Some additional applications have been proposed to exploit amplification by stimulated Brillouin scattering ("Brillouin Amplification" for short), building on the concepts introduced in the preceding article. Chief among the proposed applications is conversion of a phase-modulated optical signal to an amplitude-modulated optical or electrical signal with stability greater than that previously achievable, and without need for bias.
The difficulty of converting a phase-modulated optical signal to an amplitude-modulated optical or electrical signal arises from the essential nature of any phase-modulated signal. As illustrated in Figure 1, a phase-modulated signal includes many pairs of sidebands at integer multiples of the modulation frequency, fm, above and below the carrier frequency. For each upper sideband, there is a corresponding lower sideband of equal magnitude, and the difference between the phases of the upper sideband and the carrier is the opposite of the difference between the phases of the lower sideband and the carrier. One cannot obtain amplitude modulation through the mere detection of the phase-modulated signal because the symmetry between the upper and lower sidebands gives rise to a corresponding symmetry in the beat notes among all the sidebands and between the sidebands and the carrier.
The chief proposal for converting phase modulation to amplitude modulation involves the use of frequency-selective amplification of one of the lower sidebands to break the symmetry between this sideband and its corresponding upper sideband. Brillouin amplification offers the frequency selectivity needed for this purpose. For example, as illustrated in Figure 2, one could adjust the pump and carrier frequencies to place the first lower sideband at the Brillouin-scattering spectral peak. This adjustment would cause the first lower sideband (but not the carrier or the first upper sideband) to become Brillouin-amplified, as described in the preceding article. The beat between the carrier and the preferentially amplified first lower sideband would constitute the major part of the amplitude-modulated output signal. The amplitude modulation would be extremely stable in that it would be immune to fluctuations in the carrier frequency, the temperature, and the length of the optical fiber. In addition, the Brillouin amplification of the chosen sideband would make this technique the most efficient of all techniques for phase-to-amplitude conversion.
Other proposed applications of Brillouin amplification for conversion from phase to amplitude modulation include photonic frequency conversion, photonic frequency multiplexing, photonic generation of harmonics, and photonic amplification and shaping of optical pulses. The pulse application would be the most complex because it would involve the use of multiple pump lasers to achieve Brillouin amplification at multiple selected sidebands of a phase-modulated signal or at multiple modes of a mode-locked laser.
This work was done by Xiaotian Steve Yao of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Electronic Components & Circuits category, or circle no. 154on the TSPOrder Card in this issue to receive a copy by mail ($5 charge).
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