GB2333834A

GB2333834A - Interferometer with deadpath error compensation

Info

Publication number: GB2333834A
Application number: GB9901038A
Authority: GB
Inventors: Paul Zorabedian
Original assignee: Hewlett Packard Co
Current assignee: HP Inc
Priority date: 1998-01-29
Filing date: 1999-01-18
Publication date: 1999-08-04
Also published as: JPH11257915A; DE19840523A1; GB9901038D0

Abstract

The invention provides for automatic compensation for deadpath error in heterodyne laser displacement measuring device. The embodiment illustrated in figure 3 provides for a quarter wave plate retarder 31 to be partially inserted into the reference beam between beam splitter 30 and reference mirror 26. A similar retarder 33 is placed in the measurement beam between beam splitter 30 and measurement mirror 28 at the reset position D. Retarder 33 is partially mirrored.

Description

DISPLACE;WIEN EASURING INTERFEROit/IETER NVITH DEADPATH ERROR COMPENSATION The invention is directed to interferometry. More particularly, this invention relates to an improved laser interferometer providing correction of dead path error associated with displacement measurements.

Background of the Invention Laser interferometer systems have allowed major advances in many manufacturing technologies. Sophisticated and accurate in the measurement of distances, laser interferometers have led to the production of higher density integrated circuits, precision mechanical components, magnetic storage optical disk manufacturing, and machine tool calibration. As the demand for even greater precision increases, reducing error in the laser system becomes of greater importance.

Because interferometers measure optical path length, errors can be introduced by factors such as variation in ambient air, as well as other error sources. Deadpath error is caused by an uncompensated length of laser beam between the interferometer and the measurement reflector or mirror, when the interferometer stage is at the reset position. The reset position is also commonly lcnown as the zero point or lockup point. Deadpath distance is the difference between the optical length of the reference and measurement components of the laser beam at the zero position. These unequal components produce measurement error.

Figure 1 illustrates the unequal path lengths for a conventional interferometer.

The deadpath length is designated as "Dz", the reference component is fry and the measurement component is h.

The component fh has a longer optical path length than component fry by a distance "Dz". In performing an interferometry measurement, the measurement reflector may move a distance "L" (as shown in Fig 1B) to a new position and comes to rest. Because a laser interferometer only measures '(wavelengths of motion" which involves only distance "L", the system will not correct for the wavelength change over "Dz". This will result in an apparent shift in the zero position of the measuring device. This zero shift is deadpath error and such error occurs every time environmental conditions change during a measurement.

In ever more challenging applications which demand greater accuracy, it is desirable to eliminate as many sources of error as possible, deadpath error among them. Moreover, it is most desirable to eliminate operator involvement in error correction. In other words, the automatic elimination of an error source so that it is transparent to the machine operator is desired so that yet another error source (operator error in overcoming deadpath error, for example) is avoided.

Summarv of the Invention The invention provides for automatic compensation for deadpath error in heterodyne laser displacement measuring device. Such compensation is provided by an improvement to a conventional heterodyne laser interferometer. The improvement provides for a quarter wave plate (QWP), or other transmitting optical retarder with substantially 1/4 wave retardation, inserted into the reference beam so that a portion of the reference beam does not pass through the QWP. The improvement further provides for a second QWP, or other transmitting optical retarder with substantially 1/4 wave retardation, partially inserted into the measurement beam and a reflective element (mirror) inserted into the portion of the measurement beam not interrupted by the QWP. A measurement receiver is provided which detects the interference between the portions of the reference and measurement beams into which the QNVPs were inserted. Further, a reference receiver is provided which detects the interference between the portion of the measurement and reference beams not passing through the Q'IVP or other transmitting optical retarder.

Brief Description of the Drawings Fig 1, A through D inclusive, illustrates deadpath length.

Fig 2 depicts a conventional heterodyne laser interferometer.

Fig 3 depicts the inventive device.

Detailed Description of the Drawinas In a conventional heterodyne laser as depicted in Fig 2, a heterodyne laser source 20 emits two orthogonally polarized beams at slightly different optical frequencies 6 r and O m. A reference receiver 22 samples the initial beat frequency at the source after the sampled portions are combined by a first polarization analyzer 21. A measurement receiver 24 detects the Doppler-shifted beat frequency after the reference a) r and measurement o m beams have been reflected by, respectively, the stationary reference mirror 26 and the movable measurement mirror 280.1 and combined by a second polarization analyzer 23. Return beams are separated from outbound beams by the polarizing beam splitter 30 after quarter wave plates 32 rotate the polarization states of the reflections from the reference 26 and measurement 28 mirrors. Integration of the instantaneous frequency difference between the reference and measurement receivers during motion of the measurement mirror 28 ' yields a net phase which is indicative of total displacement.

