Abstract
Traditional electronic frequency demodulation of a microwave frequency voltage is challenging because it requires complicated phase-locked loops, narrowband filters with fixed passbands, or large footprint local oscillators and mixers. Herein, a different frequency demodulation concept is proposed based on refractive index modulation of two coupled microcavities excited by an optical wave. A frequency-modulated microwave frequency voltage is applied to two photonic crystal microcavities in a spatially odd configuration. The spatially odd perturbation causes coupling between the even and odd supermodes of the coupled-cavity system. It is shown theoretically and verified by finite-difference time-domain simulations how careful choice of the modulation amplitude and frequency can switch the optical output from on to off. As the modulating frequency is detuned from its off value, the optical output switches from off to on. Ultimately, the optical output amplitude is proportional to the frequency deviation of the applied voltage making this device a frequency-modulated-voltage to amplitude-modulated-optical-wave converter. The optical output can be immediately detected and converted to a voltage that would result in a frequency-demodulated voltage signal. Or the optical output can be fed into a larger radio-over-fiber optical network. In this case the device presents a compact, low power, and tunable route for multiplexing frequency-modulated voltages with amplitude-modulated optical communication systems. The resulting system requires modest modulation amplitudes and operates at frequencies relevant for modern communication systems. The cavity designs have realistic quality factors that are well within the range of experimental implementation. The role of modulation sidebands in reducing switching contrast is explored, and two methods for mitigating these effects are demonstrated.
Highlights
The manipulation of eigenmodes in coupled optical resonators is a well-studied component of photonic device design [1,2,3]
Spatiotemporal modulation can be used to break time-reversal symmetry resulting in optical isolation [7,8,9], optical circulation [10], and optical orbital angular momentum generation [11]
This work presents a frequency demodulator concept based on refractive index modulation of two coupled microcavities excited by an optical carrier wave
Summary
The manipulation of eigenmodes in coupled optical resonators is a well-studied component of photonic device design [1,2,3]. The microwave-frequency-controlled switch depicted in Fig. 1 amplitude modulates a cw optical carrier wave in response to an applied FM voltage. If the output optical wave is not immediately detected and converted to a voltage, it can be multiplexed with a larger radio-over-fiber optical communication network In this implementation, the proposed device functionality is attractive as few if any existing compact devices perform the specific task of directly amplitude modulating an optical wave in response to the frequency of an applied voltage. A refractive index change of n = 1.48 × 10−3 with a modulation frequency fm = 23.8 GHz are shown numerically to produce an on-off ratio of 26.4 dB These results show that this device design is consistent with well-established experimental capabilities [26,27,28,29,30,31,32]. The effect of modulation sidebands is addressed, and effective techniques for their suppression are introduced
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