Xihua Li

Beam expansion in thermo-optic-effect-induced total internal reflection and its applications in optical switches

The beam-propagation characteristics of the total internal reflection (TIR) induced by the thermo-optic effect are investigated. Based on the Fourier heat-transmission principle and the variable separation method, we derive an analytical transient expression of the thermal field for general thermo-optic devices. With the analytical expression, the time response and steady-state temperature distribution of thermo-optic devices are presented. The beam expansion rule of TIR in the thermal field is developed mathematically, and a quantitative calculation is given as well. To illustrate the application of the rule, an X-junction 2 x 2 TIR switch with high reflection efficiency is designed through theoretical calculation. The simulation shows that the structure exhibits a high reflection coefficient; the reflection loss is only -0.76 dB. The simulation results agree well with the theoretical calculation.

LiNbO3 Based S-Bend Waveguide Variable Optical Attenuator

A novel design of a variable optical attenuator based on Ti LiNbO3 has been proposed. The attenuation effect is improved with the help of radiation loss of S-bend optical waveguide. According to the simulation of BPM, the attenuation can be over 30dB when applying a negative 10V voltage, whereas, the insertion loss is less than 1.2dB. At the same time, it has the advantage of small size and easy fabricating. The experiment result shows the attenuation can be as low as 27dB when applying an 18.5V negative voltage. Keywordsoptical waveguide; variable optical attenuator(VOA); S-bend waveguide; electro-optics effect; LiNbO3

The design of a 2×2 polymer TIR switch based on thermal field analysis employing thermo-optic effect

In this paper, the beam-propagation characteristics of the total internal reflection induced by the thermo-optic effect are investigated. Based on the Fourier heat transmission principle and variable separation method, we derive the analytical transient expression of thermal field for general thermo-optic (TO) devices. Based on the analytical expression, time response and steady-state temperature distribution of thermal-optic devices are presented. The expansion rule of total internal reflection (TIR) in the thermal field is developed mathematically, and quantitative calculation is given about specific expansion value. As an illustration and application of this rule, a X-junction 2×2 TIR switch is designed to accomplish the object of high reflection efficiency. The computer simulation results show the structure presents a high reflection coefficient indeed, the reflection loss is only -0.76dB. The computer simulation results agree with the calculation well.

Liquid crystal based optical circular-polarization modulator

In this paper an Optical Circular-Polarization Modulator (CPM) based Liquid Crystal (LC) is presented. Using the electric birefringence of LC material, the device can output polarized light with two circular-polarization states. Detailed illumination of the devices structure and its testing investigation are also presented.

LiNbO3 Based 1×2 Y-Branch Digital Optical Switch Integrated with S-Bend Variable Optical Attenuator

A novel design of 1×2 Y-branch digital optical switch (DOS) integrated with S-bend variable optical attenuator (VOA) based on Ti ： LiNbO3 is proposed. The purpose of the S-bend VOA is to suppress the crosstalk of the DOS. According to the simulation result, the crosstalk can be over 30dB when 6 V voltage is applied. We realized this device by using Ti diffused LiNbO3 waveguide process. The crosstalk measured can achieve 28dB when a voltage about 15 V is applied. According to the experiment result, we can prove that the structure of 1×2 Y-branch digital optical switch integrated with S-bend variable optical attenuator is available.

Fabrication of 8×8 MMI optical coupler in BK7 by ion-exchange

The planar waveguide optical couplers are of prime importance in optical communication and optical signal processing system. Comparing with the optical fiber coupler (OFC) which fabricated by fused biconical taper technology, the planar waveguide couplers are more compact size, lower loss, better uniformity, easier manufacture and integration. Multimode interference (MMI) couplers have many advantages, such as compact size, wavelength and polarization insensitivity, fabrication tolerances and low loss, etc., which concentrate more and more attention. Conventional MMI devices are based on the uniform index waveguides. When the number of input/output waveguides becomes larger, the intrinsic propagation constant error, which will cause bad uniformity of output power, can’t be neglected. In fact, most waveguide devices are graded-index. With the enhanced compatibility of MMI coupler, the performance can be improved at the same time. Prior study shows that graded-index MMI couplers reach the best performance under certain index contrast. Among many available materials, glass is chosen to be the substrate of the coupler, because of its good features, such as low loss, ease fabrication, cheap cost, and so on. In this paper, an 8×8 MMI optical coupler is designed based on the principle of graded-index MMI. The coupler is composed of a waveguide, which is designed to support a large number of modes, and several access (usually single-mode) waveguides, which are used to launch light into and recover light from that multimode waveguide. The total length of the device is less than 3.5 centimeter, including S-bends which lead the multiple images to the output of the device with the spacing D=250μm to make the device fiber compatible. In this paper, we describe an experimental realization of the 8×8 graded-index MMI optical coupler and the measurement of its performance with the testing laser of the wavelength of 1.55μm. The device is fabricated by ion-exchange on BK7 glass substrate. During the ion-exchange process, a melting mixture of AgNO3 : (KNO3 : NaNO3) (molar ratio, 0.001:1) is used at 350~380°C for different times (range from 8 to 18 hours) to fabricate the coupler. The experimental results show that the performance of the optical coupler is quite promising. For instance, while launching light from No.5 waveguide, the uniformity of the device is approximately 0.72dB. Optimization of design and fabrication is going on to improve the total performance of the optical coupler.