The inventive device is depicted in Fig 3. The improvement to the conventional heterodyne laser interferometer provides a transmitting optical retarder with a substantially 1/4 wave retardation, such as a quarter wave plate (Q6NP), in the reference arm partially inserted into the reference beam 31. Further provided is a second transmitting optical retarder with a substantially 1/1 wave retardation, such as a quarter wave plate, which is mirrored on some portion of its surface 33 and which is positioned at the measurement mirror "reset" position D.

The reference signal is generated by interference of the non-polarizationrotated portions of the reference beam a r and the reflection from the mirrored portion of the measurement arm QWP 33. The measurement signal is generated by the interference of the portions of the reference and measurement beams which pass through their respective QWPs 31, 33.

Plane O is the plane at which the measurement and reference beams recombine. Plane R is the position of the reference mirror surface plane. Point D represents the plane of the measurement mirror at reset. Plane B is the position of the measurement mirror at the final position for a displacement measurement.

At point 0, where the reference and measurement beams split apart and recombine, the amplitude of the return reference wave/beam is given by

The wave reflected from measurement-mirror position X is given by

The measurement-receiver photocurrent is M (t) is proportional to (ER + FM )2 Thus, (3)

baseband terms + opt. freq. terms where:

(4) expresses the physical length of the reference path;

(D) expresses the index of refraction averaged over the reference path;

(6) expresses the physical length of the measurement path;

(7) expresses the index of refraction averaged over the measurement path.

An interferometric phase is associated with each point along the measurement arm:

Because phase difference is the measured quantity, it is possible to determine displacements in optical path length. For example, the optical path length between points A and B is given by:

Optical path length is a function of two independent time variables, t3 and tb, which represent the measurement instants at each endpoint of the optical path. Even when the measurement mirror is at rest, the path length may vary due to fluctuations in the index of refraction of the ambient air.

(10) The term ((tb) (to )) is the quantity obtained by counting fringes in the measurement-receiver output as the measurement mirror is translated from point A to point B. It may contain errors due to environmental fluctuations during the measurement interval.

(11) The term '(tod 0(ta)) represents a change in optical phase over the deadpath. The conventional interferometer (Fig 2) has no means by which to measure this quantity, and thereby is susceptible to deadpath error. The invention as depicted in Fig 3 provides the change in optical phase over the deadpath directly from the reference-receiver output. Thus the invention provides automatic compensation for deadpath error.

Claims

CLAIMS We claim:

1. An improved deadpath-compensating interferometer, said interferometer being comprised of a heterodyne laser source generating a vertically polarized beam and a horizontally polarized beam; means for splitting the light beam into reference and measurement beams; configuration providing for the optical transmission of said reference beam through a reference path wherein said reference path includes transmission through a polarization beam splitter, reflection off a reference mirror, and transmission through a polarization analyzer and impingement upon a reference receiver, and said measurement beam through a measurement path, wherein said measurement path includes transmission through a polarization beam splitter, a first transmitting optical retarder with substantially 1/4 wave retardation, and a measurement mirror, and a polarizing analyzer and impingement upon a measurement receiver, said improvement comprising: a second transmitting optical retarder with substantially 1/4 wave retardation partially inserted into the optical path between the polarizing beam splitter and the reference mirror; a deadpath mirror intercepting a portion of the measurement beam in the entrance plane of the first transmitting optical retarder, and which is positioned to optically intercept the measurement beam at a point corresponding to the reset point of the measurement mirror.

2. An improved interferometer useful in the performance of distance measurement, said interferometer including a light source, a means for splitting the light beam into reference and measurement beams, an optical arrangement wherein a reference mirror and a measurement mirror are optically interposed by a polarizing beam splitter, and a polarizing analyzer optically interposed between the non polarizing beam splitter and a reference receiver, and a polarizing analyzer optically interposed between the polarizing beam splitter and the measurement receiver, said improvement comprising: a second transmitting optical retarder with substantially 1/4 wave retardation, partially optically interposed between the polarization beam splitter and the reference mirror so that a portion of the reference beam passes through the transmitting optical retarder with substantially 1/4 wave retardation.

3. An interferometer as in claim 2 further comprising a first transmitting optical retarder with substantially 1/4 wave retardation optically interposed between the polarization beam splitter and the measurement mirror enabling a portion of the measurement beam to pass through the polarization rotating transmissive element.

4. An interferometer as in claim 3 wherein the first optical retarder with substantially 1/4 wave retardation is substantially at the reset point.

5. An interferometer as in claim 3 further comprising a deadpath mirror optically inserted between the polarizing beam splitter and the measurement mirror such that a portion of the measurement beam is reflected by the deadpath mirror while the remainder of the beam may pass through the first transmitting optical retarder with substantially 1/4 wave retardation.

6. An interferometer as in claim 5 wherein the deadpath mirror is located substantially at the reset point.

7. An interferometer as in claim 5 wherein an optical receiver detects the interference between a portion of the beams reflected from the reference and measurement mirrors.

8. An interferometer as in claim 5 further comprising an optical receiver operable to detect interference between a portion of the beam reflected from the reference mirror and a portion of the beam reflected from the deadpath mirror when said deadpath mirror is at the reset position.