Laser frequency translation by means of electro-optic coupling control

One of the limiting technologies affecting the utilization of lasers is the problem of efficient modulation of laser radiation. Most optical modulators have some or all of the following defects: They require a substantial amount of drive power; they have very small modulation depths; or they produce undesired sidebands in addition to the modulation of interest. We have built and tested a single-sideband modulator which overcomes these difficulties. We have placed an electro-optic modulator in a laser cavity in such a way as to couple out all the available laser power and translate its frequency. Modulators can be made for frequencies from 30 Mc to 10 Gc, with bandwidths of as much as 100 Mc in the latter case. A KDP modulator can be placed within a laser cavity to produce polarization modulation which can then be coupled out by means of the polarizing optics or the Brewster angle windows. A 3 Gc single-sideband suppressed-carrier modulator making use of this selective coupling is described. The cavity-type KDP modulator is placed in the laser cavity with the electrically induced axes of the KDP at 45° to the direction of polarization of the laser oscillation. In modulators described earlier, a birefringent plate was necessary to permit sideband suppression. In this internal modulator, we make use of the natural birefringence of the KDF to allow us to omit the birefringent plate. Thus we have a means for shifting the frequency of a laser without sacrificing any of the potentially available laser power. As compared with external modulators, the incorporation of the single-sideband modulator into the laser cavity gives almost a hundredfold increase in the amount of frequency shifted power that can be obtained from a given laser, as a function of modulation power.