wavelength of the legth is the ambient atmosphere is changed to vacuum is indicated by the laser interferometer as a dimension.

The indicated difference in length of the optical path from ambient to vacuum conditions, hereinafter called, is then used to compute the required calibration ration VA by means of the mathematical relation 1 - VA = L, where L is the known predetermined length of the optical path in the container.

One advantageous feature of the method and apparatus of the present invention is that the desired ratio VA can be determined with seven decimal place precision, while the predetermined length L, of the optical path in the container and the measured change in the length of the optical path A, need be known only to four decimal places. Using the apparatus of the invention the change of the wavelength as affected by the atmosphere can be measured to one eightieth of a wavelength, or in parts per million. Assuming a VA ratio of 0.999700, this is determinable to the seventh place, or 0.9997000: The range from ambient to vacuum air (1-VA) equals .00030000 and is determinable to + 1 part in 3000, or approximately 0.05%. If the measured change in length of the numerator is measured to one part in ten thousand, the length of the denominator L need be obtained only to the same accuracy.

In summary, the present invention provides apparatus for calibrating an optical measuring system, said system having a source of light, means for directing light from said source between first and second optical elements along an optical path, and having means for measuring and indicating the change in length of said optical path under ambient air and vacuum conditions. The calibrating apparatus of the invention comprises first and second optical elements spaced at a known, predetermined distance from each other to define a linear optical path, and an enclosure enclosing said optical path.

The enclosure further has means for admitting light from said first optical element along said optical path towards said second optical element, and means for directing light from said second optical element along said optical path toward said first optical element. The enclosure has means for admitting ambient air Into the enclosure, and also means connected to it for evacuating air from the enclosure at a rate corresponding to a suitable tracking speed.

The method of the present invention is for calibrating an optical measuring system which has a source of light, which directs light from the source between first and second optical elements along a linear optical path, and which has means for measuring and Indicating the change of length between the optical elements along the optical path. The method comprises the steps of a) spacing the first and second optical elements from each other at a predetermined, known distance L at a given temperature along a linear optical path, bl filling the space between the first and second optical elements with ambient atmosphere; c) evacuating the space between the first and second optical elements to remove the optical path from ambient environmen tal conditions; di using the measuring system to measure the change in distance between the first and second optical elements along the evacuated optical path, and recording the change of distance indicated by the measuring system; and e) computing a calibration factor VA for the measuring system by taking the indicated difference of indicated length A of the evacuated and non-evacuated optical paths, using the equation VA= 1 -E,L where VA represents the ratio of wave length of the measuring system under vacuum conditions to the wave length fo the system under the environmental conditions. The calibrating factor VA can then be used in converting the indicated measurements of the measuring system made under those ambient environmental conditions to distances which would be equivalent to those made in a vacuum. The above-noted procedure can be reversed by first evacuating the space between the optical elements, and then measuring the change of distance after ambient air has been admitted to that space.

Figure 1 shows schematic diagram of a first specific embodiment of the present invention; Figure2 depicts a schematic diagram of a second specific embodiment of the present invention adapted to be positioned in the dead path of a measuring axis of a laser interferometer; Figure 3 illustrates a schematic diagram of a third specific embodiment of the invention positioned on a separate calibration axis of a two-axis laser interferometer; The method apparatus of the present invention are applicable for use with any optical measuring device which utilizes the apparent wavelength (or relative velocity) of light in an ambient environment to measure the distance between two optical elements, however the specific embodiments of the invention illustrated in the drawing use a system much as described in the Bagley et al U.S. patent, such as the well-known commercially available Hewlett-Packard 5501A Laser Interferometer System as a measuring apparatus.

Figure 1 depicts a first and perhaps simplest embodiment of the Invention. A light source 10 is a Hewlett-Packard 5501A LaserTransducer, a high stability laser source which serves as the basis for the distance measurements. Specifically, the source 10 is a two frequency single-mode laser which produces a beam of light 12 comprising two components having frequencies i, and f2, respectively.

