DE3545456A1 - Optical device having an electrically controllable intensity transmission coefficient - Google Patents

Abstract

An optical device, in which the intensity transmission coefficient between at least one radiation bundle input and at least one radiation bundle output can be electrically controlled, contains an interferometric device having a bundle splitter (ST1) for generating two coherent component bundles which pass via corresponding component beam paths to a bundle-uniting device (ST2), where they interfere. Arranged in at least one of the component beam paths is an electrooptical device (EOK) in which the optical length of the segment, extending through it, of the relevant component beam path can be varied electrically, e.g. via the refractive index. The component bundles are united in a strictly parallel fashion, with the result that the recombined bundle has a constant phase over its entire cross-section, and the intensity can be modulated between 100% and 0% with reference to the intensity of the input bundle. <IMAGE>

Description

DESCRIPTION

In optics and especially in laser technology, this always results more often the task of a bundle of optical radiation - hereinafter also referred to as "light beam" for short by an external signal in divert its direction and / or change its intensity. If the requirements on the speed with which these changes should take place, be made, can be perform this task by mechanically or electromechanically moved optical components, e.g. B. by vibrating or rotating Mirrors or prisms. Also a piezoelectric change in the air gap between two prisms, in which a prevented total reflection occurs, fulfills this task. With higher time requirements acousto-optic deflectors and modulators are used. In the case of maximum time requirements, however, where switching or Modulation times of just a few nanoseconds are required only use electro-optical components for this task. Above all, the Pockels cell should be mentioned here, which is almost completely the old Kerr cell, which is afflicted with many disadvantages has displaced. In the Pockels cell, the refractive indices are set in an electro-optical crystal by applying an electric field for the ordinary ray and the extraordinary ray changed to different degrees, which is a "change in birefringence of the crystal causes. This allows the polarization of a light beam passing through it as a function of the length of the Crystal and the level of the applied field strength are reoriented so that arranged in a beam path behind the Pockels cell crystal-optical component, thanks to its birefringence, a change in direction and / or amplitude of the light beam can take place. In In many different embodiments, such devices are used in laser technology for various tasks. Within a laser resonator, these devices can be used, for example, to improve the quality of a resonator to change a given point in time within a few nanoseconds or periodically at intervals of a resonator cycle time

to modulate. In the first case, so-called giant pulses can be generated will; in the second, the so-called active phase coupling Ultra-short laser pulses generated. With the so-called resonator emptying (cavity dumping) can with this facility to a predetermined Time, the entire light energy stored in the resonator is coupled out and in a predetermined direction outside of the resonator be redirected. Outside of a laser resonator, such devices are often used to regulate the intensity of a laser beam or used for modulation for the purpose of transmitting messages.

A considerable disadvantage of the smallpock cells is the very high tension (generally several KiLovoLt), which are necessary for the operation of a Pockel cell is necessary and it is very difficult to place it on the pockel cell within a few nanoseconds with electronic aids is to be done.

Others use related known arrangements such as prisms made of electro-optic crystals, the light deflection of which depends on the size of the applied electrical field have the disadvantage that they are due to the slight change in angle of the light beam when applied even the highest field strengths can hardly be used in practice.

There are already purely optical logic circuits, such as AND, OR and NOT elements are known, which essentially come from a Fabry-Perot interferometer that contains a medium whose index of refraction is a non-linear function of light intensity. Here, however, you need relatively high light intensities that must be precisely controlled; also is a controller for many purposes preferable through an electrical signal.

From US-PS 35 86 416, from which the invention is based, is finally A light modulator is known, which is an interferometer with a beam splitter, two beam paths and a beam combining device Contains, in at least one of the partial beam paths an optical element, such as a KDP-KristaLL is arranged, its Refractive index can be changed by an electrical voltage. the

The union of the TeiLbünds should take place at a small angle, so that an interference fringe system arises when the bundles are merged. At the location of this interference fringe system is one corresponding mask with alternating transparent and opaque strips arranged. By changing the refractive index of the electro-optical Element, the interference fringe pattern can be shifted with respect to the mask, so that the intensity of the mask transmitted light changes between a maximum and a minimum. With such a device, however, the output intensity can only between about 80% and 20% of the intensity of the input light beam can be modulated.

It is an object of the present invention to provide the above-mentioned electro-optical Further develop the device to the effect that an intensity modulation between essentially 100% and 0%, based on the input intensity, is achievable and that in preferred embodiments with significantly lower electrical field strengths and on easier way than was previously possible.

This object is characterized by that in claim 1 Invention solved.

