High-resolution Interferometric Diagnostics for Ultrashort Pulses

2. BACKGROUNDcrometer scale features, and is finding applications in both technology and medicine.Ultrashort pulses are generally produced as a periodic train, which can be engineered to exhibitexceptional timing stability. The Fourier spectrum of such a train consists of a series of preciselydefined lines. Such frequency combs [12] have enabled revolutionary measurements of opticalfrequencies with a relative uncertainty of less than 10 −15 [13, 14].2.1.2 Need for ultrashort pulse characterisationThe measurement of ultrashort pulses, the topic of this dissertation, is a challenging task becausethe response time of electronics is tens of picoseconds at best, meaning that the temporal featuresof an ultrashort pulse, if measured directly, are blurred beyond recovery. However, ultrashortpulse characterisation is an essential component of ultrafast science and technology, because i) itilluminates the physics of ultrashort pulse generation and interactions with matter, and ii) evenwhen the physics is understood in principle, the complexity of most ultrashort pulse generationmethods combined with the sensitivity of the experiments and applications means that characterisationis a practical necessity if the entire system is to be understood, controlled and repeatable.Metrology is essential for ultrashort source development. An estimate of the pulse duration isnecessary to establish that the source is performing as desired, and gives a simple parameter tooptimise. A more detailed description of the field provides insight into the workings of the sourcethus revealing the obstacles to further pulse shortening, enabling further improvement. Therefore,ultrashort sources and appropriate characterisation technology have progressed in tandem, eachmotivating and supporting the development of the other.2.1.2.1 Metrology and the quest for shorter pulsesThe first generation of sub-100 ps lasers, using Nd:glass, Nd:YAG, or ruby as gain media, stimulatedthe development of the picosecond streak camera, which records the temporal intensity I (t )of the pulse [15]. The streak camera works by applying a time-dependent displacement to photoelectronsproduced by the pulse using a rapidly varying electric field. An image of the photoelectronstherefore has a time-to-space mapping. Streak camera measurements revealed distributionsof pulse durations and energies, as well as satellite pulses and substructure [16, 17], leading to im-8