Frequency f2 differs from f by a countable Intermediate frequency, such as 2000 KH This two frequency single-mode laser 10 can comprise a tube laser having spaced Internal mirrors which are mounted within the tube, opposite one another and perpendicularto the axis of the tube to allow ail polarizations to be ampllfied directly. and having a magnetic field superimposed along the tube to produce right-hand and left-hand circularly polarized light components of different frequency.

The laser beam 10 output beam 12 s directed to a polarized beam splltter or InterterOirPtUr 20 This IS a polarizing beam splitter which splits the beam 12 into a reference beam and a measuring beam and later combines the two to produce a beat intermediate frequency. On the other side of the interferometer 20 is a quarter-wave plate 25. Further along the optical path established by the laser source 10 is a cylindrical steel tube 30 which may be 5 inches (127 mm.) in length and which has two plane glass transparent windows 32, 34 sealed hermetically on either end. Connecting with the interior of the cyclindrical tube 30 is a mechanical vacuum pump 40 and an adjustable valve 42 which can admit ambient air or be closed to completely seal the tube 30. A plane mirror reflector 50 is positioned at the far end of the optical path.

A Receiver detector 60 is aligned with the light beam output of the interferometer 20. The detector 60 detects both the laser output beam and the interferometer output to produce an intermediate frequency of 2000 KHz., and in turn is connected to an English/Metric Pulse Output system 70, which is connected to a numeric display 80. The output electronics 70 also includes compensator circuitry which permits the insertion of the compensation VA discussed above.

The beam 12 entering the interferometer 20 is split into f and f2 frequency components which return to the receiver detector 60 after reflection by the mirror 50. The component f1 is transmitted to the mirror 50 and reflected back on itself with a slight difference in frequency due to Doppler shift "f when there is any relative movement between the interferometer 20 and the reflector 50. The quarter-wave plate 25 causes the polarization of the return frequency to be rotated through 90 degrees so that f : 'f is reflected out from the interferometer 20 a second time, and to be Doppler shifted again. The polarization off1 + 2 f is rotated again through 90 degrees so that it is now transmitted back to the receiver 60.

(The frequency components f2 is reflected directly by the interferometer 20 to the receiver 60). Resolution doubling is inherent because of the additional Doppler shift. Relative motion between the interferometer 20 and the mirror 50 causes a change in the intermediate frequency measured by the receiver 60, and this Doppler-modulated difference frequency f 2 2 Af is phase detected, multiplied and integrated and amplified by the output electronics 70 and displayed by the numeric display 80 in English or Metic units.

In operation the specific embodiment of Figure 1 Is used to determine the calibration factor VA In the following manner. Initially, the true length L of the tube 30 between the windows 32, 34 is measured accurately. The length L of the enclosed optical path between the windows 32. 34 can also be determined when the factor VA is known by another method described hereinafter. To obtain the factor VA the tube 30 is first vented to the ambient atmosphere by opening valve 42. The reading of the numeric display 80 Is recorded, or the numeric display is "zeroed" so that It reads zero. The vent valve 42 is then closed and the mechanical pump 40 its actuated to evacuate the Interior of the tube 30. The pump 40 can be of a common "forepump'' variety, and the final vacuum need only be of the order of a few tenths of millimeters of mercury. The pumping will normally take place over several minutes, so that any changes can be readily tracked by the optoelectronic measuring system, When the final steady state vacuum has been achieved the measurement system display 80 is again recorded, and the difference in measurements is the indicated difference in length A between the ambient environment and the vacuum at that time, The required calibration factor VA can then be obtained by use of the equation VA = 1 - : L, where both and L are known. The factor VA as determined is then inserted into the measurement apparatus circuitry 70, 80 so that henceforth true, calibrated measurements made between the Interferometer 20 amd the reflector 50 will be accurate under the aforementioned environmental conditions. It will be obvious that these operations can be repeated as frequently as desired in order to determine a new calibrating factor VA when changing environmental conditions so indicate.

