DE2304870A1 - OPTICAL ARRANGEMENT FOR BEAM COMPARISON - Google Patents

Description

• 31. January 1973

HASLBR AG , Bern (Switzerland)

Optical arrangement for beam comparison

The invention relates to an optical arrangement for beam comparison, which is a light source, a beam-splitting double prism, an influence section, a beam-combining double prism and a radiation receiver, wherein the two prisms of each double prism are transmissive on a semi-transparent basis Touch the boundary surface and with all prism surfaces through which light enters or exits, the same acute angle (<*) with the Form interface.

A well-known beam splitter consists of a double prism, which consists of two triangular prisms that together form a square one Form a cross-section, the dividing surface of the two prisms lying in a diagonal of the square being semi-transparent to the light is. In this known arrangement, the rays enter and exit the beam splitter perpendicular to the prism surfaces; the two beams leave the beam splitter in different directions which form a right angle with one another. In each of the two beams, a mirror is therefore required in order to guide the two beams onto the beam-combining prism.

Such arrangements are used, for example, for light modulation used to transmit messages by optical means. The light emanating from a light source becomes in one Beam splitter split into two beams, at least one of which is passed through an electro-optic material, i.e. through a material whose optical properties can be achieved by applying a

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electric field can be changed. After going through the Light modulator, the two beams are brought together again in a beam combiner and interfere with each other.

It is an object of the invention to specify an interference arrangement with a significantly simplified beam path in which the two rays run between the two prisms at any small and variable mutual distance.

In the optical arrangement according to the invention, the Boundaries of both double prisms in a plane 5 that of the light source on the beam splitting double prism incident beam and thus also the two beams of the comparison path and the beams emerging from the beam combiner run parallel to the interfaces of the two double prisms.

In the following, some embodiments of the invention are explained by way of example with reference to the figures. Show it;

Fig. 1 shows an optical arrangement for beam comparison with two double prisms,

2 shows an arrangement for beam comparison with a double prism and a mirror,

FIG. 3 shows an arrangement for beam modulation, FIG. 4 shows an arrangement for very high modulation frequencies.

Fig. 1 shows an interference arrangement. 1 the incident beam, which can come from a laser 23 and falls on the beam splitter 2. This consists of two prisms 3 and 4, which touch at the semipermeable interface 5. The lateral surfaces of the prisms through which the rays enter or emerge, form the same with the semi-transparent surface 5 angle

If the beam 1 runs parallel to the interface 5, the exiting rays 6 and 7 will also run parallel to the interface after refraction at the entrance surface, after reflection at the interface 5 or after passing through it and after refraction at the exit surfaces. The distance of the rays 6 and 7 from the plane of the interface 5 are the same and are determined by the distance of the incident ray 1 from the plane of the interface 5, since the sum of the distances between the rays 1 and 6 from the interface is constant.

The distance between the two beams 6 and 7 can thus be changed in the simplest manner by moving the beam 1 relative to the plane of the interface 5 . This beam spacing can be arbitrary

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be made small.

The rays 6 and 7 enter an influence section 8, in which they can be influenced in different ways, e.g. in such a way that that at the output of this section a phase shift between occurs in both rays.

In the beam combiner 9, which is built the same as the Beam splitter 2, the two beams interfere with one another. The beams 10 and 11 emerge from the beam combiner, with im generally only one of the beams is used and the other is blanked out. 24 denotes the beam detector.

The transverse section of the prisms 3 and 4 can instead of the one shown in Fig.l drawn trapezoidal shape also have the shape of an isosceles triangle, since the outer surfaces parallel to the separating surface 5 Surfaces are visually ineffective.

If the angle ot is chosen such that its complementary angle β is equal to the Brewster angle, one polarization component of the incoming beam will pass through the boundary surfaces without reflection, while the polarization component perpendicular to it is strongly reflected. This process occurs at all four interfaces of the beam splitters or combiners, so that in this case the arrangement acts as a polarizer.

The arrangement according to FIG. 1 can advantageously be used in all cases in which beam comparisons are carried out and νιο a relatively small distance between the two beams is advantageous is. An example is the inspection of a plane-parallel transparent plate for inhomogeneities. The plate will in this case, according to FIG. 1, introduced into the influence section 8 perpendicular to the beam direction. Rays 6 and 7 suffer because of different phase shifts and weakenings of the inhomogeneities, which are noticeable in rays 10 and 11. These interfere with the union in the prism 9, whereby converts the phase difference into an amplitude modulation.

Fig. 2 shows a further arrangement using the same jet plate. In this 12 is the incident ray, the is deflected by a reflective surface 13 so that it is parallel to the separating surface of the beam splitter 2 falls on this. He prevails this, as described in connection with FIG. 1, enters the P ^ influx section 8, and falls on the exit from this the mirror 14 is reflected and passes through the influence section 8 and the double prism 2 again. From the prism 2

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two rays 1 and 15 emerge, the first of which is the incident Beam 12 runs in the opposite direction, the second beam 15 is reflected on the surface 16, so that it is in a straight continuation of the Beam 12 emerges from the arrangement.

