Qu Li

Method of improving optical poling efficiency in polymer films

The alignment of dye molecules in optically poled polymer films is modeled. We point out that when writing beams (fundamental and second harmonic) are linearly polarized with their polarization directions parallel, the majority of the dye molecules in a poled film are axially oriented perpendicular to the writing-beam polarization, whereas the minority are polar oriented parallel to the writing-beam polarization. Only the latter, however, contribute to the second-order nonlinearity of the film. This model is verified by experiments. Based on the model, a new method of improving optical poling efficiency is put forward.

Beam propagation in nonlinear multimode interference waveguide

A variational technique is introduced to analyse the beam propagation features in nonlinear multimode interference. The differential equations for beam width and beam centre are given by the variational method. The steady-state solution of the differential equations is analysed. Detailed features of the nonlinear multimode interference when the input beam centre is at the centre and off the centre are investigated. It is found that, when the input power is high enough that the beam can be looked at as an integer, the beam will bend and experiences a zigzag path. These features are power dependent. Some results of the variational method are verified by a numerical method.

Photobleaching of polydiacetylene waveguides: a characterization of the process and patterning of optical elements

The photobleaching process of poly-3butoxyl-carbonyl-methyl-urethane (poly-3BCMU) waveguides by means of an UV lamp and the 488-nm line of an Ar laser is characterized and modeled. The limits of the theory are discussed in light of experimental results, and we stress the role of the oxygen diffusion rate on the process. Finally, we adopt the photobleaching method to pattern a guided-wave micro-optic device and holographic diffraction gratings on spun poly-3BCMU films.

Multiple coupling length nonlinear directional couplers with variable coupling coefficient

Multiple coupling length nonlinear directional couplers (NLDCs) with variable coupling coefficient (VCC) are investigated. As examples, two kinds of VCC NLDCs, exponential and Gaussian VCC NLDCs, are numerically simulated. Both couplers show multiple-switching feature and have eminent digital switching characteristic. In addition, the Gaussian VCC NLDC exhibits equi-power-interval-switching characteristic.

Nonlinear optical switching matrix

We show that nonlinear directional couplers with variable coupling coefficients can be used to compose a special kind of switching matrix. The switching matrix can be controlled by the signal itself or by a strong pump. Light routing is realized by adjustment of the power of the signal or the pump. Because light routing is based on the optical Kerr effect, ultrafast routing is possible.

Gain and noise figure characteristics of an Er 3+ -Yb 3+ doped phosphate glass waveguide amplifier with a bidirectional pump scheme and double-pass configuration

The net gain Gnet and internal noise figure INF character- istics of an Er 3+ -Yb 3+ doped waveguide amplifier EYDWA with a bidi- rectional pump scheme and double-pass configuration DPC were ana- lyzed both experimentally and numerically. A propagation-rate-equation model was used to describe the eight-level EYDWA system. The EYDWAs used in the experiments were fabricated on an Er 3+ -Yb 3+ doped phosphate glass waveguide of 3.8-cm length using a two-step ion exchange. In comparison with the backward-pump-only scheme BPOS, the equalizing bidirectional pump scheme EBPS achieved a 4.1-dB enhancement in Gnet, while the forward-pump-only scheme FPOS achieved a 6.0-dB reduction in the INF with a single-pass con- figuration SPC and 90-mW total pump power. About a 100% enhance- ment in the Gnet and 50% less deterioration in the INF were obtained in DPC compared with SPC under saturation pump power. A 4.1-dB en- hancement in Gnet and a 1.4-dB reduction in INF were observed by applying EBPS instead of BPOS or FPOS with 150-mW total pump power in DPC. Both the experiments and simulations showed that a significant improvement in the performance of the EYDWA can be achieved by optimizing the amplification configuration and pump scheme.

Switching characteristics of variable coupling coefficient nonlinear directional coupler

This paper analyzes the characteristics of the variable coupling coefficient nonlinear directional coupler (VCC NLDC) in detail. The self-switching transmission curves of three types of NLDC with different coupler length are compared. Their phase-controlled transmission curves are compared, and these device acting as pump-controlled switches are analyzed. It is found that the VCC NLDCs show digital characteristics with the increase of the input power. As phase-controlled devices, the response of the VCC NLDC is binarylike when the input power is high. As pump-controlled devices, the transmission curves of the signal are similar to their self-switching transmission curves, except for some specific details.

Experimental observation of the nonlinearity of the Pancharatnam phase with a Michelson interferometer

Three experiments are proposed for the observation of the nonlinearity of the Pancharatnam phase with a Michelson interferometer. The result of one of these experiments is reported.

Improved gain characteristics by forward-backward pumped configuration in erbium-ytterbium–doped phosphate glass waveguide amplifier

The model used for analyzing the gain characteristics of Er-Yb–doped phosphate glass waveguide amplifier (EYDWA) with forward-backward pumped configuration (FBPC) is based on the rate and optical power propagation equations, which include forward-backward pump, upconversion mechanisms, and light propagation loss. An overlap factor, which considers the difference between signal and pump field intensity distribution in channel waveguides, is introduced to evaluate the gain of EYDWA. The results show that FBPC can make the pump power light more uniform and lower than that in forward-pumped configuration (FPC), inhibiting the upconversion. Thus, FBPC increases the gain of the device by 2 to 3 dB compared with FPC, and pump conversion efficiency (PCE) is accordingly raised by 50%. The total power of amplified spontaneous emission (ASE) is also higher than that in FPC. The gain of EYDWA is affected obviously by the pump configuration, modes of pump light, upconversion coefficient, and scattering loss. Moreover the detrimental effect due to the above factors is reduced by the FBPC.

Three-core nonlinear directional coupler with variable coupling coefficient

Three core nonlinear directional couplers (TCNLDCs) with variable coupling coefficients (VCCs) are proposed. Their linear operation is examined analytically. Three types of VCC TCNLDCs are investigated numerically. Their transmission features are compared with those of their two-core counterparts. It is shown that the switching curves of VCC TCNLDCs are sharper than those of their two-core counterparts. Also the features of different types of VCC TCNLDCs are compared to each other and to the constant coupling coefficient (CCC) TCNLDC. It is found that the sidelobes in the postswitching region can be suppressed with VCC TCNLDCs and the peak transmission of VCC TCNLDCs is higher.