Initially, that is when the apparatus Is first installed, the length L of the optical path between the windows 32, 34 enclosed by the tube 30 can be determined by means of the apparatus itself. If the previously described measurement of the apparent change in length ft and the accurate calibration factor VA is known, then the path length L can be calculated from the formula L = #/(1-VA). The accurate calibration factor VA can be determined as noted before by measuring the ambient air temperature, relative humidity and atmospheric pressure, and then using these values, entering a volume of tables such as the Hewlett-Packard "Manual Compensator" (Manual Part No. 10756-90002, published by Hewlett-Packard In 19751 to arrive at a value of VA. Since by the present Invention an accurate value of VA can be obtained immediately, it is no longer necessary to use either the laborious measurement and lookup procedures required by these tables.

Assuming a5 inch 1127 mmi path length Land the resolution of 1 microinch 125.4X10-' mm,) on the determination of the value of, then an accuracy of 1X10 7 can be obtained in the determination of VA when the path length L is known exactly. The path length L can be specified as being known exactly at 20"C. At other temperatures the path length L will vary according to the coefficient of expansion of the steel tube 30. Assuming the tube 30 to he m made of a steel having a coefficient of linear expansion of 11.7X10 C. the variation of the length L over normal laboratory temperature conditions will be small.

Since the apparent change In path length @ IS due to the difference In refractive Index between vacuum and atmospheric conditions, the quality of the vacuum attained during the calibration mode will affect the results. Assuming the refractive index of air to be proportional to air density, it can be seen that a vacuum of 1 1000 atmospheric pressure will contribute errors of the same order of magnitude as a path length error 0. O. Most mechanical v3cUurll pumps can produce a vacuum of 1 X10 Tory, or 0 micron of mercury. This is approximately 0.000013 or normal atmospheric pressure, and It can be concluded that ail" si,i:i.' vncuuln nump @an be used without causing appreciable error.

Figure 2 shows a second embodiment of the invention which is adapted to be positioned in the dead path of the measuring axis of a laser interferometer. The elements of this embodiment are for the most part similar to those described in connection with Figure 1. two frequency single mode laser source 110 directs a light beam 112 into an interferometer 120. At the other side of the polarizing beam splitter interferometer 120 is a quarter wave plate 125, and to that quarter wave plate 125 is hermetically sealed a cylindrical steel tube 130 of known length L. At the other end of the tube 130 Is hermetically sealed a plane glass window 135. Connecting to the interior of the tube 130 are a mechanical vacuum pump 140 and an adjustable vent valve 142. At the far end of the optical measuring axis of the apparatus is a movable plane mirror 150. The output beat frequency beam of light from the interferometer is processed by a receiver detector 160, output electronics 170 and numeric display 180, all similar to their counterparts in the first described embodiment.

In its normal measurement mode of operation the apparatus of Figure 2 is used to measure the distance between a predetermined "zero" position on the measurement axis and the reflector 150. The distance between the center of the interferometer 120 and the "zero" position is known as the "dead path", and it can be entered Into the output electronics 170 so that it does not appreciably affect the indicated measurement of the apparatus. In the apparatus shown in Figure 2 the "dead path" is considered to be the distance between the center of interferometer 120 and the surface of the transparent window 135. The measuring path is the distance between the window 135 and the reflector 150.

The embodiment of Figure 2 can be changed from a calibration mode to a measurement mode of operation. In the calibration a removable reflector 150 can be placed anywhere In the measuring path.

The calibration procedure takes place in the manner described in connection with the first embodiment: The numeric display 180 is zeroed, the valve 142 Is closed and the vacuum pump 140 is turned on. As the interior of the steel tube 130 having a known length L is evacuated the change In indicated distance .~ is measured. At the end of the evacuation process the calibration factor VA can be calculated as before from VA 1 = L and entered Into the compensator electronics of the output circuitry 170. After the factor VA Is entered Into the electronics 170 the distance from the zero position to the moveable reflector 150 can be measured accurately, compensated for the ambient conditions.

