Yan-qing Lu

Electrically tunable liquid-crystal photonic crystal fiber

Tunable light switch using a photonic crystal fiber filled with nematic liquid crystal is demonstrated. The original band-gap-guiding fiber structure was transformed to a total internal reflection-guiding photonic crystal fiber by filling liquid crystal into the air core and cladding air holes. By applying external voltage to the liquid-crystal-filled fiber, we have demonstrated an electrically tunable fiber-optical switch with over 30dB attenuation at 60Vrms for a He-Ne laser beam. This liquid-crystal-filled photonic crystal fiber will find useful applications in fiber-optic communication systems.

Electro-optic effect of periodically poled optical superlattice LiNbO3 and its applications

The electro-optic effect of periodically poled optical superlattice LiNbO3 (PPLN) was studied. Because of the periodic electro-optic (EO) coefficient, the reciprocal vector of the periodic structure can be used to compensate for the phase mismatch between the ordinary and extraordinary waves, which is similar to the nonlinear optical frequency conversion process. If the quasi-phase-matching condition is satisfied, polarization of a light propagated in PPLN can rotate linearly with the applied electric field, which shows that PPLN may be used as a precise spectral filter or an EO switch.

Miniaturized fiber taper reflective interferometer for high temperature measurement

We present an ultra-small all-silica high temperature sensor based on a reflective Fabry-Perot modal interferometer (FPMI). Our FPMI is made of a micro-cavity (approximately 4.4 microm) directly fabricated into a fiber taper probe less than 10 mum in diameter. Its sensing head is a miniaturized single mode-multimode fiber configuration without splicing. The sensing mechanism of FPMI is the interference among reflected fundamental mode and excited high-order modes at the end-faces. Its temperature sensitivity is approximately 20 pm/degrees C near the wavelength of 1550 nm. This kind of sensor can work in harsh environments with ultra-large temperature gradient, but takes up little space because of its unique geometry and small size.

Demonstration of a compact temperature sensor based on first-order Bragg grating in a tapered fiber probe

We experimentally demonstrate an all-silica first-order fiber Bragg grating (FBG) for high temperature sensing by focused ion beam (FIB) machining in a fiber probe tapered to a point. This 61-period FBG is compact (~36.6 μm long and ~6.5 μm in diameter) with 200-nm-deep shallow grooves. We have tested the sensor from room temperature to around 500 °C and it shows a temperature sensitivity of nearly 20 pm/°C near the resonant wavelength of 1550 nm. This kind of sensor takes up little space because of its unique geometry and small size and may be integrated in devices that work in harsh environment or for detecting small objects.

Microfiber-probe-based ultrasmall interferometric sensor

We report an ultrasmall microfiber-probe-based reflective interferometer for highly sensitive liquid refractive index measurement. It has a 3.5 microm micronotch cavity fabricated by focused ion beam micromachining. A sensitivity of 110 nm/RIU (refractive index unit) in liquid is achieved with over 20 dB extinction ratio. Theoretical analysis shows this kind of device is a hybrid of Fabry-Perot and modal interferometers. In comparison with normal fiber interferometers, this probe sensor is very compact, stable, and cheap, offering great potentials for detecting inside sub-wavelength bubbles, droplets, or biocells.

Experimental demonstration of distributed feedback semiconductor lasers based on reconstruction-equivalent-chirp technology

In this paper we report, to the best of our knowledge, the first experimental realization of distributed feedback (DFB) semiconductor lasers based on reconstruction-equivalent-chirp (REC) technology. Lasers with different lasing wavelengths are achieved simultaneously on one chip, which shows a potential for the REC technology in combination with the photonic integrated circuits (PIC) technology to be a possible method for monolithic integration, in that its fabrication is as powerful as electron beam technology and the cost and time-consuming are almost the same as standard holographic technology.

Fast switchable grating based on orthogonal photo alignments of ferroelectric liquid crystals

We demonstrate a fast switchable grating based on ferroelectric liquid crystals and orthogonal planar alignment by means of photo alignments. Both 1D and 2D gratings have been constructed. The proposed diffracting element provides fast response time of around 20 μs, contrast of 7000:1 and high diffraction efficiency, at the electric field of 6 V/μm. The saturated electro-optical (EO) states up to very high frequency (≈5 kHz) are the real advantage of the proposed switchable grating, which opens several opportunities to improve the quality of existing devices and to find new applications.

Temperature sensor based on an isopropanol-sealed photonic crystal fiber in-line interferometer with enhanced refractive index sensitivity

We fabricate a simple, compact, and stable temperature sensor based on a liquid-sealed photonic crystal fiber (PCF) in-line nonpolarimetric modal interferometer. Different from other reported PCF devices, it does not need expensive polarimetric devices, and the liquid is sealed in one fiber. The device consists of a stub of isopropanol-filled PCF spliced between standard single-mode fibers. The temperature sensitivity (-166 pm/°C) increases over an order of magnitude compared with those of the previous sensors based on air-sealed PCF interferometers built via fusion splicing with the same mechanism. In addition, the refractive index sensitivity also increases. Higher temperature sensitivity can be realized by infiltrating some liquid having a higher thermo-optic coefficient into the microholes of the PCF.

Self-polarizing terahertz liquid crystal phase shifter

Using sub-wavelength metallic gratings as both transparent electrodes and broadband high-efficiency polarizers, a highly-compact self-polarizing phase shifter is demonstrated by electrically tuning the effective birefringence of a nematic liquid crystal cell. The metal grating polarizers ensure a good polarizing efficiency in the range of 0.2 to 2 THz. Phase shift of more than π/3 is achieved in a 256 μm-thick cell with a saturation root mean square voltage of around 130 V in this integrated device.

Variable optical attenuator based on polymer stabilized twisted nematic liquid crystal

A variable optical attenuator (VOA) based on polymer stabilized twisted nematic (PSTN) liquid crystal (LC) is demonstrated. Comparing with the normal twisted nematic LC-based VOA, PSTN exhibits a much faster response time. Moreover, the polymer networks effectively eliminate the backflow effect which exists in the normal TNLC cell. The attenuation mechanism of the PSTN LC was studied. Both polarization rotation and light scattering effects are found to contribute to the optical attenuation. The ratio between these two mechanisms can be adjusted by changing the polymer concentration and polymer network domain size.