−70 dB optical carrier suppression by two-beam coupling in photorefractive media

Suppression of an optical carrier from an RF modulated laser beam of wavelength 532 nm is performed using two-beam coupling in photorefractive barium titanate. A theoretical analysis reveals that perfect suppression can be achieved at a specific modulation strength, which depends on the gain and intensity ratio between the two beams. The experiments achieve a maximum of −72.9±2.4 dB carrier suppression at the theoretically ideal modulation strength, and −61.0±2.4 dB for small modulation strengths. PACS: 42.6.5-k; 42.65 Hw; 42.40.Kw Optical modulators typically transmit some amount of optical carrier power along with the imposed sidebands. Phase modulators do so inherently, yet intensity modulators in practice also transmit carrier at a typical level of −30 dB below the input power. Acousto-optic modulators are nominally single sideband, yet typically −50 dB of the carrier power leaks into the output. Optical carrier energy is often unwanted, especially in certain optical communications and signal processing applications. Our work involves the optical processing of communications signals from an active antenna array in which the presence of an optical carrier introduces undesired spurious signal correlations [1]. Methods to suppress the carrier include both linear [2–5] and nonlinear optical techniques [6–11]. Frankel et al. [4] have reported better than −40 dB carrier suppression using a Sagnac fiber loop, and Loayssa et al. report a better than −55 dB result using a Brillouin-erbium fiber laser [9]. In optical communication links, for example, a balanced modulator scheme using a Mach–Zehnder interferometer can be used to convert phase-modulation into intensity modulation, as illustrated in Fig. 1. The modulators in each arm of the interferometer are driven in anti-phase. The recombining beamsplitter serves to subtract the two carriers while it adds the sideband energy. The optical path length difference between the two arms must be controlled in order to ∗Corresponding author. (Fax: +1-303/492-5504, E-mail: [email protected]) ω ω