Study of all-optical logic devices

为了实现光控制的波导光开关器件，进行了研究，其功率可由激光二极管获得。本文的主要工作如下：理论上估算了直接带隙半导体的强度依赖折射率和非线性吸收系数。通过数值计算，估算出在1550 nm波长下，控制GaInAsP波导DFB晶体管所需的功率约为几十mW;对于传统的均匀耦合系数DFB结构，其通态透过率随输入光强的增加而降低。通过在DFB结构中引入加权耦合系数，克服了这一缺点。数值计算表明，采用加权分布反馈结构的光开关器件具有较好的通态透过率和较低的阈值光强。此外，加载有布拉格反射器的非线性定向耦合器（BR-NLDC），其作为具有与输入端口分离的两个输出端口的MEMS器件工作，被提出用于光信号处理应用。与传统的非线性定向耦合器相比，非对称BR=NLDC的开关功率降低了1/10。最后，在GaInAsP波导中成功实现了分布反馈结构的全光开关。在与光集成电路兼容的波导结构中首次观察到了全光置位-复位操作。通过本实验，光开关的阈值功率在20 ~ 80 mW之间，虽然实验证实器件的响应时间小于2.5nsec，但没有研究开关速度的限制。为了将来的实际应用，必须这样做。

The research was carried out in order to realize the waveguide optical switching devices controlled with light whose power is obtainable by a laser diode. The results obtained throughout this research are summarized as concluding remarks in the following.The intensity-dependent refractive index and the nonlinear absorption coefficient were theoretically estimated in direct bandgap semiconductors. Through the numerical calculation, the required power to control a waveguide DFB bistable device composed of GaInAsP was estimated to be a few tens mW at a wavelength of 1550nm.In case of conventional DFB structures with a uniform coupling coefficient, the on-state transmittance decreases as input intensity increases. This drawback, however, was overcome by introducing a weighted coupling coefficient in DFB structure. Through numerical calculations, it has been shown that optical bistable device with the weighted DFB structure exhibits improved on-state transmittance together with reduced threshold intensity. Also, nonlinear directional couplers loaded with a Bragg reflector (BR-NLDC), which work as a bistable device having two output ports separated from the input one, were proposed for optical signal processing applications. The swiching power of an asymmetric BR=NLDC is typically 1/10 less compared with a conventional nonlinear directional coupler.Finally, all-optical switching was successfully demonstrated in the GaInAsP waveguide with a distributed feedback structure. The first observation of all-optical set-reset operation has been obtained in the waveguide structure compatible with optical integrated circuits. Through the present experiment, the threshold power for the optical swiching operation ranges from 20 to 80 mW.Although it is experimentally confirmed that the response time of devices is less than 2.5nsec, the limitation of the switching speed was not investigated. This must be done for practical applications in the future.