Microparticle Photonics: Frequency Control and Linewidth Quenching of Semiconductor Lasers Using Optical Feedback from Spherical Micro-Cavities

9818596格雷半导体激光器的频率稳定度和光学线宽是影响相干和光通信系统性能、基于波分复用(VMM)的光子交换和通信系统、室温光谱烧孔光存储器、干涉传感、路径长度测量和高分辨率光谱等广泛应用的关键参数。对于许多应用，单纵模(SLM)激光器，如分布反馈激光器(DFB)或分布布拉格反射器激光器(DBR)，其工作频率不够稳定。它很容易受到热涨落的影响，载流子浓度的强度和空间变化会引起啁啾。此外，工作在1.5微米的典型DFB激光器具有3-10 MHz数量级的线宽，而大多数相干光通信系统需要亚MHz线宽。过去使用了几种技术来锁定工作频率和减小光线路宽度，其中包括电反馈或光反馈。利用体外腔半导体激光器成功地将光线宽减小到kHz范围。另一种方法是将激光腔耦合到适当失谐的外部高精细谐振器。利用半导体激光器与高精确度法布里-珀罗标准具耦合，实现了线宽约1000倍的锁频。然而，尽管激光噪声的降低令人印象深刻，但这些技术中的大多数都很笨重，需要庞大的实验室Lype光学设置，并且对不稳定的参数很敏感，例如反射信号的相位，而反射信号的相位受到微声效应的强烈影响。此外，由于设置所需的腔长，纵模间距很小，很难实现SLM操作。因此，这些技术主要用于实验装置。在本项目中，我们提出并开展了一种在小尺寸几何结构中实现半导体激光器锁频和线宽缩减的新方案的理论和实验研究。这种方法包括使用一种新型的光学谐振系统，该系统基于一个非常高Q的微球腔。众所周知，微米大小的介质球充当高Q谐振器，电磁能量以球腔模式的形式存储在球体周围，靠近球体表面。在轴对称的介质体中使用这种模式会产生尖锐的共振峰，称为“形态相关共振”或MDR。这些共振峰的Q因子在T=77=度K时接近108，在T=4K时超过109。作为比较，由两个相距10 cm、反射率为98%的反射镜组成的法布里-珀罗标准具腔体的Q因子为3×107。这些微球的共振峰出现在球体尺寸参数X的特定值，其中X=2*PI*a/lambda；A是球体的半径，而lambda是光学波长。利用这一现象，已经报道了光学双稳和激光等腔效应。在我们的例子中，微球可以用作半导体激光器的失谐加载，使振荡场锁定在其中一个MDR上，同时抑制其线宽。这种方法可以在稳定、经济高效且不比激光本身大的系统中实现极窄的线宽。此外，锁定和测量单个固定微球的MDR的能力开辟了在键合到微球表面的物种和周围溶液中的试剂之间执行极其灵敏的吸附和反应测量的前景。在适当的Q=106的情况下，可以在r=3d的10微米的微球上探测到具有亚原子厚度(~0.1埃)的层。由于S的这一厚度比典型抗原分子的分子尺寸小得多，因此观察到单层的小部分是合理的，从而为新的免疫学测试技术铺平了道路。

9818596GriffelFrequency stability and the optical linewidth of semiconductor laser are critical parameters, affecting a wide range of applications such as coherent and optical communication system performance, wavelength division multiplexing (VMM) based photonic switching and communication systems, room-temperature spectral-hole-burning optical memory, interferometric sensing, path length measurement, and high-resolution spectroscopy. For many applications the operating frequency of a single longitudinal mode (SLM) laser such as the distributed feedback laser (DFB),.or the distributed Bragg reflector laser (DBR), is not stable enough. It is susceptible to thermal fluctuations, and intensity as well as spatial variations of carrier concentration cause chirping. In addition, a typical DFB laser, operating at 1.5 microns has a linewidth of the order of 3- 1 0 MHz, while most coherent optical communication systems require sub MHz linewidth. Several techniques have been used in the past to lock the operating frequency and reduce the optical linewidth, among which are electronic or optical feedback. Reduction of the optical linewidth to the kHz range has successfully been demonstrated using bulk external cavity semiconductor laser. Another approach is to couple the laser cavity to a properly detuned external high finesse resonator. Frequency locking with dramatic linewidth reduction by a factor of ~I 000, using coupling of a semiconductor laser to high-finesse Fabry-Perot etalon, has been demonstrated. However, in spite of impressive reduction of the laser noise, most of these techniques are cumbersome, require a bulky, laboratory-Lype optical set-up, and are sensitive to cantankerous parameters, such as the phase of the reflected signal, which is strongly affected by niicrophonic effects. In addition, due to the length of the cavity required for the set-ups, the longitudinal mode spacing is small and SLM operation is difficult to realize. As a result, these techniques were primarily used only for experimental set-ups.In this project, we propose to develop and carry out theoretical and experimental studies of a novel scheme for realizing frequency locking and linewidth reduction of semiconductor lasers in a small scale geometry. The approach involves the use of a new type of optical resonator system, based on a very high Q microsphere cavity. It has been known for some time that micrometer sized dielectric spheres act as high-Q resonators, with electromagnetic energy stored in form of spherical cavity modes confined around the sphere, near its surface. Employment of such modes in an axially-symmetric dielectric body results in sharp resonance peaks named "morphology dependent resonances" or MDR'S. The Q-factor of these resonances is projected to approach 10 8 at T = 77=degrees K, and exceed 10 9=at T=4 K. For comparison, a Fabry-Perot etalon type resonator, comprised o= f two mirrors of 98% reflectivity separated 10 cm apart would result in a Q factor of 3X10 7. These resonance peaks of the micro-spheres occur at specific values of the sphere size parameter X, where X = 2*Pi*a/lambda; a is the radius of the sphere, and lambda is the optical wavelength. Using this phenomenon, cavity effects such as optical bistability, and lasing have been reported. In our case, the micro-sphere can be used as a detuned loading for a semiconductor laser, causing the oscillating field to lock on to one of the MDR's and quench its linewidth at the same time. This approach may achieve extremely narrow linewidth in a system that is stable, cost effective, and not larger than the laser itself.In addition, the ability to lock into and measure MDR's of single stationary micro-spheres opens up the prospect of performing extremely sensitive adsorption and reaction measurements between species bonded to the micro-sphere surface and reagents in a surrounding solution. With modest Q's of 10 6, a layer having a subatomic thickness (~O.1 Angstrom) may be detected on a micro-sphere with r=3D 10 microns. Since this thicknes= s is considerably less than the molecular size of a typical antigen molecule, the possibility of observing small fractions of a monolayer is reasonable and thus paves the way to new immunological testing techniques.