Xinhai Zou

Self-calibrating measurement of high-speed electro-optic phase modulators based on two-tone modulation

We propose a self-calibrating method for high-frequency response measurement of electro-optic phase modulators based on two-tone modulation. The method utilizes the electrical domain measurement of heterodyning spectrum between the two-tone modulation optical signal and the frequency-shifted optical carrier, and eliminates the need to correct the responsivity fluctuation in the photodetection. High-frequency modulation depth and half-wave voltages are measured and compared to those with the traditional optical spectrum analysis method in the experimental demonstration. The proposed method enables calibration-free and accurate frequency response measurement of electro-optic phase modulators by using high-resolution electrical spectrum analysis.

Optical Frequency-Detuned Heterodyne for Self-Referenced Measurement of Photodetectors

A self-referenced frequency response measurement of high-speed photodetectors (PDs) is proposed and demonstrated based on optical frequency-detuned heterodyne method. Our method provides a narrow linewidth and wide-bandwidth optical stimulus consisting of acoustooptic frequency shifting and two-tone phase modulation in a Mach-Zehnder interferometer, and achieves the self-referenced frequency response measurement of high-speed PDs without the need for correcting the power variation of optical stimulus. Moreover, it allows multiplied measuring frequency range and avoids any bias drifting problem. Frequency responses are experimentally measured for a commercial PD, which agree well with the results obtained by the conventional methods.

High-Speed Optical Phase Modulator Based on Graphene-Silicon Waveguide

A high-speed optical phase modulator based on graphene-silicon waveguide (GSW) is presented. Two graphene flakes, sandwiched by insulating dielectric spacers, are embedded in a silicon waveguide to enhance the interaction between the graphene and light. By applying a bias on the graphene flakes, the refractive index variation of TE mode in the waveguide shows a large quasi-linear dynamic range, which is ideal for achieving optical phase modulation. Based on this structure, a π phase shift can be achieved by a 75.6 μm-long GSW. Calculations show that the 3 dB modulation bandwidth of the optical phase modulator can be as high as 119.5 GHz with a low consumption of 0.452 pJ/bit.

Extinction-ratio-independent electrical method for measuring chirp parameters of Mach-Zehnder modulators using frequency-shifted heterodyne

An extinction-ratio-independent electrical method is proposed for measuring chirp parameters of Mach-Zehnder electric-optic intensity modulators based on frequency-shifted optical heterodyne. The method utilizes the electrical spectrum analysis of the heterodyne products between the intensity modulated optical signal and the frequency-shifted optical carrier, and achieves the intrinsic chirp parameters measurement at microwave region with high-frequency resolution and wide-frequency range for the Mach-Zehnder modulator with a finite extinction ratio. Moreover, the proposed method avoids calibrating the responsivity fluctuation of the photodiode in spite of the involved photodetection. Chirp parameters as a function of modulation frequency are experimentally measured and compared to those with the conventional optical spectrum analysis method. Our method enables an extinction-ratio-independent and calibration-free electrical measurement of Mach-Zehnder intensity modulators by using the high-resolution frequency-shifted heterodyne technique.

Graphene-Assisted Electroabsorption Optical Modulator Using D-Microfiber

We report on an electroabsorption optical modulator using graphene-on-D-microfiber configuration. Two-lamellae of graphene embedded in the dielectric spacer are incorporated with a side-polished microfiber to enhance the graphene-light interaction efficiency measured in terms of effective mode index change. The effects of the side-polished microfiber on different radial distances are analyzed on an effective mode index change and energy flow through the fiber for transverse electric mode by applying bias to the graphene lamellae. The theoretical simulated results show that the effective mode index change is maximized from a value of 0.006 to 0.018, which is almost three times larger than the existing fiber optic modulator. Calculations also show that the electroabsorption optical modulator can be realized with extinction ratio as high as 17.80 dB, modulation bandwidth of 97.26 GHz and operates at C-band of optical communication with the active length of 80 μm long at the expense of 24.30 fJ/bit.

