Transversal electrooptical modulator

The invention relates to a transverse electrooptic modulator (EOM) drivable by electrical amplifiers, said modulator consisting of at least two modulator elements for modulating the light wave of a laser beam, preferably for television image projection.

The use of laser radiation sources in television image projection is well known (German Patent 43 06 797).

Currently, acoustooptic and electrooptic modulators are employed in the projection of television images. In the case of the former, the low cost of the system is advantageous, while the poor efficiency is disadvantageous. In order to achieve high modulation bandwidth, a high degree of focusing is required; however this results in low diffraction efficiency. Additionally, the quality of the wave fronts is degraded, and the coupling efficiency in optical fibers is thereby also degraded.

With electrooptic modulators, the attainable high optical efficiency is advantageous. Currently offered electrooptic modulators are available which have optical losses of less than 10% (e.g. Data Sheet LM 0202 from Gesanger Optoelektronik). The disadvantage here is the high driving voltage, typically several 100V. Consequently, expensive electrical amplifiers and driving devices are required which result in a high system price.

The goal of the invention is thus to create an EOM according to the species, of small size and inexpensive to fabricate, which is suitable for a contrast ratio of at least 1:1000 and which has the highest possible optical efficiency at a modulation bandwidth of up to 90 MHz.

This goal is achieved according to the invention by arranging the modulator elements in series, locating a focusing device in front of and behind preferably every other modulator element, associating the output of each electrical amplifier with each modulator element, and connecting the inputs of all the electric amplifiers to the same control circuit for the control signal.

By arranging several optical modulator elements in series, the driving voltage for each modulator element may be reduced. This series connection with high optical efficiency can only be ensured if a focusing device is located in front of and behind each, preferably every other, modulator element so that a narrowing of the beam falls within each modulator element. In this case, for example with four modulator elements, a driving voltage of less than 50V is possible so that simple-to-fabricate, or commercially available electrical amplifiers, for example, video-output-amplifier chips (hereafter abbreviated as “video chips”) may be used, whereby one video chip drives one modulator element, and the same control signal is applied to the inputs of all video chips.

It has proven especially advantageous if, according to the invention, such modulator elements are provided which exhibit at least two or three of the following characteristic data:

The electrical capacitance of one of the modulator elements is in the range of 5 pF to 20 pF.

The length of the modulator element is between 5 mm and 30 mm.

The dimension of the modulator element in the electrooptically effective range is 0.1 mm to 1 mm in the direction of the electrical field of the control signal, i.e. that of the thickness of the crystal.

This design allows for a considerably smaller size than that attainable in the prior art—with the result that the number of possible uses is considerably greater. Specifically, the EOM according to the invention may be used in exposure or projector systems. In addition, the crystal consumption of the new modulator is smaller by a factor of 100 than with current prior art EOMs due to the smaller size of said modulator.

The fundamentally critical mounting of the crystal on the carrier substrate must ensure both good electrical contact and good thermal conductivity. Said mounting must cause the smallest mechanical strains possible in each crystal since the contrast ratio obtainable for the EOM is impaired by mechanical strains. If the individual modulator elements are thus designed, according to the invention, to be wider than necessary and oriented on the substrate such that the electrooptically effective area nevertheless remains limited to an approximately square cross-section, the mounting of the crystal may be effected in a zone which lies far outside of the electrooptic modulation cross-section.

Additional useful embodiments further developments of the invention are identified in the subclaims.

The following explains in more detail one embodiment of the invention with reference to the drawing. In this drawing,

FIG. 1 shows the transverse electrooptic modulator in a schematic diagram of the driving circuit including the associated electrical amplifiers.

Numeral 10 is the transverse electrooptic modulator (EOM) comprising a total of four modulator elements 11 which are aligned with the laser beam designated overall as 12 . Polarizer 13 is located at the input of light beam 12 , while in similar fashion polarizer 13 a is located at the end. A biconvex lens 14 designed as a focusing device is located in front of and behind each modulator element 11 such that a narrowing of the beam falls within the range of each modulator element.

The first modulator element 11 is provided with electrodes 15 , 16 which are connected via connectors 20 , 21 with outputs 22 , 23 of first electrical amplifier A 1 whose two inputs 24 , 25 are connected with the driving circuit designated overall as 26 for the control signal of first amplifier A 1 .

In the same way, an electrical amplifier A 2 , A 3 , and A 4 (for the fourth and last modulator element 11 ) are accordingly associated with each modulator element 11 .

During operation, laser beam 12 , focused in each case in the range of each modulator element 11 , is driven by the same control signal from control signal circuit 26 , but through separate electrical amplifiers A 1 through A 4 , which method has the advantage that as a result of the enhanced effective passage length, the switching voltage for each modulator element is correspondingly reduced so that conventional video chips may be utilized.