Developments and advantageous embodiments of the invention Establishments are the subject of subclaims.

The device according to the invention has the great advantage that it allows a practically one hundred percent modulation and controlled in preferred embodiments and with relatively small voltages that can be easily mastered with semiconductor components and can accordingly be changed quickly. In addition, the present device is characterized by a relatively simple and robust structure.

In advantageous embodiments of the present device the electro-optical device changing the optical path length, e.g. a crystal whose refractive index depends on the field strength of

pass through one or both partial beam paths several times, so that that for a given change in intensity of the output radiation bundle (s) or for a given switching function required voltage swing is particularly small.

In the following, embodiments of the invention are referred to explained in more detail on the drawings.

Show it:

Fig. 1 is a schematic representation of a first embodiment the present optical device;

Fig. 2 electrical equivalent circuit diagrams for two switching states of the Device according to FIG. 1, when this is operated as an "optical switch";

3 shows a schematic representation of a second embodiment the invention;

Fig. 4 electrical equivalent circuit diagrams for two switching states of the Device according to FIG. 3;

FIG. 5 shows a further development of the device according to FIG. 1; 6 shows a further development of the device according to FIG. 3;

7 shows a schematic representation of a fourth, preferred embodiment of the invention;

Figure 8 is a representation of the index ellipsoid of one in the facility 7 used electro-optically active crystal, and

9 shows a schematic illustration of a modification of the device according to FIG. 7.

The principle of the invention can be explained most simply with reference to FIG. 1, which shows a simple arrangement for realizing the idea of the invention. An input light bundle comes from direction (A), which is ^{split into two coherent partial light bundles (1) and (1 1} ) of equal intensity by a 50% beam splitter (STD). The partial bundle (1) runs over a mirror ( SpD and through an electro-optical element (EOD on the one hand beam splitter (ST2), while the partial beam (V) first reaches the beam splitter (ST2) through an electro-optical element (E02) via a mirror (Sp2). or the beam splitter (ST2) operating the beam splitter, the partial beams are combined strictly parallel to one another, so that the phase of the recombined radiation beam is constant over its entire cross-section (perpendicular to the direction of propagation). This also applies to the following exemplary embodiments. Depending on the relative path difference between the two partial beams These interfere in such a way that part of the light goes in direction (C) and another part in direction (D), with no loss of light through the mask n or other interference fringe-selective optical spatial filters, as they have to be provided in the above-mentioned prior art in the path of the recombined bundle. The electro-optical elements contain materials whose refractive index depends on the applied voltage or field strength. If, for example, the voltage applied to the electro-optical element (EOD is increased, its refractive index changes and thus the optical path length for the partial bundle (D; if the total change in voltage is large enough, a path length change of several wavelengths can occur A change in the refractive index can, for example, be achieved at output (C) with a time-dependent intensity curve of the light that can be described by a (sin) function, while at output (D) there is a curve of the intensity that can be described by a (cos) function applies to the change in the optical path length in the partial beam (V) by applying a voltage to the electro-optical element (E02). By push-pull operation of the two electro-optical elements, the difference between the optical path lengths (with a given voltage swing) can be doubled Amount can be changed.

Of course, it is also sufficient to only pass a partial beam through To lead electro-optical element and in the other partial beam to Path length compensation a corresponding, not electro-optically active Glass or the like to be provided. It is now noteworthy that with exactly functioning beam splitters (STD and (ST2) and when applying a suitable voltage to one or both electro-optical elements the full intensity occurring at (A) either entirely to the output (C) or can be steered entirely to the exit (D). The same applies if input (B) is used instead of input (A). Included it is now so that in each case in the state in which the at (A) entering intensity at (D) exits, at the same time the intensity entering at (B) would exit at (C). In the reverse If the intensity entering at (A) exits at (C), the intensity entering at (B) would exit at (D). One can see that here the radiation beam, depending on the switching state, is similar to that Current in electrical engineering can be led from two inputs to two outputs with a two-pole changeover switch (Fig. 2). This arrangement can therefore be referred to as an electro-optical light switch.

Those skilled in the art will recognize that the arrangement in FIG. 1 is a Mach-Zehnder interferometer acts in which either one or the two separate coherent ones in the beam path Part of the bundle an electro-optical element was used. It is now immediately apparent that a Michelson interferometer with at least one electro-optical element can be provided of the type described, so that an input (output) is then optionally in a first operating or switching state (A) is connected to an output (input) (B), while in the other switching state (A) and (B) are separated from each other because each at (A) or (B) irradiated light is reflected back in itself and not to the other output or input. Such an “electro-optically active” Michelson interferometer is shown in FIG. 3 shown schematically; 4 shows the corresponding electrical equivalent circuit diagram.

