Xihua Zou

An Approach to the Measurement of Microwave Frequency Based on Optical Power Monitoring

A novel approach to the measurement of microwave frequency based on optical power monitoring is proposed and demonstrated. The microwave signal with its frequency to be measured is modulated on two optical carriers with their wavelengths set at one peak and one valley of the spectral response of a sinusoidal filter. The modulation is performed by a Mach-Zehnder modulator that is biased to suppress the optical carriers. A mathematical expression relating the optical powers from the two wavelength channels and the microwave frequency to be measured is developed. By simply monitoring the optical powers at the outputs of the two wavelength channels, the microwave frequency can be evaluated. A proof-of-concept experiment is implemented. Frequency measurement with good accuracy for microwave signals at different power levels is realized.

An Optical Approach to Microwave Frequency Measurement With Adjustable Measurement Range and Resolution

We propose an approach for the measurement of microwave frequency in the optical domain with adjustable measurement range and resolution. In the proposed approach, two optical wavelengths with a large wavelength spacing are modulated by an unknown microwave signal in a Mach-Zehnder modulator (MZM). The optical output from the MZM is sent to a dispersive fiber to introduce different chromatic dispersions, leading to different microwave power penalties. The two wavelengths are then separated, with the microwave powers measured by two photodetectors. A fixed relationship between the microwave power ratio and the microwave frequency is established. The microwave frequency is estimated by measuring the two microwave powers. The frequency measurement range and resolution can be adjusted by tuning the wavelength spacing. Different frequency measurement ranges and resolutions are demonstrated experimentally.

Analytical Models for Phase-Modulation-Based Microwave Photonic Systems With Phase Modulation to Intensity Modulation Conversion Using a Dispersive Device

Recently, optical phase modulation has been widely used in microwave photonics (MWP) systems, such as radio over fiber systems, photonic microwave filters, optical microwave and millimeter-wave signal generators, and optical subcarrier frequency up-converters. An optical phase-modulated signal can be converted to an intensity-modulated signal in a dispersive optical fiber. Due to the intrinsic nonlinearity of optical phase modulation, for linear applications such as microwave signal distribution and filtering, the modulation index should be kept small to minimize the unwanted modulation nonlinearity. However, for nonlinear applications such as microwave frequency multiplication and subcarrier frequency upconversion, the modulation index should be large to maximize the frequency multiplication and upconversion efficiency. In this paper, for the first time to our knowledge, we develop a thorough theoretical framework for the characterization of phase-modulation-based MWP systems, in which the phase modulation to intensity modulation conversion is realized using a dispersive fiber. Analytical models for the distributions of single-tone and two-tone microwave signals and for microwave frequency multiplication and subcarrier frequency upconversion are developed, which are verified by numerical simulations. The analytical models for single-tone and two-tone transmissions are further confirmed by experiments. The developed analytical models provide an accurate mathematical tool in designing phase-modulation-based MWP systems.

Microwave Frequency Measurement Based on Optical Power Monitoring Using a Complementary Optical Filter Pair

An approach to the measurement of a microwave frequency based on optical power monitoring using a complementary optical filter pair is proposed and investigated. In the proposed system, a microwave signal is applied to a Mach-Zehnder modulator, which is biased at the minimum transmission point to suppress the optical carrier. The carrier-suppressed optical signal is then sent to the complementary optical filter pair, with the powers from the complementary filters measured by two optical power meters. A mathematical expression that relates the microwave frequency and the optical powers is developed. Experiments are performed to verify the effectiveness of the proposed approach. The performance of the proposed system in terms of the frequency measurement range, operation stability, and robustness to noise is also investigated.

Photonic generation of triangular-shaped pulses based on frequency-to-time conversion.

An all-fiber approach to generate triangular-shaped pulses based on frequency-to-time conversion is proposed and demonstrated. Two filter modules that have sinusoidal spectral responses are cascaded to create a triangular-shaped optical spectrum. Through the frequency-to-time conversion in a dispersive fiber, periodic triangular pulses with the same shape as the optical spectrum are obtained. The repetition rate and pulse width of the generated signals can be tuned by adjusting the modulation rate and the dispersion value, respectively.

