【作者】 焦雨清；

【导师】 何赛灵； 戴道锌；

【作者基本信息】 浙江大学 ， 光学工程， 2013， 博士

【摘要】 在各种光学相干层析成像系统的研究方向中,基于1.6至1.8微米波长范围的扫频光学相干层析成像系统在集成性、高性能以及低成本等方面都具有很大的优势。本文研究的集成光电子器件就是着眼于这种长波长扫频光学相干层析成像系统的应用。本工作首先采用1.7微米附近的波长范围,与之前常用的较短波长相比,此长波段能减少光在生物组织中的瑞利散射,从而可以改进成像深度。本研究工作也采用了成熟的基于磷化铟半导体材料的光电子集成技术,从而使得1.6至1.8微米波长范围的扫频光学相干层析成像系统的单片集成化成为可能。首先,要实现产生和探测这种新颖的长波长波段的光,需要新型有源材料的研究与应用作为支持。新型的有源材料必须能在1.7微米附近的波长范围内提供足够的光增益或光吸收。尤其是扫频光学相干层析成像系统,为了达到成像深度方向上足够的成像分辨率,需要可调谐激光器的调谐范围与光探测器的光谱响应都至少达到100纳米以上。与此同时,实时成像的应用需求则对可调谐激光器扫描整个调谐光谱范围的重复速率、以及与可调谐激光器配套工作的光探测器的响应带宽都提出了一定的要求。一种基于五层堆叠量子点有源材料的可调谐激光器已经在之前得到了一定的研究。本文首先将此激光器用于自由空间光学层析成像平台,并在玻璃板和Scotch(?)半透明胶带上进行一些验证性的成像实验,成像实验结果较为成功。在成像实验过程中,也发现一些针对量子点可调谐激光器和商用光探测器性能方面的问题。对于激光器而言,问题主要在光功率、调谐范围与调谐速度上。而对于商用光探测器,性能局限性主要是噪声水平、响应度和响应带宽。这些问题的发现给本文之后的研究工作提供了直接的参考和推动。在光学相干层析成像实验中,还对激光器谐振腔中采用的可调谐滤波器的校准流程进行了一些优化,并通过实验验证其对激光器性能的改进。本文接着研究了具有更高光增益的不同有源材料对激光器性能的影响。制作并测试了基于四层堆叠应变量子阱有源材料的可调谐激光器,并与五层堆叠的量子点激光器作比较。此量子阱激光器的波导设计与量子点激光器完全相同,所用的半导体层堆栈结构也互相兼容。量子阱激光器的测试结果显示,在注入电流密度为1875A/cm2时,基于量子阱的光放大器能提供约19cm-1的模式增益,这个数值远远高于量子点光放大器在3000A/cm2电流密度下所得到的6cm。由于量子阱的高增益,测得量子阱激光器的阈值电流为500毫安,远低于量子点激光器的1500毫安。量子阱激光器在两个激光波长间切换所需的时间为140纳秒,而量子点激光器的波长切换时间为500纳秒。测得的140纳秒切换时间已经接近控制电路的实际极限。另一方面,量子阱激光器的波长调谐范围仅有10纳米,与量子点激光器之前实现的60纳米相比,主要原因是由于量子阱增益材料相对较窄的增益带宽。这一系列工作直观地反映了有源材料的特性对于可调谐激光器性能的影响。本文接着提出了一种量子点可调谐激光器的改进方案,并加以制作和测试。方案的改进主要集中在激光器的设计布局上以增加输出光功率、改进光反馈机制以增强环形激光器的单向性、设计一个新的16臂的多模干涉树状滤波器以增加调谐范围等,并用一个分段式环形激光器模型来仿真和分析此激光器的改进方案所能增加的调谐范围。同时,也通过实验验证了新的多模干涉树状滤波器的通带半高全宽为11至12.5纳米,波长调谐范围至少为60纳米。制作的激光器无法成功地产生激光,主要原因是由于晶片生长的质量原因导致量子点光放大器的低模式增益和无源波导中的高传输损耗。本文接下来的部分是关于量子点波导型光探测器、以及将其用于光学相干层析成像系统可行性的研究。此光探测器采用与第2章中研究的量子点可调谐激光器一样的五层堆叠量子点有源材料。这种量子点能吸收1.7微米附近波长范围的光。此光探测器的半导体层堆栈结构和波导结构都与量子点激光器采用的有源-无源混合集成技术完全兼容。因此它能与量子点激光器一起集成在同一个芯片上。对量子点光探测器的测试结果显示,对于280微米长的器件,施加3伏的反向偏置电压时,它具有低暗电流(约15纳安)、平坦光谱响应(整个300纳米的光谱范围内响应度都高于0.5A/W)以及足够的响应带宽(75兆赫)等特性。用修改过的速率方程模型来仿真量子点波导型光探测器的性能,并通过模型理解量子点吸收光的机理。还用等效电路模型来分析响应带宽的限制因子(主要是金属电极的电容)。本文的最后研究了一种在1.6至1.8微米波长范围具有高增益的单层砷化铟量子阱上砷化铟量子点结构的有源材料。与之前的3000A/cm2下仅有6cm-1模式增益的量子点放大器相比,采用这种有源材料的光放大器具有显著改进的模式增益(3000A/cm2下11cm-l)。对于长波长应用领域的量子点激光器或光探测器来说,这种量子点材料将是一种很好的选择。在测试过程中发现的当电流密度或温度变化时,增益谱峰值对应的波长漂移的现象,用一个改进的速率方程模型来解释。本文包含的整个研究工作展示了将用于扫频光学相干层析成像系统的可调谐激光器、光探测器和其他无源光器件集成在单个芯片上的可行性。基于1.7微米波长范围的单片集成扫频光学相干层析成像系统的发展在本文的研究工作中得到了充实与显著的进展。更多还原

