Technical field of the invention:

The present invention relates to a high resolution heterodyne interferometric method and system, which can be used for measuring displacement of the target with high resolution and no periodic nonlinearity.

Technical background

The heterodyne interferometer has been widely used in many precision machines and calibration services due to its large measuring range, high signal-to-noise ratio and high measurement precision. Nowadays, high resolution, high speed and high accuracy measurements are required for many applications in order to produce small features. This initiates an improvement of the performance of the heterodyne interferometer to satisfy industrial demands.

The periodic nonlinearity caused by frequency and polarization mixing effects limits the accuracy and resolution of a heterodyne interferometer. A lot of research has been done to reduce the periodic errors, However, periodic nonlinearity is originated from the frequency and polarization mixing effects in the light path which is inherent to the traditional heterodyne interferometer. As a result, for traditional heterodyne interferometers, it is hardly possible to realized high resolution and high precision displacement measurement.

A heterodyne interferometer with an acousto-optic modulator as a beam splitter was proposed by T. L. Schmitz and J. F. Beckwith in "Ascousto-optic displacement-measuring interferometer: a new heterodyne interferometer with Anstrom level periodic error" Journal of Modern Optics 49, pages 2105-2114. In this method can reduce the frequency and polarization mixing effects, the periodic nonlinearity is reduced and the precision and resolution are improved consequently. Nonetheless, its specific and complicated configuration limits the typically possible applications for measuring displacement.

A simpler heterodyne interferometer was proposed by Ki-Nam Joo, Jonathan D. Ellis, et al in (High resolution heterodyne interferometer without detectable periodic nonlinearity. Optics express/Vol.18, Issue 2/1159-1165 ) . In this interferometer has a reference beam and a measurement beam spatially separated to each other. This interferometer can eliminate the frequency and polarization mixing effects. The periodic nonlinearity is also eliminated and the precision is improved. In addition, the interferometer has a simple structure and which make it applicable to precision industrial engineering. Despite the benefit of the interferometer, the optical structure is not balanced and is therefore sensitive to the environment temperature variation.

In conclusion, all of the above existing methods and apparatuses are not suitable for high resolution and high accuracy measurements.

Summary of the invention

One objective of the invention is to provide a heterodyne interferometric method and system, which is suitable for high resolution and high accuracy measurements.

According to one aspect of the invention, a high resolution heterodyne interferometric method is proposed which comprises the steps of:

(a) providing a first linearly polarized laser beam with a first frequency fi and a second linearly polarized laser beam with a second frequency _ , wherein the first linearly polarized laser beam and the second linearly polarized laser beam is provided in parallel and spatially separated to each other;

(b) orienting the two linearly polarized laser beam diagonally in an incident plane of a beam splitter;

(c) dividing the first linearly polarized laser beam into a first reference beam and a first measurement beam by the beam splitter;

(d) dividing the second linearly polarized laser beam into a second reference beam and a second measurement beam by the beam splitter;

(e) directing the first reference beam to a first retroreflector by means of which it is reflected back to the beam splitter;

(f) directing the second reference beam to a second retroreflector by means of which it is reflected back to the beam splitter;

(g) directing the first measurement beam to a third retroreflector by means of which it is reflected back to the beam splitter;

(h) directing the second measurement beam to a fourth retroreflector by means of which it is reflected back to the beam splitter;

(i) adjusting the first retroreflector and the fourth retroreflector such that the first reference beam interferes with the second measurement beam to form a first measurement signal I _ml ;

(j) adjusting the second retroreflector and the third retroreflector such that the second reference beam interferes with the first measurement beam to form a second measurement signal l _m 2,

(k) detecting the two measurement signals I _m i, I _m 2 by a first photodiode and a second photodiode respectively, and calculating the displacement of the subject being measured.

According to a second aspect the present invention, a high resolution heterodyne interferometric system is proposed.

