JPH06250125A - Device for forming linear polarization orthogonally intersecting two-frequency - Google Patents

Abstract

PURPOSE:To provide an optical device converting a two-frequency laser beam being elliptic polarization approximately intersecting orthogonally to linear polarization perfectly intersecting orthogonally and eliminating cross-talk in a heterodyne interference device. CONSTITUTION:The two-frequency laser beam is separated to two optical paths, and the one side polarization is converted to the linear polarization by using phase shifters 2, 6 in respective optical paths, and thereafter, the beams without cross-talk are taken out by polarizers 4, 10 in a direction perpendicular to the linear polarization to be synthesized by a polarization beam splitter 5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for producing a two-frequency laser beam having a good orthogonality required as a light source for an optical heterodyne interferometer having a small nonlinearity based on crosstalk.

[0002]

2. Description of the Related Art Two-frequency lasers include a Zeeman laser, a synthetic two-frequency laser using an optical acoustic modulator, and a two-frequency laser using a longitudinal mode. In both cases, polarizations of light having different frequencies are orthogonal to each other. It is considered that there is, but this orthogonality is not perfect. Therefore, when a heterodyne interferometer is configured by using these two-frequency lasers as a light source and the interferometric measurement is performed, nonlinearity occurs between the phase of the heterodyne signal and the measured length, which is one of the causes of the measurement error. That is, if the orthogonality is perfect, no optical beat is observed when the two-frequency laser light is detected by a detector having no anisotropy, but if the orthogonality is not perfect, an optical beat appears. This is crosstalk, and this crosstalk causes the above-mentioned nonlinearity.

Conventionally, it has been attempted to measure the amount of crosstalk in advance, calculate a non-linear amount given by this amount, and correct the measured length. However, there is a problem in terms of real-time property at the measurement site, and it is still better to create a dual frequency laser light source with good orthogonality.

[0004]

Of the two-frequency lasers,
The Zeeman laser has a complicated crosstalk due to the influence of a magnetic field, and the problem using the photoacoustic element is different. Therefore, the present invention provides a crosstalk of a two-frequency laser using a longitudinal mode having a simple crosstalk shape. Intended for talk.

[0005]

SUMMARY OF THE INVENTION In the internal resonator type adjacent longitudinal modes,
It is said that the polarized lights of frequencies f1 and f2 are linearly polarized lights which are orthogonal to each other, but when viewed in detail, they are neither linearly polarized light nor orthogonal. Schematically, as shown in FIG. 1A, the light is elliptically polarized light and the orthogonality is not perfect. In the present invention, such polarized light is split into two, and one of the split light is (b) → (c) → (d) → (e) and the other is (b ′) → (c Operations such as')-> (d ')->(e') are performed, and finally two lights are superposed to form orthogonal linearly polarized light as shown in (f).

The polarized light as shown in (b) and (b ') is converted into a completely linear polarized light by using a retarder having a specific direction, for example, a quarter-wave plate (λ / 4 plate), though it is only one polarized light. Can be changed. The light of frequency f2 was linearly polarized (c), and the light of frequency f1 was linearly polarized (c ').
Is. Next, with respect to the light of (c) and (c '), the component (d) of the azimuth orthogonal to the azimuth of the polarized light that is linearly polarized
Take out θ1 and θ2 in (d '), and for the other half, adjust the azimuth of the polarized light using a wave plate (λ / 2 plate) or the like to make linear polarized light like (e) and (e'). A splitter will be used to superimpose the two lights.

[0007]

2 is a block diagram showing an apparatus according to an embodiment of the present invention.

A laser beam is divided into two light beams, a transmitted light beam and a reflected light beam, by a cube beam splitter 1 in two rounds of frequencies f1 and f2. The transmitted light passes through the λ / 4 plate 2 whose azimuth is adjusted so that the light of the frequency f2 becomes linearly polarized light and enters the polarization beam splitter 3. Since the transmitted light of the splitter 3 is a projection component of the f2 linearly polarized light and the f1 elliptically polarized light to the polarization direction of f2, the light f including the optical beat that is the source of crosstalk is generated.
Since it is 2 + b, it is discarded. Since the reflected light is a component of the elliptically polarized light of f1 in the direction orthogonal to the polarization direction of f2, the optical beat is not included. The azimuth of this component of f1 is further adjusted by the polarizer 4, and the polarization beam splitter 5
to go into.

On the other hand, the polarization of the reflected light from the cube beam splitter 1 is rotated by 90 ° by the λ / 2 plate 7, and then λ / 4.
The light of f1 is linearly polarized by the plate 6. The component of the azimuth of the linearly polarized light of f1 contains an optical beat and is discarded as the transmitted light of the polarized beam splitter 8. The reflected light of the splitter 8 is a polarization component orthogonal to the polarization direction of the light of f1 and therefore does not include an optical beat. This reflected light is rotated again by 90 ° in the λ / 2 plate 9 and then the azimuth is adjusted by the polarizer 10 to enter the polarization beam splitter 5 where it is superimposed on the light on the transmission side to produce orthogonal linearly polarized light. A dual-frequency laser beam is created.

FIG. 3 shows an embodiment having another structure. Figure 2
In this embodiment, the emitted light is moved in parallel with the incident light, whereas in the present embodiment, the two are on the same line, and in the embodiment of FIG. In contrast to the components of the dividers 3 and 8, in FIG. 3, they are completely equivalent except that the S component is discarded and the P component is used. That is, the λ / 4 plates 2 and 6 are phase shifters for converting one polarization into linearly polarized light, and the λ / 2 plates 7 and 9 are
Is a retarder for adjusting the polarization direction, and the polarizer 4 is also for adjusting the polarization direction. Also, the prisms 11 and 1
Reference numeral 2 is a simple reflecting mirror.

Further, although the return light is a problem in the racer,
In this optical system, it is easy to insert an optical isolator using a Farade element.

[0012]

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing the order of polarization operations of the present invention.

FIG. 2 is an example of the present invention.

FIG. 3 is another embodiment of the present invention.

[Explanation of symbols]

 f1, f2 Frequency θ1, θ2 Polarizer azimuth 1, Cube beam splitter 2, λ / 4 plate 3, Polarizing beam splitter 4, Polarizer 5, Polarizing beam splitter 6, λ / 4 plate 7, λ / 2 Plate 8, polarized beam splitter 9, λ / 2 plate 10, polarizer 11, reflection prism 12, reflection prism

Claims (1)

[Claims] 1. A two-frequency laser beam consisting of an elliptically polarized light having a frequency of f1 and two elliptically polarized lights substantially orthogonal to the elliptically polarized light and having a frequency of f2
Means for splitting into a second light flux and a light flux of frequency f1 in the first light flux, and a frequency f in the second light flux.
A means for converting the second light into a linearly polarized light, and a polarized light having an azimuth orthogonal to the azimuth of the linearly polarized light having a frequency f1 in the first light flux and the linearly polarized light having a frequency f2 in the second light flux. A dual-frequency orthogonal linearly polarized light producing device comprising means and means for making the two linearly polarized light orthogonal and superimposing them.