In Fig. 5, a further advantageous embodiment of the basic idea of FIG. 1 is illustrated, by the use of two further mirrors (SPV) and (SP2 '), the sub-beams respectively deflected (1) and (1: ¹⁾ again to 90 < and then cross at 90 degrees. As shown, there is an electro-optical element (EO) in the intersection area of the two partial beams. The material of the electro-optical element is such that, by applying the voltage, the refractive index is reduced in the direction of the partial bundle (1), for example, while it is increased in the direction of the partial bundle (V). This arrangement makes double use of the expensive electro-optic element, generally consisting of a properly cut crystal. A corresponding electro-optical Michelson interferometer with double utilization of the electro-optical element is shown schematically in FIG. 6.

A particularly advantageous practical implementation of the principle explained with reference to FIG. 1 is shown in FIG. In this embodiment of the invention, the partial bundles have to traverse a crystal serving as an electro-optical element several times, as a result of which a large path difference can be achieved with small voltages. An electro-optical crystal (EOK) can be used as an active element, which consists of a cuboid of deuterated calcium dihydrogen phosphate (KD * P), polished on all sides, for example with the dimensions 35 35 χ 7 mm. It is cut so that the optical axis is perpendicular to the plane of the paper and the crystallographic axes X ₁ and x- lie in the diagonals of the square. Evaporated gold films serve as electrodes on the base and top surface, which are contacted by resilient pressure with supply lines of a control voltage source (not shown). The electro-optical crystal is located in a trough (T) made of quartz glass or another suitable transparent material which, as shown in the figure, is assembled from four identical individual pieces (KD, (K2), (K3) and (K4) by cementing which in plan view have approximately the shape of right triangles with cut corners on the hypotenuse

the optically polished outer sides of the corner standing cavity, in that of the crystal forming the electro-optical element (EOK) if possible fits exactly. The technically unavoidable space of about 1/10 mm between the quartz glass trough and the electro-optical crystal to avoid reflection losses with an immersion liquid filled with a refractive index between that of the quartz glass and that of the KDP crystal. Serve as beam splitter (STD and (ST2) two quartz glass blocks of the shape shown, which are arranged side by side and form an air gap between them of such a thickness that at the operating wavelength an exact lossless beam splitting in Ratio 1: 1 is achieved by hindered total reflection. This As a result, the air gap can be set very precisely in the desired manner that a film of the desired thickness made of a suitable material, e.g. SiO-, is vaporized. The two beam splitters, the trough and the electro-optisehe crystal are by means of an adjustable mechanical Holder held on an optically flat base plate made of quartz glass or Zerodur by pressing it on. In Fig. 7 is the beam path of the light beam incident at input A and the partial beam passing through the beam splitter (STD drawn out, that of the (STD reflected partial beam, on the other hand, is shown in dashed lines. One recognises, that by total reflection on the walls of the glass trough for each of the two partial beams a five-time passage through the electro-optical Crystal is achieved in such a way that the directions of the solid and the dashed partial beam, respectively are perpendicular to each other, which has the effect that when an electric field is applied, the optical path length for one partial beam correspondingly greatly increased, for the other is reduced to the same extent.

When calculating the voltage change Delta u, which is required to switch completely from output (C) to output (D) or vice versa, it should be noted that, according to the index ellipsoid shown in FIG. 8, the refractive index η moves in the direction of the crystallographic axes ( x ') and (x') changed when a voltage is applied to n _Q + Delta η or n _Q - Delta η, whereby it is known from the crystal-optical manuals that the optical path length change

α * i f.-

354S456

L DeLta η in a crystal, in which the path length of a partial beam L is given by

η > r, · Λυ ♦ 1 (1.) 1 · Δη »_ ° ⁶³

2-d

where r., the electro-optical constant of the crystal in question

OJ

for the operating wavelength and d is the thickness of the crystal in Direction of the electric field. This path length change for a partial beam must be exactly lambda / 4, so that a complete one Switching from one output to the other takes place. With the above The relationship thus results for the required change in switching voltage Delta u

Δυ =
(2)

For example, the arrangement should be used in a neodymium-YAG laser (wavelength 1059 nm), then for KD * P η _ 1.491, r ,, = 26.4 pm / V, so that for the specified dimensions d = 7 mm and I = 175 mm (corresponding to five passes through the 35 mm long crystal) the switching voltage Delta u = 242 volts. These low switching voltage can be easily managed with semiconductor components. For shorter, e.g. visible wavelengths, the Switching voltage even lower.