The system is constructed of

a beam splitter provided for dividing two parallel laser beams with different frequencies _ and _/½ into reference beams and measurement beams;

a first retroreflector provided to reflect the reference beam with frequency _ back to said beam splitter;

a second retroreflector provided to reflect the reference beam with frequency _ back to said beam splitter;

a third retroreflector provided to reflect the measurement beam with frequency _ back to said beam splitter;

a fourth retroreflector provided to reflect the measurement beam with frequency f2 back to said beam splitter;

a first photodiode provided to detect one measurement signal of the heterodyne interferometric system;

a second photodiode provided to detect the other measurement signal of the heterodyne interferometric system.

The features and advantages of this invention are shown as detailed below:

(1) The measurement beam and the reference beam in this method and system are spatially separated, which eliminates the frequency and polarization mixing effects. As a result, the periodic nonlinearity is eliminated.

(2) The measurement signals have opposite Doppler shift for the same target movement. The resolution of the heterodyne interferometer is two times higher than the traditional heterodyne interferometer.

(3) The structure of the proposed heterodyne interferometer is balanced and not sensitive to the environment temperature change. BRIEF DESCRIPTION OF THE DRAWINGS

IN THE DRAWINGS,

Fig. 1 : construction of the high resolution heterodyne interferometric system proposed in preferred embodiment;

Fig. 2: front view of the heterodyne interferometric system in Fig. 1;

Fig. 3: side view of the heterodyne interferometric system in Fig. 1;

Specification of piece numbers in Fig. 1 :

1 beam splitter,

2 second retroreflector,

3 first retroreflector,

4 third retroreflector,

5 fourth retroreflector,

6 first photodiode,

7 second photodiode,

8 laser beam with frequency },

9 laser beam with frequency _ .

PREFERRED EMBODIMENTS OF THE INVENTION

As shown in Fig.l, a high resolution heterodyne interferometric system, which comprises: a beam splitter 1 provided to divide two parallel laser beams with different frequencies fi and _ into reference beams and measurement beams; a first retroreflector 3 provided to reflect the reference beam with frequency fi back to said beam splitter 1; a second retroreflector 2 provided to reflect the reference beam with frequency _/½ back to said beam splitter 1; a third retroreflector 4 provided to reflect the measurement beam with frequency _ back to said beam splitter 1; a fourth retroreflector 5 provided to reflect the measurement beam with frequency _ back to said beam splitter 1; a first photodiode 6 provided to detect one measurement signal of the heterodyne interferometric system; a second photodiode 7 provided to detect the other measurement signal of the heterodyne interferometric system.

As shown in Fig.1—3, a high resolution heterodyne interferometric method, which comprise the steps of:

(a) providing two parallel laser beams which are linearly polarized laser beam and with different frequencies fi and f ,

(b) orienting the two laser beams in diagonal position in the incident plane of a beam splitter, as shown in Fig.2;

(c) dividing the laser beam with frequency _ into a reference beam and a measurement beam by the beam splitter;

(d) dividing the laser beam with frequency _ into a reference beam and a measurement beam by the beam splitter;

(e) directing the reference beam with frequency ] to a first retroreflector by means of which it is reflected back to the beam splitter;

(f) directing the reference beam with frequency _/} to a second retroreflector by means of which it is reflected back to the beam splitter;

(g) directing the measurement beam with frequency fi to a third retroreflector by means of which it is reflected back to the beam splitter;

(h) directing the measurement beam with frequency _/½ to a fourth retroreflector by means of which it is reflected back to the beam splitter;

(i) adjusting the first retroreflector and the fourth retroreflector such that the reference beam with frequency fi interferes with the measurement beam with frequency whereby a first measurement signal l _m i is generated;

(j) adjusting the second retroreflector and the third retroreflector such that the reference beam with frequency _/½ interferes with the measurement beam with frequency _ whereby a second measurement signal I _m2 is generated;

(k) detecting the two measurement signals I _m i, I _m2 by a first photodiode and a second photodiode respectively, and calculating the displacement of the measuring subject.