Xiao-shi Qian

Tunable one-way cross-waveguide splitter based on gyromagnetic photonic crystal

We report our design of a tunable one-way cross-waveguide splitter, resulting from the broken time-reversal symmetry, based on the edge modes of gyromagnetic photonic crystal. Mode control is accomplished by altering the radius or refractive index of a single central electro-optical rod. The on-off switch of the channels can be manipulated by external electric or magnetic field. Coupled-mode theory between defect modes and waveguide modes was used to characterize the transmission efficiency of the channels.

Acousto-optic interaction in photonic crystals with defects

The acousto-optic (AO) effects of photonic crystals (PCs) were studied. Both the PCs’ periodicity and their index distribution could be modulated instantly by the propagating acoustic wave. As a consequence, the PCs’ band structure becomes tunable. In addition to band gap shift in an ideal PC, AO frequency modulation was observed in a PC with single defect, which is quite different from normal AO tunable filters and gives rise to some interesting applications. Furthermore, in dual-defect situation, synchronized and desynchronized modulations were realized at different acoustic wavelengths. Interesting phenomena such as dual frequency sweeping and dual frequency Q-switching were demonstrated.

A three-beam path photonic crystal fiber modal interferometer and its sensing applications

Photonic crystal fiber (PCF) modal interferometer is studied for sensing applications. There is a trade-off between high sensitivity and wide sensing range due to the intrinsic two-beam interference. We propose and experimentally demonstrate a three-beam path reflective interferometer by exciting mode conversion at the end surface of a dual-mode PCF. Approximately 5% mode conversion is taken place at the PCF end surface according to our model. In comparison with the normal dual-mode interference in a PCF, the mode conversion induced three-beam interference generates sharper spectrum and expanded sensing range. Gas sensing experiment is implemented with a sensitivity of 3019 nm/RIU. The advantage of expanded sensing range is also experimentally demonstrated.

A bidirectional tunable optical diode based on periodically poled LiNbO 3

We propose a bidirectional tunable optical diode based on a periodically poled lithium niobate (PPLN) with defect. An acoustic wave propagates together with the light beam so that a collinear photon-phonon interaction happens, which affects the nonlinear optical processes in PPLN. The fundamental wave exhibits an optical diode effect, i.e., the light only may travel toward a single direction while the opposite way is isolated. However, the acoustic wave could be used to adjust the contrast of optical isolation from -1 to 1. A direction-optional operation is thus realized. Moreover, the advantages of our tunable PPLN optical diode are also discussed.

Left-handed and right-handed one-way edge modes in a gyromagnetic photonic crystal

We theoretically reveal that a two-dimensional magneto-optical photonic crystal shows two pairs of one-way edge Dirac cones, whereby only clockwise and anticlockwise circle wave scattering (facing the direction of external applied magnetic field) are allowed in the second and third band-gaps, respectively. These two different chiral pathways originate from the different exchange of Chern numbers, which are attributed to the broken time-reversal symmetry. Moreover, below and above the edge Dirac point, the edge modes show the left-handed (counter propagation of group velocity and phase velocity) and right-handed characters, respectively.

Tunable broadband isolator based on electro-optically induced linear gratings in a nonlinear photonic crystal

We theoretically propose a broadband optical isolator based on second-harmonic generation in a one-dimensional quadratic nonlinear photonic crystal (NPC) with an embedded defect. An external electric field along the z axis is applied to modulate the NPC refractive index periodically. Complete optical isolation always could be reached with the help of an external field. Influences of the defect position and thickness are discussed. The spectral and power tolerances of the isolator also have been investigated and show high contrast within a wide wavelength range at different power levels.

Electro-optic tunable optical isolator in periodically poled LiNbO3

We proposed a design of a tunable optical isolator. The electro-optic (EO) effects in periodically poled lithium niobate (PPLN) with a defect were studied. EO effects and second harmonic generation (SHG) allow three-wave coupling in this nonlinear optics process. A defect is inserted in an asymmetrical position in PPLN to break the reciprocity of the fundamental-wave transmission in two reversed propagating directions. We numerically demonstrated that the contrast of this optical isolator can be tuned from −1 to 1 by simply changing the external dc electric field. With EO effects, the position of the defect can be designed more flexibly maintaining a high contrast of the optical isolation.

Aberration analysis and efficiency improvement of a bidirectional optical subassembly

An approach to improve the coupling efficiency of bidirec- tional optical subassembly BOSA modules is proposed and experimen- tally demonstrated. We analyzed the wavefront aberration coefficients of a typical BOSA. It was found that the 45-deg wavelength filter induces coma and astigmatism, and then it further deteriorates the laser diode to fiber coupling. We measured the BOSA efficiencies based on a series of different filters. For a typical 0.5-mm filter, 25% coupling efficiency im- provement was achieved by optimizing the filter parameters.

The efficiency improvement of a bidirectional optical subassembly

An approach to improve the coupling efficiency of bidirectional optical sub-assembly (BOSA) modules is proposed and experimentally demonstrated. We analyzed the wave-front aberration coefficients of a typical BOSA. It was found that the 45° wavelength filter induces coma and astigmatism, and then it further deteriorates the laser diode (LD) to fiber coupling. We measured the BOSA efficiencies based on a serial of different filters. For a typical 0.5 mm filter, 25% coupling efficiency improvement was achieved by optimizing the filter parameters.