Songnian Fu

Ultrawideband monocycle generation using cross-phase modulation in a semiconductor optical amplifier

We propose a novel scheme to generate ultrawideband (UWB) monocycle pulses based on cross-phase modulation (XPM) of a semiconductor optical amplifier (SOA). The proposed system consists of a SOA and an optical bandpass filter (OBF). Due to the XPM, a continuous wave (CW) probe signal is phase modulated by another optical Gauss pulse in the SOA. The OBF will convert the phase modulation to intensity modulation. A pair of polarity-reversed UWB monocycle pulses is achieved by locating the probe carrier at the positive and negative linear slopes of the OBF. Both cases conform to the UWB definition of the Federal Communications Commission.

Performance comparison of different modulation formats over free-space optical (FSO) turbulence links with space diversity reception technique

The transmission performance of a free-space optical (FSO) link could be severely degraded due to atmospheric turbulence, which causes the temporal and spatial fluctuation of light intensity. Both the space diversity reception technique (SDRT) and advanced modulation formats can successfully mitigate the transmission impairments of the atmospheric turbulence. The purpose of this paper is to study and compare the bit-error-rate (BER) performance of several widely used modulation formats under different atmospheric turbulence scenarios with and without SDRT. The modulation formats studied in this paper include on-off keying (OOK), differential phase-shift keying (DPSK), and differential quadrature phase-shift keying (DQPSK). We derive a series-form formula for evaluating the BER performance of the DPSK format in the Gamma-Gamma distributed channel with SDRT. We use both theoretical analysis and simulation to examine the BER performance of OOK, DPSK, and DQPSK formats with and without SDRT. It is found that, in the strongly turbulent scenario, the OOK and DPSK formats can have as large as 19.5 and 20.3 dB of SDRT gains at the BER of 10-3, respectively. Using SDRT, the modulation gains of the DPSK format over the OOK format are 3.2 dB in the strongly turbulent scenario and 4.5 dB in the weakly turbulent scenario, respectively. In addition, in the moderately and strongly turbulent scenarios, it is found that the DPSK and DQPSK formats have almost the same BER performance under the same symbol rate.

Ultrafast All-Optical Signal Processing Based on Single Semiconductor Optical Amplifier and Optical Filtering

We propose and demonstrate several all-optical signal-processing technologies, including wavelength conversion (WC), leading/trailing edge detection, and logic gates based on a semiconductor optical amplifier (SOA) and a detuning optical bandpass filter (OBF). The system is based on pump-probe scheme. When the pump signal is return-to-zero (RZ) format, both inverted WC and noninverted WC at 40 Gb/s are obtained, and the polarity variation is highly dependent on the OBF detuning. When the pump signal is nonreturn-to-zero (NRZ) format, both the inverted WC and leading/trailing edge detection are obtained. In the inverted WC, the OBF detuning is small, and it helps to accelerate the amplitude recovery. However, in the noninverted WC and pulse edge detection, the OBF detuning is relatively large, and it acts as frequency-amplitude conversion. Finally, we present all- optical reconfigurable logic gates based on various nonlinearities in an SOA. The logic gates including AND, NOR/NOT, and OR are obtained by four-wave mixing, cross-gain modulation, and transient cross-phase modulation in the SOA, respectively. The NOR and OR gates are capable of operating at 40 Gb/s with the help of the OBF detuning. The XNOR gate is implemented by combining the AND output and NOR output.

High-energy laser pulse with a submegahertz repetition rate from a passively mode-locked fiber laser

We demonstrate an ultralong cavity, all-fiber, all-normal-dispersion Yb-doped fiber laser that is passively mode locked by a semiconductor saturable absorber mirror (SESAM). Without any discrete dispersion-compensation components or conventional spectral filters, the SESAM works together with the strongly chirped pulse and the nonlinearity induced spectrum broadening to perform a filtering-equivalent function, thus stabilizing the mode locking. The laser generates 4.3 nJ stable mode-locked pulses with a 397 kHz fundamental repetition rate at a 1068 nm central wavelength.

