A proposed electro-optical apparatus would shift the frequency of a laser beam by a controllable amount within the range of about ±10 GHz. It would be a wide-band alternative to an acousto-optical tunable filter, which is limited to a frequency shift of about 100 kHz. The power efficiency of the proposed apparatus would be nearly 100 percent; the main insertion loss would be only about 2 dB and would be associated with fiber-optic input and output.
The apparatus would produce a constant frequency shift by exploiting the phase shift of the laser beam in an electro-optical medium. The effect, upon frequency, of a steadily increasing or decreasing phase shift is equivalent to the Doppler frequency shift produced by a steadily advancing or retreating mirror. Of course, there is no practical way to realize, in a compact apparatus, a steadily advancing or retreating mirror. Similarly, there is no practical way to realize its functional equivalent in a phase modulator in which the phase shift could be made to increase or decrease steadily without limit.
However, it is possible to drive two electro-optical modulators with identical sawtooth waveforms that differ in phase by about 180°. It is also possible to switch a laser beam electro-optically so that it repeatedly passes through one modulator and then the other. If this switching were done in proper synchronization with the sawtooth modulation waveform, then laser beam could be made to undergo either an increasing or a decreasing phase shift at all times. The proposed apparatus is based on this concept.
The apparatus (see figure) would be made from standard lithium niobate integrated optics. A Mach-Zehnder switch driven by a rectangular waveform synchronized with the sawtooth modulating waveform would direct the laser beam alternately along optical paths to two phase modulators. The amplitude of the sawtooth waveform would typically be chosen to obtain a peak-to-peak phase modulation of 4π radians. The shift in frequency of the laser beam would equal the phase-modulation rate, which would be proportional to the amplitude and repetition frequency of the sawtooth waveform; for example, at a typical repetition frequency of 1 GHz, the frequency shift would be about 2 GHz.
The amplitude and phase relationships between the two sawtooth modulating waveforms would be chosen so that during the brief switching intervals in which parts of the laser power were passing partly through both phase modulators, the difference between the phase shifts produced by the two modulators would be 2π radians. This choice would ensure coherent addition of the outputs of the two modulators and prevent the introduction of a spurious phase modulation associated with the switching.
This work was done by Roman C. Gutierrez of Caltech for NASA's Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
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Refer to NPO-20531