Xiaoqiong Qi

Photonic Microwave Applications of the Dynamics of Semiconductor Lasers

In this paper, major photonic microwave generation and modulation methods are described. Their advantages and limitations are identified and compared through the analysis of the key characteristics relevant to practical photonic microwave applications. The focus of this paper is on the photonic microwave based on the nonlinear dynamics of optically injected semiconductor lasers. Both single-beam optically injected semiconductor lasers and dual-beam optically injected semiconductor lasers are considered. In particular, the three dynamic states of stable injection locking, period-one oscillation, and period-two oscillation are considered and demonstrated for photonic microwave generation, modulation, and conversion applications. Several prospects for future research and development in this area are discussed.

Fiber Dispersion and Nonlinearity Influences on Transmissions of AM and FM Data Modulation Signals in Radio-Over-Fiber System

Transmission properties of data amplitude modulation (AM) and frequency modulation (FM) in radio-over-fiber (RoF) system are studied numerically. The influences of fiber dispersion and nonlinearity on different microwave modulation schemes, including double side band (DSB), single side band (SSB) and optical carrier suppression (OCS), are investigated and compared. The power penalties at the base station (BS) and the eye opening penalties of the recovered data at the end users are both calculated and analyzed. Numerical simulation results reveal that the power penalty of FM can be drastically decreased due to the larger modulation depth it can achieve than that of AM. The local spectrum broadening around subcarrier microwave frequency of AM due to fiber nonlinearity can also be eliminated with FM. It is demonstrated for the first time that the eye openings of the FM recovered data can be controlled by its modulation depths and the coding formats. Negative voltage encoding format was used to further decrease the RF frequency thus increase the fluctuation period considering their inverse relationship.

Dynamics Scenarios of Dual-Beam Optically Injected Semiconductor Lasers

The nonlinear dynamics of a dual-beam optically injected semiconductor laser are studied. Most of the observed dual-beam injection dynamics can be understood from competitions of single-beam injection dynamics that are associated separately with each injection beam. Three general scenarios are identified and mapped for various combinations of detuning frequencies and injection strengths of the two injection beams. In Scenario A, simultaneous injection of two beams completely suppresses the dynamics due to separate single-beam injection and replaces them with the dynamics resulting from strong nonlinear interactions of the two injection beams. In Scenario B, simultaneous injection of two beams does not suppress the dynamics due to single-beam injection of one of the two beams but modifies such dynamics through nearly degenerate four-wave mixing with the other injection beam associated with the suppressed dynamics. In Scenario C, simultaneous injection of two beams does not suppress the dynamics due to single-beam injection of either of the two beams but allows the two dynamics to coexist and mix nonlinearly.

Tunable Carrier Generation and Broadband Data Upconversion for RoF Systems Based on Stimulated Brillouin Scattering

We present a frequency-tunable RF carrier generation and broadband data upconversion technique for radio-over-fiber (RoF) systems. A dual-parallel Mach-Zehnder modulator is used to realize single-mode modulation (SMM). On the other hand, frequency-tunable RF carrier generation with single-sideband (SSB) modulation is performed using stimulated Brillouin scattering (SBS) with three types of configuration. The optical carrier-to-sideband ratio of the SMM-SSB modulated signal can be adjusted to achieve the best received sensitivity performance of the RoF system by simply modifying the pump power in the SBS process. Finally, the transmission performance of the RoF downlink system is examined. The power penalty is less than 1 dB at the bit-error rate of 10-9 after 25-km single-mode fiber transmission.

Novel Method for Frequency Response Measurement of Optoelectronic Devices

We develop a novel method to measure the frequency response of the optoelectronic devices. In this method, an electro-absorption modulator is utilized not only as a modulator but also as a photodetector with the identical frequency response. Based on this property, the frequency responses of the optoelectronic devices can be obtained and the results show good consistency with the measurement results by traditional methods. Instead of using broadband modulators or photodetectors, the frequency response of high bandwidth optoelectronic devices can be measured by using the low bandwidth optoelectronic devices.