Instantaneous Microwave Frequency Measurement Using Photonic Technique

We propose a novel photonic technique for measuring microwave frequency instantaneously over a wide bandwidth. In our approach, an optical carrier is divided into two parts. Both parts are modulated by an unknown microwave signal; one part is phase modulated while the other one is intensity modulated. The two differently modulated optical signals are then launched into single-mode fibers with the same lengths to introduce microwave signal power fading. After photodetection, the radio-frequency powers of the two parts are used to generate an amplitude comparison function which provides a frequency-to-power mapping. The proposed scheme is simple and is experimentally verified over a frequency range of 13.5 GHz with a measurement error less than plusmn0.3 GHz.

Photonic measurement of microwave frequency based on phase modulation

A photonic approach for microwave frequency measurement is proposed. In this approach, an optical carrier is modulated by an unknown microwave signal through a phase modulator. The modulated optical signal is then split into two parts; one part passes through a spool of polarization maintaining fiber (PMF) and the other one, through a dispersion compensation fiber (DCF), to introduce different microwave power penalties. After the microwave powers of the two parts are measured by two photodetectors, a fixed frequency-to-power mapping is established by obtaining an amplitude comparison function (ACF). A proof-of-concept experiment demonstrates frequency measurement over a range of 10.5 GHz, with measurement error less than +/-0.07 GHz.

Photonic ultrawideband monocycle pulse generation using a single electro-optic modulator

A compact approach to photonic-assisted ultrawideband (UWB) monocycle pulse generation is proposed and experimentally demonstrated based on the wavelength dependence of the half-wave voltage of a Mach-Zehnder modulator (MZM). By employing a single MZM with dual-wavelength injection at around 1310 and 1550 nm, a pair of polarity-reversed monocycle pulses with the full width at half-maximum of about 80 ps and the fractional bandwidth of greater than 160% can be generated. The experiment results agree well with the theoretical prediction. The simple setup and the convenience to control the monocycle pulse polarity are favorable for future applications.

Photonic-assisted microwave frequency measurement with higher resolution and tunable range

A photonic-assisted approach to microwave frequency measurement is proposed based on frequency-to-power mapping with the help of the so-called amplitude comparison function. The key component is a dual-output Mach-Zehnder modulator (MZM) working at chirped modulation. The proposed scheme is characterized as having simplicity, higher resolution, and tunable measurement range. Owing to experimental constraint, an equivalent experiment has been carried out using a common single-output MZM at different bias points to prove the concept.

40 Gb/s all-optical NRZ to RZ format conversion using single SOA assisted by optical bandpass filter

We propose a novel all-optical format conversion from nonreturn-to-zero (NRZ) to return-to-zero (RZ) based on single semiconductor optical amplifier (SOA) and a detuning optical bandpass filter (OBF). By adopting the ultrafast SOA model associated with intraband mechanism, the 40Gb/s NRZ-to-RZ format conversion is successfully demonstrated with numerical simulation. We investigate the influence of the filter detuning, extinction ratio, conversion efficiency, output pulsewidth, and filter bandwidth. These simulation results confirm the feasibility of our scheme. The proposed scheme is robust and potential for applications in future optical networks.

Nonlinear Polarization Rotation in Semiconductor Optical Amplifiers With Linear Polarization Maintenance

We propose a geometrical model based on the concept of dynamic eigenstates of polarization to describe the behavior of nonlinear polarization rotation (NPR) arising in semiconductor optical amplifiers (SOAs). The rotation axis with respect to either the bias current or the optical power variation is demonstrated on the Poincare sphere (PS), meanwhile a procedure to find the rotation axis is presented. Thus, the SOA-based NPR with linear polarization maintenance (zero polarization ellipticity angle) can be achieved experimentally. The rotation of polarization azimuth on the PS with respect to the bias current, the probe signal power, and the pump signal power variation is measured experimentally. The 180deg phase difference between the transverse electric and the transverse magnetic modes can be all achieved with linear polarization maintenance.