A third specific embodiment of the Invention is shown in Figure 3. This embodiment Is incorporated into a two-axis laser measurement system. A laser source 210 of the type described before directs Its output beam 212 to a beam splltter 214. which splits the light Into two perpendicular beams 2 16, 218, one of which i2161 proceeds to 3 calibration axis, and the other (218) goes to the measurement axis of the apparatus.

The calibration beam 216 goes to an interferometer 220, quarter-wave plate 225 and then to a plane mirror reflector 250. A cylindrical steel tube 230 of known length is positioned between the quarter-wave plate 225 and reflector 250, both of which are hermetically sealed to the tube 230. A vent valve 242 and a vacuum pump 240 are also connected to the interior of the tube 230. The output beam from the interferometer 220 goes to a detector 260 connected to electronics 270 and numeric display 280, similar to the comparable elements described above.

The measurement beam 218 is transmitted to another interferometer 320, quarter-wave plate 325 and then over a measurement path to a movable plane mirror reflector 350. The output light from this other interferometer 320 goes to another detector 360 which is connected to output electronics 370 and a numeric display 380, all similar to the comparable elements described before.

In order to calibrate the measurement axis of this apparatus, the calibration axis tube 230 Is first vented by opening the vent valve 242 to admit ambient air to the interior of the tube 230. The valve 242 is then closed and the vacuum pump 240 is started, to remove air from the tube 230. When the desired vacuum is attained the change In the measured distance displayed on the numeric display 280 is noted as the value rn. Since the length of the tube 230 is already known as L, the calibration ratio VA can again be found from the equation VA = 1 -AaL. This value of VA can be Inserted Into the compensator electronics of the measurement output circuitry 370 in order to calibrate the measurement axis measurements of the apparatus for the environmental conditions so that it measure In ambient air as if it were measuring in vacuum.

CLAIMS 1. Apparatus for calibrating an outical measuring system, said system having a source of light. means for directing light from said source between first and second optical elements along an optical path and means for measuring and indicating tne change of distance between said optical element, along the length of said optical path, said apparatus ornp Is- ing: first and second optical elements spaced at a known, oredetermined distance from each other at ; given temperature to define an optical path. 3n enclosure enclosing said optical @atr@ said enclosure having: a means for admitting light from said first optical element along said optical pats, toward second element; bl rneans for admitti iq - ignt from said second optical element along aaiu optical oath toward said first optical element; c) means connected to said enclosure for admitting ambient air into said enclosure; and di means connected to said enclosure for evacuating ambient air from saiL) enclosure.

Claims (1)

1. 2. Apparatus according to Claim l wherein sa id first and second optlc31,elel11elats lre plane transp.l- rent wlnrlo\nrs.

   3. Apparatus according to Claim 1 wherein said first optical element is a quarter-wave plate and said second optical element is a plane reflector.

   4. Apparatus according to Claim 1 wherein said first optical element is a quarter-wave plate and said second optical element is a plate transparent window, said apparatus further having a removable plane mirror reflector positioned against said window.

   5. A method for calibrating an optical measuring system, said system having a source of light, means for directing light from said source along an optical path, and means for measuring and indicating the change of distance between said optical elements along the length of said optical path, the method comprising the steps of: a) spacing said first and second optical elements from each other at a known, predetermined distance L at a given temperature along an optical path; b) admitting ambient air into the space between said first and second optical elements; c) evacuating the space between said first and second optical elements to remove said optical path from the ambient environment; d) using said system to measure the change in distance A between said first and second optical elements along said optical path between the ambient and evacuated conditions; and el computing a calibrator factor VA for said system by using the equation VA= 1 -E,L, where VA represents the ratio of indicated wave length of the measuring system in vacuum to the indicated wave length of the measuring system under said ambient environmental conditions, whereby said factor VA can be used in converting other indicated measurements of said measuring system to the equivalent distance measured in a vacuum.

   6. Apparatus for calibrating an optical measuring system substantially as hereinbefore described with reference to and as illustrated in Figure 1, Figure 2 or 3 of the accompanying drawings.

   7. A method for calibrating an optical measuring system substantially as hereinbefore described with reference to the accompanying drawings.