The thesis consists of theoretical and experimental studies on characteristics of the laser diode under optoelectronic feedback, as well as a new analytic treatment of solitary laser diode dynamics. As applications of these studies, we realize a new optical short-pulse generator and optical multistable devices that may be useful in high speed optoelectronic signal processing. Large signal modulation response, relaxation oscillation, bistability, period doubling routes to chaos, and self-pulsing in both the solitary laser diode and the laser diode under optoelectronic feedback are also investigated in detail.
Understanding dynamic behavior of the solitary laser diode is fundamental to studies of the laster diode under optoelectronic feedback. It is well known that dynamics of the laser diode is critically dependent on spontaneous emission, gain saturation, Auger recombination, and saturable absorption. For the analytic treatment of the solitary laser diode that takes into account these effects, we transform the nonlinear rate equations and use a standard perturbation theory, i.e. the method of multiple scale expansions. Phenomena covered here include resonance frequency shift, relaxation oscillations, bistability, and period doubling bifurcations in a directly modulated laser diode. Based on this analytic treatment, we also explain mechanism of period doubling route to chaos in a directly modulated laser diode. Furthermore, we derive analytically the basic limit of optical pulse width that can be achieved from gain-switching of the solitary laser diode. To overcome some of this limit and to explore new device configurations, we seek utilization of optoelectronic feedback.
We model the laser diode under optoelectronic feedback by using a rate-equation formulation. The optoelectronic feedback that has a negligible feedback time delay may be divided into two, i.e. the positive and the negative feedback. It is well known that the laser diode under positive optoelectronic feedback may exhibit optical bistability. Also the delayed optoelectronic feedback has been proposed for optical short-pulse generation. However, the effect of negative optoelectronic feedback in the laser diode has not drawn much attention despite its potential importance.
To study instabilities of laser diodes under negative optoelectronic feedback, we solve our rate equations for typical optoelectronic feedback networks such as one-pole low pass networks, two-pole low pass networks, and band pass networks. Based on this study, we propose a new method of optical short-pulse generation. The proposed method generates picosecond optical pulses with repetition rates of several gigahertz, and the required gain bandwidth product of the feedback network is a few tens of gigahertz. The interaction of the feedback signal and the dynamics of the stable laser diode forms picosecond optical pulses. This method does not require large external modulation signal, and it is not to the critical optical alignment. Furthermore, the generated pulse is stable. Feasibility of the proposed method is also demonstrated experimentally. The effect of delayed optoelectronic feedback is also discussed briefly. Furthermore, we propose a simple graphical method for static solutions and check their stability by using the gain and the phase conditions.
Utilizing positive optoelectronic feedback, we realize an optical multistable device as well as an improved version of optical bistable device. In this device the number of stable state is determined by the number of transistor pairs. The optical logic device has a good tuning capability and operate at a low optical input power. Also they are insensitive to polarizations and wavelengths of optical input. These devices have cascading capability. A graphical solution, as well as stability analysis, is presented to explain the operational principle of this device. As a by-product of this study, an optical flip-flop using laser diodes with the optoelectronic feedback is demonstrated.
Finally, we study large signal behavior of the laser diode under optoelectronic feedback. By transforming the equations of motion of the system, we show that the two-component rate equations can describe the effects of optoelectronic feedback. It is found that the optoelectronic feedback decreases the damping constant of the system and the laser diode becomes more unstable due to the feedback. We also predict bistabilities and chaos in laser diodes with optoelectronic feedback. For a delayed optoelectronic feedback, we observe experimentally spectral bistability and chaos.
본 논문에서는 레이저 다이오드의 정특성과 동특성에 미치는 광 전궤환의 영향을 이론적 및 실험적으로 고찰하였다. 이들의 영향을 체계적으로 고찰하기 위하여 단일 레이저 다이오드의 동특성도 자세히 고찰하였다. 비율 방정식을 generalized coordinates 로 치환하여 자연방출, 이득포화, Auger recombination, 포화흡수의 영향을 통합적으로 고찰하였다. 치환된 방정식을 multiple scales expansion 방법으로 풀어서 레이저 다이오드에서의 완화발진, self-pulsing, 직접변조한 레이저 다이오드에서의 쌍안정성, 공진주파수 천이, 주기증배 등을 이론적으로 고찰하였다. 또, 이득 스위칭과 대신호 변조시에 얻을 수 있는 최소 펄스폭을 산출하였다. 그리고, 레이저 다이오드, 광검출기, 증폭기로 구성되고 광전궤환으로 결합되어 있는 system를 비율 방정식 공식화로 모델하고, 광전궤환의 영향을 체계적으로 고찰하였다.
System의 정적인 해를 위하여 간단한 도식적인 해법을 제안하였다. 정적인 해의 안정성을 판별할 수 있는 이득조건과 위상조건도 제안하였다. 광전궤환의 영향의 초석으로서 one-pole low pass feedback network, two-pole low pass feedback network, band pass feedback network를 자세히 고찰하였다. 광전궤환의 응용으로 부궤환을 이용하는 극초단 광펄스생성법과, 정궤환을 이용하는 광다안정소자 및 광논리회로를 제안하고 이들을 실험적으로 구현하였다. 제시한 극초단 광펄스 생성법에서는 외부변조신호를 필요치 않고, 정밀한 광학적 정열을 요구치 않으며, 쉽게 펄스의 반복율을 tuning 할 수 있는 장점을 가지고 있다. 또한 광다안정소자도 낮은 입력 power 와 어떤 입력 분극에서도 동작한다. 더구나 이소자들은 직렬연결이 가능하다. 이러한 소자들은 초고속 광통신, 초고속 광신호처리 등에 응용이 기대된다.
끝으로 레이저 다이오드의 synergetics 로서 레이저 다이오드에 광전궤환이 가해진 system의 대신호 변조특성을 고찰하였다. 계의 동특성을 기술하는 방정식을 generalized coordinates 로 치환함으로서 통합된 비율방정식이 광전궤환의 영향도 기술하는 것을 보였다. 광전궤환은 감쇄정수를 감소시켜서 레이저 다이오드를 보다 불안정하게 한다. 또, 광전궤환이 가해진 레이저에서의 쌍안정성, 불안정성, 혼돈현상 등을 예측하였다. 실험적으로는 지연 광전궤환이 가해진 레이저 다이오드에서의 spectral bistability 와 chaos 를 관측하였다.
