Rongqing Hui

Subcarrier multiplexing for high-speed optical transmission

The performance of high-speed digital fiber-optic transmission using subcarrier multiplexing (SCM) is investigated both analytically and numerically. In order to reduce the impact of fiber chromatic dispersion and increase bandwidth efficiency, optical single-sideband (OSSB) modulation was used. Because frequency spacing between adjacent subcarriers can be much narrower than in a conventional DWDM system, nonlinear crosstalk must be considered. Although chromatic dispersion is not a limiting factor in SCM systems because the data rate at each subcarrier is low, polarization mode dispersion (PMD) has a big impact on the system performance if radiofrequency (RE) phase detection is used in the receiver. In order to optimize the system performance, tradeoffs must be made between data rate per subcarrier, levels of modulation, channel spacing between subcarriers, optical power, and modulation indexes. A 10-Gb/s SCM test bed has been set up in which 4 /spl times/ 2.5 Gb/s data streams are combined into one wavelength that occupies a 20-GHz optical bandwidth. OSSB modulation is used in the experiment. The measured results agree well with the analytical prediction.

Intensity-dependent phase-matching effects on four-wave mixing in optical fibers

A new phase-matching factor is derived for four-wave mixing (FWM) that includes the effects of self-phase and cross-phase modulation in optical fibers. Theoretical results predict that the wavelength of peak FWM efficiency shifts away from the fiber zero-dispersion wavelength and indicate that the conventional phase-matching factor may induce significant errors in FWM calculations. Experiments are presented to verify the new phase-matching factor and the related theoretical results. The measured results agree well with those predicted by the new phase-matching factor.

Cross-phase modulation in multispan WDM optical fiber systems

The spectral characteristics of cross-phase modulation (XPM) in multispan intensity-modulation direct-detection (IM-DD) optical systems are investigated, both experimentally and theoretically. XPM crosstalk levels and its spectral features are found to be strongly dependent on fiber dispersion and optical signal channel spacing. Interference between XPM-induced crosstalk effects created in different amplified fiber spans is also found to be important to determine the overall frequency response of XPM crosstalk effects. XPM crosstalk between channels with different data rates is evaluated. The crosstalk level between higher and lower bit rate channels is found to be similar to that between two lower bit rate channels. The effect of dispersion compensation on XPM crosstalk in multispan optical systems is discussed and per span dispersion compensation was found to be the most effective way to minimize the effect of XPM crosstalk.

Injection locking in distributed feedback semiconductor lasers

Injection locking properties of distributed feedback semiconductor lasers are studied systematically. Due to the high side mode suppression, these devices show different locking properties when compared to lasers with Fabry-Perot structures. The main result is the identification of four regimes for different injection levels. In particular, a symmetrical locking band at low optical injection level is confirmed. The presence of this symmetrical band can be exploited in some applications. As examples, the measurement of the linewidth enhancement factor and the phase-shift-keying modulation capability are reported. >

An adaptive first-order polarization-mode dispersion compensation system aided by polarization scrambling: theory and demonstration

An adaptive polarization-mode dispersion (PMD) compensation system has been developed to cancel the effects of first-order PMD by producing a complementary PMD vector in the receiver. Control parameters for the PMD compensation system comprised of a polarization controller and a PMD emulator are derived from the nonreturn-to-zero (NRZ) signal in the channel to be compensated. Estimates of the links differential group delay (DGD) and principal states of polarization (PSPs) based on this signal are reliable when the signal power is equally split between the links two PSPs; however this condition cannot be assumed. To meet this requirement, we scramble the state of polarization (SOP) of the input signal at a rate much greater than the response time of the PMD monitor signal so that each sample represents many different SOP alignments. This approach allows the effective cancellation of the first-order PMD effects within an optical fiber channel.

Modulation instability and its impact in multispan optical amplified IMDD systems: theory and experiments

The nonlinear interaction between amplified spontaneous emission noise and copropagating signal in a dispersive optical fiber is investigated both theoretically and experimentally. A transfer matrix formulation is used to solve the nonlinear propagation equation. A continuous wave (CW) input optical signal format isolates the effect of modulation instability (MI) from the signal self-phase modulation. The change of the relative intensity noise (RIN) spectrum at an optical receiver caused by MI has been analyzed extensively in multispan optical amplified systems, and thus the impact of MI in the performance of intensity modulation and direct detection (IMDD) systems is evaluated. Performance of systems with distributed dispersion compensation (DC) and a lumped DC at the receiver are compared in terms of the effect of MI, the results show that the highest efficiency of DC to reduce the effect of MI can be achieved by putting a lumped DC module at the receiver side. Excellent agreement between theory and experiment demonstrates a good understanding of the mechanism of MI and its impact in the performance of terrestrial optical transmission systems.

GaN-based waveguide devices for long-wavelength optical communications

Refractive indices of AlxGa1−xN with different Al concentrations have been measured in infrared wavelength regions. Single-mode ridged optical waveguide devices using GaN/AlGaN heterostructures have been designed, fabricated, and characterized for operation in 1550 nm wavelength window. The feasibility of developing photonic integrated circuits based on III-nitride wide-band-gap semiconductors for fiber-optical communications has been discussed.

Two-photon microscopy with wavelength switchable fiber laser excitation

Two-photon scanning fluorescence microscopy has become a powerful tool for imaging living cells and tissues. Most applications of two-photon microscopy employ a Ti:sapphire laser excitation source, which is not readily portable or rapidly tunable. This work explores the use of two-photon fiber laser excitation (TP-FLEX) as an excitation source for scanning two-photon microscopy. We have further demonstrated the use of a photonic crystal fiber (PCF) for facile tuning of the excitation wavelength over the range from 810 nm to 1100 nm. We generated two-photon fluorescence images at excitation wavelengths from 850 nm to 1100 nm detected on a scanning-stage microscope. By PCF wavelength tuning the dye BODIPY fl was selectively excited at 1000 nm whereas MitoTracker red was excited preferentially at 1100 nm. We discuss the potential for fiber laser sources coupled with PCF wavelength tuning as an attractive tunable excitation source for two-photon scanning fluorescence microscopy.

Phase noise of four‐wave mixing in semiconductor lasers

A simple theoretical analysis shows that the linewidth of the conjugate wave produced in four‐wave mixing in semiconductor lasers is equal to the linewidth of the probe plus four times the linewidth of the pump. Experimental results in good agreement with the theory are presented. This result implies an enormous enhancement in the phase noise of the conjugate wave and sets a limitation on some practical applications of four‐wave mixing.

Fiber-optic acoustic pressure sensor based on large-area nanolayer silver diaghragm

A fiber-optic acoustic pressure sensor based on a large-area nanolayer silver diaphragm is demonstrated with a high dynamic pressure sensitivity of 160  nm/Pa at 4 kHz frequency. The sensor exhibits a noise limited detectable pressure level of 14.5  μPa/Hz(1/2). Its high dynamic pressure sensitivity and simple fabrication process make it an attractive tool for acoustic sensing and photo-acoustic spectroscopy.