True-time delay line with separate carrier tuning using dual-parallel MZM and stimulated Brillouin scattering-induced slow light

We experimentally demonstrate a novel tunable true-time delay line with separate carrier tuning using dual-parallel Mach-Zehnder modulator and stimulated Brillouin scattering-induced slow light. The phase of the optical carrier can be continuously and precisely controlled by simply adjusting the dc bias of the dual-parallel Mach-Zehnder modulator. In addition, both the slow light and single-sideband modulation can be simultaneously achieved in the stimulated Brillouin scattering process with three types of configuration. Finally, the true-time delay technique is clearly verified by a two-tap incoherent microwave photonic filter as the free spectral range of the filter is changed.

A QEPAS-Based Central Wavelength Stabilized Diode Laser for Gas Sensing

A quartz-enhanced photoacoustic spectroscopy-based central wavelength stabilized distributed feedback diode laser is demonstrated in this letter. A quartz tuning fork is used as a sharp transducer to generate an error signal for wavelength tuning and stabilization without photodetector, which contributes to low cost, compact size, and freedom from ambient acoustic noises. The central wavelength of the diode laser obtains quickly fine-tuning at various modulation depths because it varies in time with the driving current. The wavelength stability of the diode laser is thus enhanced effectively. The wavelength stabilized diode laser is suited for absorption spectroscopy-based gas sensors.

Comparison of T-matrix calculation methods for scattering by cylinders in optical tweezers

The T-matrix method, or the T-matrix formulation of scattering, is a framework for mathematically describing the scattering properties of an object as a linear relationship between expansion coefficients of the incident and scattering fields in a basis of vector spherical wave functions (VSWFs). A variety of methods can be used to calculate the T-matrix. We explore the applicability of the extended boundary condition method (EBCM) and point matching (PM) method to calculate the T-matrix for scattering by cylinders in optical tweezers and hence the optical force acting on them. We compare both methods with the discrete-dipole approximation (DDA) to measure their accuracy for different sizes and aspect ratios (ARs) for Rayleigh and wavelength-size cylinders. We determine range of sizes and ARs giving errors below 1% and 10%. These results can help researchers choose the most efficient method to calculate the T-matrix for nonspherical particles with acceptable accuracy.

Dynamics Maps and Scenario Transitions for a Semiconductor Laser Subject to Dual-Beam Optical Injection

Compared with single-beam injection, dual-beam optical injection increases the complexity and enriches the nonlinear dynamics of a semiconductor laser. The nonlinear dynamics of a dual-beam optically injected semiconductor laser system under various operating conditions are numerically studied by comprehensive comparisons of the output spectra of single-beam injection and dual-beam injection. Complete mapping of the different scenarios induced by dual-beam optical injection is shown for various combinations of detuning frequencies and injection strengths of the two injection beams. Complex dual-beam injection dynamics are mapped, and the dynamics in different scenarios are linked through transitional regions. Characteristics of the transition from one scenario to another are traced and analyzed by comparing the resonance frequency of each dual-beam injection optical spectrum with that of the dominant single-beam injection optical spectrum. Two different types of chaos, each following a period-doubling route, are identified in two different scenarios.

Frequency-Pushing Effect in Single-Mode Diode Laser Subject to External Dual-Beam Injection

The frequency-pushing effect in single-mode diode lasers subject to single-beam injection and dual-beam injection is studied experimentally. The physical process of the frequency-pushing effect has been analyzed in detail using a medium-gain model in a single-beam injection system. A ¿pushed to lock¿ phenomenon has been observed in dual-beam injection system and has also been analyzed using the medium-gain model. This model can also be used to explain the maximum of conversion efficiency in an injection-locked frequency convertor. First, we produce a conventional mapping of nonlinear dynamic regions, including stable locking, period doubling, chaos, undamped relaxation oscillations, and bistability, as functions of the detuning frequency and injection ratio in single-beam injection system. Then, a pushed stable locking range is observed when two masters are injected simultaneously. The direction of pushing is towards negative detuning frequencies and low injection ratios, which allows a larger detuning frequency and lower injection ratio in choosing the lasers used as pump source in injection-locked frequency convertor.