【Abstract】 The presented research work in this thesis focuses on the development of integrated photonic devices which are used in integrated swept-source optical coherence tomography (SS-OCT) system operating at the1.6to1.8μ m wavelength region. Since the Rayleigh scattering in biological tissue can be reduced by using longer wavelength compared to more commonly used regions at shorter wavelengths, the wavelength region around1.7μ m is chosen as the operating wavelength band. Thus an improvement in the imaging depth is expected by using this long wavelength. The use of indium phosphide (InP)-based photonic integration technology makes the monolithically integrated SS-OCT system in the1.6to1.8μ m wavelength region very possible and promising.In order to obtain light around1.7μ m, the research and analysis of novel active material which can offer light generation, amplification and absorption around this long wavelength is desired. The SS-OCT requires the tuning range for the tunable laser and spectral responsivity for the photodetector to be wide enough in order to achieve sufficient image resolution along the depth. The demand for real-time imaging also sets up the requirement on the repetition rate of the laser wavelength sweep over its entire tuning range as well as the electrical bandwidth of the photodetector to support the laser.In this thesis, a tunable laser based on five-layer quantum dot (QD) gain material was firstly used in a free-space OCT setup for demonstrative OCT imaging experiments on glass dish and Scotch" tape. Successful OCT images on glass dish and Scotch tape have been obtained. During the OCT imaging experiments, several issues have came up concerning the performance of the QD laser and the commercial photodetectors. The QD laser used in the experiments had problems with the optical power, tuning range and tuning speed. The commercial photodetectors also showed serious limitations in noise level, sensitivity and electrical bandwidth. These issues directly motivated the following work presented in this thesis. During these OCT imaging experiments several improvements on the calibration routine of the intra-cavity tunable filters in the laser have been proposed and experimentally demonstrated.Several approaches to the improvement of laser performance have been studied. First, the influence of a different gain material with higher modal gain value to the performance of the laser has been studied. A tunable laser with amplifiers based on four-layer strained quantum well (QW) active material has been characterized and compared to the five-layer QD laser. The layout of the QW laser was identical to that of the QD laser and was fabricated with a fully compatible layerstack. The measurement results have shown that the much higher modal gain in the QW amplifiers could significantly reduce the threshold current of the QW laser compared to the QD laser with much lower modal gain. The huge improvement on the wavelength switch time in the QW laser has also been demonstrated compared to the slow switch time in the five-layer QD laser. The measured tuning speed in the QW laser has already approached the limitation in the control electronics. On the other hand the tuning range of the QW laser was observed to be much narrower due to the relatively narrow gain bandwidth in the QW amplifiers compared to the five-layer QD laser. Overall this part of work has provided a clear idea of how the characteristics of the active material influence the performance of the tunable laser.Next, an improved design of the QD tunable laser has been proposed, fabricated and preliminarily characterized. The improvements include the redesign of the laser layout to increase the output power, the redesign of the optical feedback scheme to enhance unidirectional lasing and the improved new design of the MMI-tree filter to extend the tuning range. The improved tuning range of the improved laser design has been analyzed and demonstrated using a segmented ring laser model. The MMI-tree filter also has been experimentally demonstrated. However, the lasing of the improved laser chip was not successful. We attributed this to the low modal gain in the QD amplifiers and high passive loss in the passive waveguides which was caused by the bad quality of the wafer growth.Then the QD waveguide photodetectors for the long wavelength OCT application are studied in this thesis. The photodetectors use the identical five-layer QD active material as has been used in the QD laser. The layerstack and waveguide structure of the photodetectors are also fully compatible to the active-passive integration technology for the QD laser. Thus they can be integrated with the QD laser on a single chip. The light absorption at1.7μm wavelength region provided by the QDs is analyzed both experimentally and theoretically. The characterization of the photodetectors has shown low dark current, flat spectral response and sufficient electrical bandwidth. A modified rate equation (RE) model has been used to simulate the performance of the QD photodetector and to understand and explain the mechanisms of light absorption in the QDs. An equivalent electrical circuit model has also been applied to figure out the limitation (metal pad capacitance) of the electrical bandwidth.Finally in this thesis a high-gain single-layer InAs QD on InAs thin QW active material in the1.6to1.8μm wavelength range has been studied. The amplifiers using this new QD material have shown significantly higher modal gain than the modal gain in the previous QD amplifiers, while still maintaining the same wide gain bandwidth as the previous QD material. This type of new QD material has been demonstrated to be a very promising candidate to be used in the next generation QD tunable lasers or QD waveguide photodetectors for long-wavelength OCT applications. Then an improved RE model has been optimized and applied on the new high gain QD amplifiers to simulate the gain behavior of this material as well as the carrier dynamics in the new QDs.The overall work in this thesis has revealed the very high potential of integrating tunable lasers, photodetectors and any other passive photonic waveguide components in a single chip for the SS-OCT application. The presented results have shown a significant progress towards the monolithically integrated SS-OCT system operating at the1.7μ m wavelength region.更多还原