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Journal of the Optical Society of America B

Performance evaluation of a non-resonant electro-optic phase modulator using quantum interference

Arpan Adak, Ayan Ray

Published Jul 13, 2026• 0 citations
Publisher

Abstract

Phase modulation of light plays a key role in modern-day optical communication and processing systems. For this purpose, electro-optic phase modulators are indispensable because of their unique operational flexibility in manipulating the output light signal and providing a large modulation bandwidth. We propose a characterization of the performance of a non-resonant electro-optic phase modulator using what we believe to be a novel quantum-interference-based method. The evaluation is done to study (i) sideband generation, (ii) V π voltage, (iii) modulation parameter, and (iv) capability to introduce controlled phase noise in the system. Electromagnetically induced absorption, a product of constructive quantum interference, is used to study the above-mentioned aspects. These results are compared with those obtained from scanning Fabry–Perot Etalon fringes. In the case of a non-resonant modulator, the efficiency of analysis based on electromagnetically induced absorption is proved to be better than that of Etalon fringe analysis. The result of the noise through the phase modulator in the atom-field system is cross-verified with the resulting heterodyne beat signal obtained between two similar external cavity lasers (free-running, passive stabilization using an extra layer of thermal and vibrational isolation), where one of them is quieter than the other and experiences the introduction of noise through the electro-optic phase modulator.