It should be noted that in the arrangement according to FIG. In each case the first and last total reflection in the glass trough on one Partial area takes place, each from the points marked with arrows (a), (b), (c) and (d) to the adjacent putty point opposite the adjacent wall is angled inwards by about 5 degrees in the manner shown. This is the coupling and coupling out the partial beams into or out of the glass trough

made possible because the radiation bundle occurs at the first or last impact There is no total reflection on the wall, but a reflection-free one Passage through the wall when polarized light is used, the electric vector of which oscillates parallel to the plane of the drawing. As well The passage of light through the walls is free of reflection the bundle dividers, which run perpendicular to the air gap. On the other hand, inclined entrance walls must be anti-reflective.

FIG. 9 finally shows how the arrangement according to FIG. 7 can be modified is to obtain a corresponding electro-optic Michelson interferometer. For this purpose, instead of the bundle splitter (STD in FIG. 7 a mirror block (S) is used, which consists for example of a glass prism whose surfaces (SD and (S2) are mirrored so that the both partial bundles are each precisely reflected back in itself. The switching voltage Delta u for this embodiment is exactly half as large as that required in FIG. 7, since each sub-bundle is now ten times going through the crystal.

It is now easy to see that any state can be achieved by applying a certain bias, for example with reference to FIG. 7, that a bundle entering at (A) exits completely at (D) or at (C) or else bundles of the same are in each case Intensities emerge at (C) and (D). By superimposing an alternating voltage of suitable amplitude and frequency, for example the intensities at the outputs (C) and (D) can be modulated in phase opposition or by applying square pulses between the Outputs (C) and (D) are toggled, and there are also Liebige Superimpositions of different pulse shapes are conceivable, so that an extraordinary number of applications with the most varied Requirements by the described invention is possible.

Instead of the types of interferometric devices described Of course, other known interferometric measurements can also be used Use devices in which the partial beam paths are sufficiently separated so that an electro-optic device of the type described can be used.

Claims (10)

Claims 1. Optical device with electrically controllable intensity transmission between at least one radiation beam entrance and at least one radiation beam exit at which - each input radiation beam (A, B) is optically coupled to an input of a beam splitter (STI) of an interferometric device, wherein each supplied thereto optical radiation beam into two coherent partial beams (1, 1 ¹⁾ disassembled, - the beam splitter (STD via two partial beam paths, each lead one of the sub-bundles, with corresponding sub-bundle inputs a beam combining device (ST2) is optically coupled, which at least one output for a by interference of the Has recombined radiation beam generated by sub-beam, - in at least one of the partial beam paths an electro-optical ^ Device (E01, E02) is arranged, in which the optical length of the piece of the relevant part running through it is radiated ganges is electrically changeable, and - Each radiation beam exit (C, D) of the device via one Output beam path is optically connected to an output of the beam combining device (ST2), characterized in that the partial beam paths (1, V) and the beam combining device (ST2) are arranged so that the partial beams are combined in parallel with one another and an essentially constant phase results over the entire cross section of the recombined radiation beam. 2. Device according to claim 1, characterized in that the interferometric device in the manner of a Mach-Zehnder interferometer is constructed (Fig. 1). 3. Device according to claim 1, characterized in that the interferometric device in the manner of a Michelson interferometer is constructed (Fig. 3). 4. Device according to claim 1 or 2, characterized / that the partial beam paths contain a mirror arrangement (Sp1, Sp ^- T, Sp2, Sp2 ') which deflects the partial beams so that they cross at least once and that an electro-optical device of the specified type is arranged in at least one of these crossing points (Fig. 5, 6 and 7). 5. Device according to claim 4, characterized in that the electro-optical device (EOK) has a square cross-section and is arranged at a corner in a trough (T) made of transparent material which tightly encloses it and forms the mirror arrangement. 6. Device according to claim 5, characterized in that parts (a, b, c, d) of the outer sides of the trough (T) for coupling and decoupling the sub-bundles into or out of the trough (T) with respect to the remaining parts of the outer sides are angled. 7. Device according to one of the preceding claims, characterized in that the electro-optical device contains a material whose refractive index is a function of the electric field strength. 8. Device according to claim 7, characterized in that the electro-optical device a preferably deuterated potassium dihydrogen phosphate crystal contains. 9. Device according to claim 1, characterized in that the beam splitter has two radiation inputs and / or the beam combining device has two radiation outputs. 10. Device according to one of the preceding claims, characterized in that the output bundle beam path is free of interference fringe-selective optical space filters.