Some experiments of optical performance monitoring (OPM) with uniform or nonuniform
quasi-phase matching (QPM) gratings waveguides have already been demonstrated during the last few
years. Theoretical analyses based on coupled-wave equations are carried out for nonuniform QPM
gratings. The generation process of SHG in nonuniform QPM gratings is simulated and the outcome
comparison between uniform and nonuniform QPM gratings, which demonstrate that nonuniform QPM
gratings are more suitable for the OPM systems, is carried out in this paper. The value of grating chirp
coefficient of the chirped PPLN has influence on the efficiency and band width of the output pulse. The
relation between them is also discussed. These are new simulative attemptation to apply the
nonuniform quasi-phase matching gratings into optical performance monitoring system.
Progress in optical network has stimulated interest in optical performance monitoring (OPM). Optical sampling
technique is believed to be a promising candidate to monitor the physical state of the network. The theoretical model of a
monitoring system based on optical sampling in semiconductor optical amplifier (SOA) and software synchronized
algorithm is constructed. Compared with the results obtained by Optsim, the monitoring system model is proved. For
10Gb/s NRZ (RZ) optical signals, the differences on Q values between the results obtained by SOA and the ideal
sampling processes are 0.195dB (0.247dB), 0.988dB (0.594dB), and 1.707dB (0.596dB) for pump energy equal to, ten
times of and fifty times of the optical data signal energy respectively. The sampling device can induce degradations of
the sampling results. It is mainly because of the gain saturation and the nonlinear effects in SOA. The high input power
can make the gain saturated deeply and further influence the probe and conjugate outputs. At the same time, the pump
power should not be too low. The proper pump power will obtain better sampling linearity and better sampling results.
A theoretical model for description of the cSFG/DFG is developed in this paper. The factors influence the poled period of
PPLN is studied and the result shows that the poled period decreases while the temperature increases, and the poled
period also changes while we choose different signal wavelengths. In our simulation, a pulsed light is injected into a
PPLN waveguide as the signal, and then the performances of the wavelength conversion structures with two continuous
waves and two pulsed lights are compared in detail by numerical simulation. The output of these two situations are
educed while depletion, walk-off and nonlinear effect are all considered. The walk-off effects of output and conversion
efficiency are studied in both cases. The results demonstrate that there is an obvious walk-off between input signal and
output in the pulse pumped case, and in the CW case the converted wave width is boarder than that of input signal due to
pulse dispersion. Factors that influence the conversion efficiency are also analyzed including the power of the pump light,
the length of the PPLN waveguide and the experiment structure.
Many experiments of optical sampling based on the second-order optical nonlinear interactions in periodically poled
LiNbO<sub>3</sub> (PPLN) quasi-phase-matched (QPM) waveguides have already been demonstrated during the last few years.
There are several processes in PPLN that may be used for sampling optical signals, including sum-frequency generation
(SFG), difference-frequency generation (DFG), cascaded second harmonic generation and difference-frequency
generation (cSHG/DFG), and cascaded sum-frequency generation and difference-frequency generation (cSFG/DFG).
The comparisons are carried out, which are necessary for designing the optical sampling system that satisfies the
requirements of optical performance monitoring (OPM) of the optical networks. The SFG and cSFG/DFG based
conversions are much more suitable for constructing OPM systems. The SFG of either two pulsed waves or one pulsed
wave and one continuous wave is the basic nonlinear process for sampling optical signals using PPLN waveguide.
Theoretical analyses based on coupled-wave equations are carried out for the pulsed-SFG process.
A stokes-vector theory is introduced to simulate the azimuth and ellipticity trajectories of the both probe and conjugate
pulses in a four-wave mixing (FWM) scheme. Based on a new polarization-dependent pulsed FWM model which is
derived from the three-level system, the theoretical analyses are carried out for the optical sampling of picosecond
optical pulses in a lattice-matched unstrained semiconductor optical amplifier (SOA) by use of strong ultrashort pump
pulses. We take the TE- and TM-gain dependent effects into account. The polarization characteristics of the pulses
involved in optical sampling process are analyzed in detail.
Based on a three-band model, a polarization-dependent pulsed
four-wave mixing (FWM) model which can be used to
analyze the optical sampling process in semiconductor optical amplifier (SOA) is presented. The polarization-dependent
characteristics and cross-polarization modulation (XPolM) of pump, probe and conjugate pulses are investigated in detail.
The maximum sampling efficiency occurs, when pump and probe pulses are linearly co-polarized and parallel to the TE
axis. When the pump, probe pulses with initial linear polarization states interacting in an SOA, their polarization states
do not just rotate and the conjugate pulse is not just linearly polarized but with complicated polarization states during the
propagation along the length of SOA.
Based on a three-band model, a new theoretical model is derived to describe the self-polarization modulation (SPolM)
effect in semiconductor optical amplifier (SOA) on femtosecond time-scale. In the case of SPolM, the phase difference
between the TE and the TM modes is created by the signal itself and results in changes of polarization states of the signal.
Numerical experiments are carried out to analyze the gain and the signal induced phase shift when an ultrashort optical
pulse injected into a tensile strained SOA. The higher injected power can induce a larger phase shift. The
polarization-dependent effect has much more influences on strong optical pulses.