Microwave frequency measurement (MFM) is to estimate frequencies of intercepted microwave signals, which is critical to modern military and civil radio-frequency (RF) systems, such as wireless communications, electronic countermeasure (ECM), radar warning and electronic intelligence systems. In this paper, a photonic-assisted MFM method based on harmonic down-conversion with semiconductor optical amplifiers (SOAs) is proposed. Two optical harmonic intensifiers consisting of an electro-optic intensity modulator and a SOA are used to generate high-order optical harmonics based on cascaded four-wave mixing in the SOA, which has low-frequency and tunable spacing. It enables ultra-wide harmonic down-conversion of microwave signals under test in the electrical domain with low-frequency local oscillator (LO). The microwave frequency is therefore unequivocally determined by cross-referencing two pairs of harmonic down-converted tones within the LO frequency. It enables multi-tone frequency measurement and eliminates the trade-off between the measurement range and frequency-resolution. Moreover, it avoids the limitation of deadband by the cross-referenced frequency discrimination.
Although autocollimators enable noncontact measurement, their performance is limited in large-scale and long-distance applications because of errors caused by nonideal point light sources. Therefore, we analyze this type of measurement error, including the error source and the equations used to describe the irradiance distribution of the light spot. A two-dimensional exponential approximation formula is used to express the light irradiance distribution and compensate the spot imaging errors. Experimental results show that the measurement error could be reduced by sixfold. Therefore, the proposed compensation algorithm can be applied to autocollimator measurements over distances.
An electrical method is proposed for the absolute time-delay characterization of optical delay components based on the frequency-shifted self-heterodyning. The method utilizes the electrical spectrum of the heterodyne products between the delayed optical signal and the frequency-shifted optical carrier, and achieves the intrinsic absolute time-delay measurement from the notch frequencies of the spectrum at microwave region. Moreover, our method enables highresolution and wide range measurement with low-frequency electrical spectrum analysis. The theoretical analysis is supported by experimental results.
A two-tone spectrum analysis method is proposed for self-referenced microwave characterization of high-speed electro-optic phase modulators (PMs) based on the frequency-shifted heterodyning. The method avoids correcting the roll-off responsivity of photodetection and alleviates the bandwidth requirements of the high-speed photodetector and the electrical spectrum analyzer. Moreover, it achieves very high frequency resolution and high stability measurement by the use of the two-tone frequency-shifted heterodyning.
We demonstrated a self-referenced electrical method for measuring frequency response of high-speed Mach-Zehnder modulators (MZMs) based on two-tone modulation. The modulation index and half-wave voltage can be extracted from the heterodyne ratio of two desired components by properly adjusting bias voltage. The method achieves the electrical domain measurement of the frequency-dependent modulation indices and frequency-dependent half-wave voltages of MZMs without any extra calibration for the responsivity fluctuation in the photodetection. Moreover, it reduces half bandwidth requirements of photodetector and electrical spectrum analyzer by carefully choosing a half frequency relationship of two-tone microwave signals. The consistency between our method and the optical spectrum analysis method verifies the simple but accurate measurement.
A self-calibrated electrical method to measure magnitude response of optical filters is proposed based on dual-frequency-shifted heterodyne. The combined response of optical modulation, filtering, and photodetection is determined from the first frequency-shifted heterodyne, while the reference response of optical modulation and detection is simultaneously obtained from the second frequency-shifted heterodyne, with which the magnitude response of optical filter under test is extracted and self-calibrated. Our method eliminates the extra separate calibration to correct the responsivity fluctuation in the optical modulation and detection. Moreover, it extends double frequency range due to both upper and lower frequency direction measurement at every swept frequency. Magnitude response of optical filter is experimentally measured with our method and compared to that with the conventional method to check its consistency.
An electrical method is proposed for the microwave characterization of dual-drive Mach–Zehnder modulators based on heterodyne mixing. The proposed method utilizes the heterodyne products between the two-tone modulated optical sidebands and frequency-shifted optical carrier, and achieves calibration-free and bias-drift-free microwave measurement of dual-drive Mach–Zehnder modulators with high resolution electrical-domain techniques. Our method avoids the extra calibration for the photodetector and reduces half the bandwidth requirement for the photodetector and the electrical spectrum analyzer through carefully choosing a half frequency relationship of the two-tone modulation. Moreover, our measurement avoids the bias drifting problem due to the insensitivity to the bias phase of the modulator under test. The frequency-dependent modulation depths and half-wave voltages are measured for a commercial dual-drive Mach–Zehnder modulator with our method, which agree well with the results obtained by the conventional optical spectrum analysis method.
We investigate the phase modulation to intensity modulation conversion in dispersive fibers for measuring frequency
responses of electro-optic phase modulators, and demonstrate two typical measurements with cascade path and fold-back
path. The measured results achieve an uncertainty of less than 2.8% within 20 GHz. Our measurements show stable and
repeatable results because the optical carrier and its phase-modulated sidebands are affected by the same fiber
impairments. The proposed method requires only dispersive fibers and works without any small-signal assumption,
which is applicable for swept frequency measurement at different driving levels and operating wavelengths.
All-optical sampling attracts considerable attention due to its crucial applications in high-speed optical analog-to-digital conversion. We successfully demonstrated an all-optical sampling scheme using nonlinear polarization rotation in a single semiconductor optical amplifier at 40 GSa/s and 160 GSa/s, respectively. The scheme requires only a single semiconductor optical amplifier and has low power consumption, which shows much potential for the high-speed optical analog-to-digital conversion.
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