The arrangement according to FIG. 2 is thus similar to the arrangement of FIG. 1 with the change that the single prism and the. Influence section 8 are traversed twice by light. This doubles the effect on the rays in the influence section 8.

An absorption or scatter measurement can be carried out in a known manner the beams 6 and 7 or 10 and 11 alternately - e.g. by interrupt a perforated disc. So that comes alternately at the detector 24. Share of ray 6 and ray 7 of what a differential measurement enables.

3a and b shows an arrangement of the influence section 8, in which the small distance between the beams 6 and 7 offers particular advantages. 3b shows a view in the direction I of the Fig. 3a.

The two beams 6 and 7 each pass through an electro-optical modulator 18 and 19, respectively. Rods are used as modulators electro-optically active material used. The optical properties of this material are applied by a transverse Electric field changed, which can be generated by the capacitor plates 20 and 21 or 21 and 22. At these plates is a modulation voltage is placed in such a way that in the two rods either the respective fields or the optical axes of the two Rod materials are antiparallel to each other. This results in a phase difference between the two beams ara output of the modulator and thus an interference in the beam combiner 9 and when changing light modulation of the electric fields.

It is of particular advantage in the illustrated arrangement that the two plates 18 and 19 are made of optically active material right next to each other and in good thermal contact with each other are located. The phase shift is mostly temperature-dependent, but this is compensated if both bars are at the same temperature being held. For this it is advantageous if both rods are very thin and the beam spacing is only a little larger than is the diameter of the beam.

For very high modulation frequencies for which the dimensions are no longer small compared to a wavelength of the modulation wave, the arrangement sketched in FIGS. 4a and b is more advantageous. Included

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Fig. 4b shows a view of Fig. 4a in direction II. The electro-optical modulator in the influence section 8 is similar to that of Fig. 3, but the shape of the electrodes has found a special design. 6, 7 are the two light beams lying one above the other, which pass through two transparent rods 28 and 29, which are also lying one above the other, made of electro-optically active material. 30, 31, 32, the three superposed electrodes corresponding to the electrodes 20 _f 21 and 22 of FIG. 3. They form a "triplate", ie a symmetrical high-frequency line with ärei flat conductors, which is fed with the modulation voltage from the side designated by 35 and is closed without reflection at the end in the direction 36. If the angle t between the direction of the modulation wave passing through the electro-optically active body inside the rod, which does not have to coincide with the direction 35-36 due to the different dielectric properties of the rod and its surroundings, and the light beams 6, 7 is chosen such that cos t = V \, / Vr, where Vw is the phase velocity of the electrical and Vr is the group velocity of the light wave, then the modulation reduction and distortion due to the different travel velocities of the electrical and light wave is compensated as a first approximation, whereby the distortion is reduced and the degree of modulation of the light wave is improved.

Claims (7)

Claims 1. Optical arrangement for beam comparison, which is a light source, at least one double prism for beam splitting and beam combining, contains a comparison section and a radiation receiver, the two prisms of the double prism on one touch the semi-translucent interface and all faces of a prism through which air enters or exits are the same Form an angle («*) with the interface, characterized by that the beam (1) incident from the light source (23) on the beam-splitting double prism (2) and thus also the two beams (6, 7) of the comparison section (8) run parallel to the interfaces (5) of the beam-splitting double prism (2). 309832/0979 2. Arrangement according to claim 1, characterized in that a double prism (2) for beam splitting and a second double prism (9) is provided for beam union, and that the boundary surfaces (5) of the two double prisms (2, 9) in one plane lie (Fig. 1). 3. Arrangement according to claim 1, with a double prism which acts as a beam splitter and beam combiner, characterized in that that a flat mirror (14) is provided, the The plane is perpendicular to the aforementioned interface (5), and that the comparison path (8) lies between the prism (2) and the mirror (14). 4. Arrangement according to claim 1, characterized in that the distance between the beam (1) impinging on the beam-splitting prism (2) and the interface (5) is adjustable. 5. Arrangement according to claim 1, characterized in that the angle of entry (β) of the beam (1) into the prism (2) is equal to the Brewster angle. 6. Arrangement according to claim 1, characterized in that in the comparison section (8) two parallel rods (18, 19) optically active material, each of which lies in the longitudinal direction "is traversed by a beam that the rods carry electrodes (20, 21, 22) which are connected to a modulation voltage generator are connected in such a way that opposite phase shifts of the light are caused in the two rods, and that the rods are in thermal contact with one another. 7. Arrangement according to claims 1 and 6, characterized in that the electrodes form two microwave lines, which fed from one end with the modulation voltage and at the other end are closed without reflection, with the quasi-flat electrical waves in the electro-optically active rods and the light waves overlap at an angle (ft) such that that COs (^) is equal to the ratio of the phase velocities of the electrical and the group velocity in the electro-optically active material. 309832/0979