Calibration-free measurement of high-speed Mach-Zehnder modulator based on low-frequency detection.

A calibration-free electrical method is demonstrated for measuring the frequency response of high-speed Mach-Zehnder modulators (MZMs) based on low-frequency detection. The method achieves the high-frequency modulation index and half-wave voltage measurement of MZMs by the low-frequency electrical spectrum analysis of the two-tone and bias-modulated optical signal. Moreover, it eliminates the need for correcting the responsivity fluctuation in the photodetector through setting a specific frequency relationship between the two-tone and bias modulation. Both absolute and relative frequency response of MZMs are experimentally measured with our method and compared with those obtained with conventional methods to check for consistency.

Calibration-Free Electrical Spectrum Analysis for Microwave Characterization of Optical Phase Modulators Using Frequency-Shifted Heterodyning

A novel calibration-free electrical spectrum analysis method for microwave characterization of electrooptic phase modulators is proposed and experimentally demonstrated based on frequency-shifted optical heterodyning. The method achieves the electrical domain measurement of the modulation efficiency of phase modulators without the need for correcting the responsivity fluctuation in the photodetection. Moreover, it extends double the measuring frequency range through setting a specific frequency relationship between the driving microwave signals. Modulation depth and half-wave voltage of phase modulators are experimentally extracted from the heterodyning spectrum of two phase-modulated signals with and without frequency shifting, and the measured results are compared to those obtained with the traditional optical spectrum analysis method to check the consistency. The proposed method provides calibration-free and accurate measurement for high-speed optical phase modulators with the high-resolution electrical spectrum analysis.

Polarization-Independent Modulator by Partly Tilted Graphene-Induced Electro-Absorption Effect

A polarization-independent graphene-based electro-absorption optical modulator concept is presented. The device is based on a slanted silicon stripe upon which two graphene flakes reside; on this surface, another silicon waveguide is capped, forming a rectangular silicon-graphene waveguide (SGW). Simulation results show that effective mode index of both TE and TM modes in the SGW undergo almost the same variations under different biases across a broad wavelength range. For a 30-μm-long silicon-graphene waveguide, throughout the S, C, and L communication bands, extinction ratio (ER) higher than 20 dB can be obtained by proper choosing of switching points for “ON” and “OFF” states. The ER discrepancy between TE and TM modes is smaller than 2.3 dB, and the polarization dependency is lower than 0.1 dB for “ON” state, which fulfills the requirement of polarization-independent modulation. Calculations show that the 3-dB modulation bandwidth higher than 100 GHz of our modulator is possible.

Calibration-free absolute frequency response measurement of directly modulated lasers based on additional modulation.

A calibration-free electrical method is proposed for measuring the absolute frequency response of directly modulated semiconductor lasers based on additional modulation. The method achieves the electrical domain measurement of the modulation index of directly modulated lasers without the need for correcting the responsivity fluctuation in the photodetection. Moreover, it doubles measuring frequency range by setting a specific frequency relationship between the direct and additional modulation. Both the absolute and relative frequency response of semiconductor lasers are experimentally measured from the electrical spectrum of the twice-modulated optical signal, and the measured results are compared to those obtained with conventional methods to check the consistency. The proposed method provides calibration-free and accurate measurement for high-speed semiconductor lasers with high-resolution electrical spectrum analysis.

Fiber chromatic dispersion measurement with improved measurement range based on chirped intensity modulation

A novel method for accurately measuring chromatic dispersion of optical fibers is proposed based on the use of chirped intensity-modulated signals. Unlike the conventional method, the proposed method utilizes the configurable transfer function of optical fibers caused by the residual chirp of intensity modulation, which not only eliminates the chirp error but also improves the measurement range through adjusting the chirp parameter of the intensity modulator. Our method is applicable for measuring both the magnitude and sign of chromatic dispersion of optical fibers or other dispersive devices at different operating wavelengths by using a vector network analyzer.