Unidirectional travelling wave operation of a mode-locked Nd:glass ring laser
Abstract The development of unidirectional mode-locked pulses in a Nd:glass ring laser is studied for various combinations of active and passive mode-locking and pulse direction discrimination elements.
The generation of ultrashort laser pulses
Ultrashort laser pulses are finding increasing application in numerous branches of science and technology. This article is an attempt to provide a comprehensive review of the whole field of ultrashort pulse generation, covering both the experimental and the theoretical aspects of the subject. By far the commonest method of generating pulses in the picosecond and subpicosecond regime is the technique known as 'mode-locking' and it is with this that the review is therefore largely concerned. After introductory remarks and a simple treatment of some of the general principles, the central sections of the review deal with the main generic techniques, namely spontaneous mode-locking, active mode-locking (including mode-locking by synchronous pumping) and passive mode-locking. Within each section the material is organised into three main categories, namely (a) general background, (b) historical survey (mainly relating to experimental work and densely packed with references to published work) and (c) theoretical methods (presented with a strongly pedagogical flavour). A major section at the end deals with a mass of important (and mostly recent) work that does not fit naturally into the main framework of the review. This includes USP generation in semiconductor lasers, a number of special methods such as injection mode-locking (and others where two or more mode-locking techniques are combined), as well as chopping and switching techniques.
Atmospheric drag effects on the KOMPSAT-1 satellite during geomagnetic superstorms
We examine the atmospheric drag on the low earth-orbiting satellite, KOMPSAT-1 in a sun-synchronous orbit at ~685-km altitude starting in 1999, during a 3-month (October-December) period in 2003. This 3-month interval includes the October 29-30 and November 20 magnetic superstorms and weak to moderate storms. We observed that the daily KOMPSAT-1 drag acceleration transiently responses to transient storm-time disturbances. That is, there is an one-to-one correspondence between the KOMPSAT-1 drag accelerations and the storm events. We find that the drag acceleration correlates strongly with the level of geomagnetic disturbance. This indicates that the trajectory of KOMPSAT-1 is significantly perturbed during extremely disturbed intervals because of atmospheric density increase. The main contributor to the density increase is Joule heating associated with the geomagnetic activity rather than the solar EUV radiation, as reported by previous studies. We suggest that understanding how the upper atmosphere responses to the geomagnetic-associated heating is important to predict space weather impacts on low earth-orbiting satellites.