Junjun Li

Drop filters in a rod-type photonic crystal based on self-collimation ring resonators

We design a rod-type drop filter (RTDF) in a two-dimensional photonic crystal (2D PhC) employing self-collimation (SC) effect. The perfect 2D PhC consists of a square-lattice of cylindrical silicon rods in air. The dielectric constant and the radius of host rods are ε=12.25 (correspondingly the refractive index n = 3.5) and r=0.40a respectively, where a is the lattice constant. In such a PhC, self-collimation phenomenon occurs for transverse-magnetic (TM) light beams with frequencies between 0.176c/a and 0.192c/a. The proposed RTDF based on a self-collimation ring resonator (SCRR) consists of two beam splitters and two mirrors. The performances of the SCRR are investigated with the finite-difference time-domain (FDTD) simulation technique. The calculation results show that the transmissivity spectrum at the drop port has nearly equal peak spacing which will decreases when the geometrical length of the SCRR is increased. Moreover, the full width at half maximum (FWHM) and thus quality (Q) factor of peaks can be easily tuned by changing the reflectivity of two beam splitters.

Tunable drop filters based on photonic crystal self-collimation ring resonators

A tunable drop filter (TDF) based on two-dimensional photonic crystal (PC) self-collimation ring resonator (SCRR) is proposed. The PC consisting of square-lattice air cylinders in silicon has square-shaped equal frequency contours (EFCs) in the wavevector space at the frequencies between 0.172-0.188c/a for TE modes. The SCRR includes two mirrors and two splitters. The air holes inside the SCRR are infiltrated with a kind of liquid crystal whose ordinary and extraordinary refractive indices are 1.522 and 1.706, respectively. The effective refractive index neff of liquid crystal depends on the applied electric field. Simulated with the FDTD method, the transmission spectra at the drop port of SCRR are in the shape of sinusoidal curves with uniform peak spacing between 0.172-0.188c/a. Transmission peaks will shift to the lower frequencies when neff is increased. When neff changes from 1.522 to 1.706, the peaks will experience red-shift over 0.003c/a. So this SCRR can work as a tunable drop filter. For the operating wavelength around 1550nm, its dimensions are only tens of microns.

1×3 optical drop splitter in a rod-type silicon photonic crystal

We report an 1×3 optical drop splitter (ODS) based on a self-collimation ring resonator (SCRR) in a rod-type silicon photonic crystal. The proposed 1×3 ODS consists of four beam splitters which are formed by changing the radius of one row of silicon rods. When the self-collimated light with resonance frequency is launched into the ODS, the light beam can be split into three parts come out from three drop ports while no light coming out from the through port. The splitting ratio of the three drop beams can be controlled by tuning the radii of the beam splitters. The FDTD method is employed to calculate the transmission of the 1×3 ODS. For the drop wavelength of 1550 nm, the free spectral range is 28.7 nm, which almost covers the whole optical communication C-band window. This 1×3 ODS may have applications in photonic integrated circuits.

Drop and split filter in a hole-type photonic crystal

A drop and split filter (DSF) based on a self-collimation ring resonator (SCRR) in a hole-type photonic crystal is proposed and demonstrated. The DSF performance was investigated with the finite-difference time-domain (FDTD) simulation technique. By forming three splitters of the SCRR properly, self-collimated light can come out from two drop ports equally while no light come out from the through port. For the drop wavelength of 1550 nm, the free spectral range is 29.7nm, which covers the whole optical communication C-band window. This DSF may have applications in photonic integrated circuits.

Ultracompact channel filters based on race-track photonic crystal ring resonators

We proposed a new channel drop filter (CDF) based on race-track photonic crystal ring resonator (PCRR) composed of square-lattice cylindrical silicon rods in air. Two representative scenarios, parallel and perpendicular, related to the direction of race track and bus channel waveguide, are comparably studied by using 2D finite-difference time-domain technique. A good set of parameters with adequate modal spacing can be determined by adjusting the amount and size of scatterers. By optimizing the relationship among surrounding periods, race-track ring size and coupling strength, more than 150 spectral quality factor and 93.9% dropped efficiency can be achieved at 1370-nm channel for one single race-track PCRR. These findings enhance and enrich the PCRRs as an alternative to current micro-ring resonators for ultra-compact WDM components and high density photonic integration.

A novel design of dispersion and dispersion slope compensator for LEAF fiber in WDM systems

Dispersion and dispersion slope management is a problem for LEAF fiber, because LEAF fiber has a low dispersion but a significantly high relative dispersion slope. This article demonstrates a design of dispersion and dispersion slope compensator for LEAF fiber in WDM systems. It can compensate dispersion and dispersion slope respectively by combining broadband nonchannelized linear chirped fiber Bragg grating and a pair of nonlinear chirped fiber Bragg gratings, which can be inversely cascaded in a four-port optical circulator. The design can effectively improve dispersion linearity and increase dispersion matching ability of compensator, and a numerical experiment of compensating a 50-kmlong LEAF fiber in WDM system proves the feasibility of this approach.

Fabry-perot interferometer in a rod-type photonic crystal based on self-collimation

Low-loss Fabry-Perot interferometer consisting of two reflectors in a rod-type photonic crystal based-on the self-collimation effect is proposed and investigated. Free spectral range and peaks frequencies of its transmission decrease when the resonant cavity length increases. By raising the reflector reflectivity, the quality factor of peaks can be easily improved.

Folded Mach-Zehnder interferometer based on photonic crystal self-collimation effect

A Folded Mach-Zehnder interferometer (FMZI) in a two-dimensional photonic crystal is proposed. The FMZI consists of one splitter and several mirrors. Light propagates between them employing self-collimation effect. Its two interfering branches have different path lengths. The two complementary transmission spectra at two FMZI output ports are both in the shape of sinusoidal curves and have a uniform peak spacing in the frequency range from 0.255c/a to 0.270c/a. The peak spacing becomes smaller when the length difference between the two branches is increased. As self-collimation light beams can cross each other without coupling, this FMZI is much smaller than non-folded interference-type filters in photonic crystals. This FMZI may work as a wavelength division demultiplexer in high-density photonic integrated circuits.