Instantaneous Microwave Frequency Measurement With Improved Measurement Range and Resolution Based on Simultaneous Phase Modulation and Intensity Modulation

A novel approach to implementing instantaneous microwave frequency measurement based on simultaneous optical phase modulation and intensity modulation with improved measurement range and resolution is proposed and experimentally demonstrated. The simultaneous optical phase modulation and intensity modulation are implemented using a polarization modulator (PolM) in conjunction with an optical polarizer. The phase- and intensity-modulated optical signals are then sent to a dispersive element, to introduce chromatic dispersions, which results in two complementary dispersion-induced power penalty functions. The ratio between the two power penalty functions has a unique relationship with the microwave frequency. Therefore, by measuring the microwave powers and calculating the power ratio, the microwave frequency can be estimated. Thanks to the complementary nature of the power penalty functions, a power ratio having a faster change rate versus the input frequency, i.e., a greater first-order derivative, is resulted, which ensures an improved measurement range and resolution. The proposed approach for microwave frequency measurement of a continuous-wave and a pulsed microwave signal is experimentally investigated. A frequency measurement range as large as 17 GHz with a measurement resolution of plusmn 0.2 GHz for a continuous-wave microwave signal and plusmn0.5 GHz for a pulsed microwave signal is achieved.

Photonic approach for multiple-frequency-component measurement using spectrally sliced incoherent source

A photonic approach to the instantaneous measurement of multiple frequency components using a spectrally sliced incoherent source (SSIS) is proposed. In the proposed system, a broadband incoherent source is spectrally sliced using an etalon to generate an SSIS. Each channel of the SSIS is externally modulated by a microwave signal containing multiple frequency components. The modulated SSIS is then sent to a second etalon. Thanks to the difference between the free spectral ranges of the two etalons, multiple frequency components are simultaneously estimated from the power distribution at the output channels of the second etalon. Compared with the use of a laser source array, the use of an SSIS provides a simpler way to perform multiple-frequency-component measurement.

Photonic-Assisted Microwave Channelizer With Improved Channel Characteristics Based on Spectrum-Controlled Stimulated Brillouin Scattering

A photonic-assisted microwave channelizer with improved channel characteristics based on spectrum-controlled stimulated Brillouin scattering (SBS) is proposed and experimentally demonstrated. In the proposed system, N lightwaves from a laser array are multiplexed and then split into two paths. In the upper path, the lightwaves are modulated by a microwave signal with its frequency to be measured. In the lower path, for each lightwave, the wavelength is shifted to a specific shorter wavelength via carrier-suppressed single-sideband modulation and the spectrum is then shaped. The wavelength-shifted and spectrum-shaped lightwaves are used to pump a single-mode fiber to trigger SBS. Thanks to the SBS effect, multiple gain channels at the N wavelengths are generated. The channel profile of each channel, determined by the designed spectral shape of the pump source, is improved with a flat top and a reduced shape factor. The characteristics including the bandwidth, channel spacing, and channel profile can be controlled by adjusting the spectral shape of the pump source. A proof-of-concept experiment is performed. A microwave channelizer with a shape factor less than 2, a tunable channel bandwidth of 40, 60, or 90 MHz, and a tunable channel spacing of 50, 70, or 80 MHz, is demonstrated.

Wideband Unpredictability-Enhanced Chaotic Semiconductor Lasers With Dual-Chaotic Optical Injections

The unpredictability degree and bandwidth properties of chaotic signals generated by semiconductor lasers subject to dual chaotic optical injections (DCOI) are investigated numerically. The unpredictability degree is evaluated quantitatively via permutation entropy. Compared with the slave laser (SL) subject to single chaotic optical injection, both the chaotic bandwidth and the unpredictability degree can be enhanced significantly for SL with DCOI. The effects of injection strength, frequency detuning as well as feedback strength are considered. It is shown that, with the increase of injection strength, the unpredictability degree of chaotic signals generated by SL increases firstly and then decreases until saturates at a constant level. Positive frequency detuning is preferred to achieve wideband unpredictability-enhanced chaos, and higher bandwidth and unpredictability degree can be further expected by adopting two differently detuned master lasers. The physical mechanisms behind the wideband unpredictability-enhanced chaos are also revealed. The wideband unpredictability-enhanced chaotic signals generated by SL with DCOI are extremely useful for high speed random number generators, as well as for high capacity security-enhanced chaotic communications.

All-fiber optical filter with an ultranarrow and rectangular spectral response

Optical filters with an ultranarrow and rectangular spectral response are highly desired for high-resolution optical/electrical signal processing. An all-fiber optical filter based on a fiber Bragg grating with a large number of phase shifts is designed and fabricated. The measured spectral response shows a 3 dB bandwidth of 650 MHz and a rectangular shape factor of 0.513 at the 25 dB bandwidth. This is the narrowest rectangular bandpass response ever reported for an all-fiber filter, to the best of our knowledge. The filter has also the intrinsic advantages of an all-fiber implementation.