DOI
10.1364/josab.605798

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Phase modulation of light plays a key role in modern-day optical communication and processing systems. For this purpose, electro-optic phase modulators are indispensable because of their unique operational flexibility in manipulating the output light signal and providing a large modulation bandwidth. We propose a characterization of the performance of a non-resonant electro-optic phase modulator using what we believe to be a novel quantum-interference-based method. The evaluation is done to study (i) sideband generation, (ii) V π voltage, (iii) modulation parameter, and (iv) capability to introduce controlled phase noise in the system. Electromagnetically induced absorption, a product of constructive quantum interference, is used to study the above-mentioned aspects. These results are compared with those obtained from scanning Fabry–Perot Etalon fringes. In the case of a non-resonant modulator, the efficiency of analysis based on electromagnetically induced absorption is proved to be better than that of Etalon fringe analysis. The result of the noise through the phase modulator in the atom-field system is cross-verified with the resulting heterodyne beat signal obtained between two similar external cavity lasers (free-running, passive stabilization using an extra layer of thermal and vibrational isolation), where one of them is quieter than the other and experiences the introduction of noise through the electro-optic phase modulator.
A novel phase-modulation technique which permits subkilohertz-laser stability and new levels of precision in laser spectroscopy was reported recently. For spectroscopy, the basic arrangement consists of a combination of an optical pump and a probe field which is phase modulated. The pump prepares the atomic sample by burning a narrow hole within the atom's inhomogeneous line shape, and the probe beam samples the prepared hole when its modulation sidebands are swept into resonance. Off resonance, the probe is balanced as pairs of sidebands produce heterodyne beat signals of opposite phase which just cancel. On resonance, the balance is upset and yields a nonvanishing beat signal with a Lorentzian absorption or dispersion line shape and with residual noise approaching the shot noise limit. Here we investigate the theory of phase-modulation spectroscopy. We treat the nonlinear response of an atomic two-level quantum system subject to an intense pump and a weak copropagating or counterpropagating phase-modulated probe beam. The density-matrix equations of motion are solved by a Laplace-transform method and by the novel use of a translation operator which allows the infinite hierarchy of coupled equations to close. A solution equivalent to the rate-equation result is developed and coherence corrections are found which predict new resonances that have just been detected in this laboratory. The delayed pump-probe technique encountered in solid-state laser spectroscopy is analyzed in this context for two- and three-level quantum systems. The response of a Fabry-Perot cavity to a phase-modulated light wave is examined also and reveals an unexpected absorption feature.
We present a quantum mechanical model to evaluate the time evolution of quantum states of light in an electro-optic phase modulator (EOPM). The phase modulator is analogous to a multilevel atomic system with equally spaced energy levels, each of which corresponds to the sidebands of the phase modulated optical field. Using perturbation theory, we solve the equations governing the interaction of the optical field with a modulating field both with and without phase velocity mismatch. A unitary operator that evolves any given quantum state allows us to evaluate the response of EOPM for single photon, coherent and squeezed states. We apply these results to the analysis of frequency coded quantum key distribution scheme.
Quantum analysis has been done for the traveling-wave (TW) electro-optic phase modulator in the presence of the phase noise of a radio frequency (RF) oscillator and a laser source, in which the Hamiltonian for an active section of the phase modulator system is derived. The effect of RF phase noise from the oscillator and a phase velocity mismatch (PVM) between optical photons and the RF field are included in the Hamiltonian. The time evolution of the field operator of the laser in the TW electro-optic phase modulator is derived by the Heisenberg equation of motion. The action of a TW electro-optic phase modulator in the presence of the two distinct weak noises—classical phase noise because of RF and quantum phase noise caused by a laser—has been evaluated and explained in terms of the second-order temporal correlation function and power spectrum. The power spectrum has been written as a function of the semiconductor laser linewidth, an enhancement factor in the laser linewidth, and the RF oscillator linewidth. The results are applied to analyze the basic model of a frequency-coded quantum key distribution (FC-QKD) system. The quantum bit error rate (QBER), the first-order optical intensity correlation function, and the spectrum of intensity fluctuation have been calculated for the partially coherent regime to evaluate the performance of the FC-QKD system.
We present a detailed analysis of the quantum description of electro-optical phase modulation. The results define a black-box type model for this device, which may be especially useful in the engineering steps leading to the design of complex quantum information systems incorporating one or more of these devices. By constructing an explicit representation of the phase modulation scattering operator, it is shown that an approach based entirely on its classical description leads to unphysical modes associated to non-positive frequencies. After modifying this operator, phase modulation is described, for the first time to the best of our knowledge, in terms of a unitary scattering operator Ŝ defined over positive-frequency modes. The modifications introduced by Ŝ in the process of sideband generation by phase modulation are shown to be insignificant when the radiation belongs to optical bands, thus being consistent with the classical description. Finally, the model is employed to characterize the important case of multitone radio- frequency modulation of an optical signal.
Phase-coherent multi-tone lasers play a critical role in atomic, molecular, and optical physics.Among them, the Raman opeartion laser for manipulating atomic hyperfine qubits requires gigahertz bandwidth and low phase noise to retain long-term coherence.Raman operation lasers generated by directly modulated and frequency-multipled infrared lasers are compact and stable but lack feedback control to actively suppress the phase noise, which limits their performance in practical applications.In this work, we employ a fiber electro-optical modulator driven by a voltage-controlled oscillator (VCO) to modulate a monochromatic laser and employ a second-harmonic generation process to convert it to the visible domain, where the beat note of the Raman operation laser is stabilized by controlling the output frequency of VCO with a digital phase-locked loop (PLL).The low-frequency phase noise is effectively suppressed compared to the scheme without active feedback and it reaches -80 dBc=Hz@5 kHz with a 20 kHz loop bandwidth.Furthermore, this compact and robust scheme effectively reduces the system's complexity and cost, which is promising for extensive application in atomic, molecular, and optical physics.
A compact and robust laser system is essential for mobile atom interferometers. Phase modulation can provide the two necessary phase-coherent frequencies without sophisticated phase-locking between two different lasers. However, the additional laser frequencies generated can perturb the atom interferometer. In this article, we report on a novel method to produce a single high-power laser beam composed of two phase-coherent sidebands without the perturbing carrier mode. Light from a diode laser is phase-modulated by using a fiber-coupled electro-optic modulator driven at 3.4 GHz and passes through a Fabry-Perot cavity with a 6.8 GHz free spectral range. The cavity filters the carrier mode to leave the two first-order sidebands for the two-photon Raman transition between the two hyperfine ground states of 87 Rb. The laser beam is then fed to a single tapered amplifier, and the two sidebands are both amplified without mode competition. The phase noise is lower than that of a state-of-the-art optically phase-locked external-cavity diode laser (−135 dBrad 2 /Hz at 10 kHz) at frequencies above 10 Hz. This technique can be used in all-fiber-based laser systems for future mobile atom interferometers.