High Power, High Frequency Mode-locked Semiconductor Lasers

半导体激光器是紧凑、低成本的短脉冲光源，这在许多应用中都有应用，例如CD播放器（CD激光器工作频率为几兆赫）和光通信。在光通信的应用中，超短脉冲需要非常高的重复频率，即几十千兆赫（每秒数百亿周期）/并且在未来可能需要更高的重复频率。通过使用脉冲电流源，可以直接开关半导体激光器的开关，频率高达约40千兆赫，但不会高得多。未来的光通信系统可能需要更高的重复频率，而这些频率可以通过使用一种称为锁模的方法来达到，在这种方法中，激光腔内的特殊吸收部分有助于形成脉冲，相邻脉冲之间的时间由腔的往返时间控制。但是这种锁模激光器产生的脉冲平均功率水平相对较低，时间也相对较长（通常是几皮秒）。对于光通信应用，需要开发具有更短脉冲和更高输出功率水平的激光器。锁模激光器的另一个潜在用途是产生太赫兹辐射。电磁频谱的太赫兹部分位于可见光光谱和微波光谱之间。这种非电离辐射能够穿透不透光的材料，如纸、塑料、布和皮肤，因此它可以用于安全和医疗应用。它可以探测爆炸物和肿瘤——它比x射线更安全，因为它不会电离穿过的物质，而且在区分不同类型的软组织方面做得更好。太赫兹波通常是由脉冲光源转换产生的，目前这种光源体积大，购买和运行都很昂贵。该项目的目标之一是开发能够产生非常高频率（300至2000千兆赫）脉冲的半导体激光器，其输出功率足够用于产生太赫兹和次太赫兹波，效率大大提高。激光器需要以更高的重复频率和更高的功率输出水平发射更短的脉冲。这种需求可以通过使用已经设计用于发射高功率的结构，然后将其调整为脉冲操作来满足。我们将把高功率半导体激光器与锁模操作结合起来，开发出以世界纪录的重复频率发射短而高功率脉冲的激光器。我们还将研究可以与激光集成的结构，从而进一步压缩光脉冲，并研究光如何与半导体材料相互作用，从而找到适合特定应用的激光结构的最佳设计。

Semiconductor lasers are compact, low-cost sources of short pulses of light - and this is used in many applications, e.g. CD players (CD lasers operate at frequencies of a few MHz) and optical communications. For applications in optical communications ultra-short pulses are required at very high repetition frequencies, i.e. tens of GigaHertz (tens of billions of cycles per second) / and in the future even higher repetition rates are likely to be required. It is possible to switch semiconductor lasers on-and-off, directly, at frequencies up to about 40 GHz by using a pulsed current source but not much higher. Future optical communications systems are likely to need higher repetition frequencies - and these frequencies can be reached by using a method called mode-locking, where a special absorbing section within the laser cavity helps to form pulses, with the time between adjacent pulses being controlled by the round trip time for the cavity. But such pulses from mode-locked lasers have relatively low average power levels and are relatively long (typically a few picoseconds). For optical communications applications, lasers with shorter pulses and higher output power levels need to be developed.Another potential use of mode-locked lasers is in the generation of terahertz radiation. The Terahertz part of the electromagnetic frequency spectrum lies between the spectra for visible light and for microwaves. This non-ionising radiation is able to penetrate through materials that are opaque to light, such as paper, plastic, cloth and skin / so it can be used in security and medical applications. It can detect explosives and tumours - it is safer than x-rays because it does not ionise the material through which it passes and is better at differentiating between different types of soft tissue. Terahertz waves are typically generated by conversion from pulsed light sources that presently are both large and expensive to buy and to run. One of the aims of this project is to develop semiconductor lasers that produce pulses at very high frequencies (300 to 2000 Gigahertz) with enough output power to be used to generate Terahertz and sub-Terahertz waves with much increased efficiency. Lasers are needed that emit shorter pulses at higher repetition frequencies and with higher power output levels. This need can be met by using structures already designed to emit high powers and then adapting them for pulsed operation. We shall bring together high-power semiconductor lasers with mode-locked operation to develop lasers that emit short higher-power pulses at world-record repetition frequencies. We shall also investigate structures that can be integrated with the laser that can compress the light pulses even further - and also investigate exactly how the light interacts with the semiconductor material, thereby finding optimum designs of the laser structures for specific applications.