Table of Contents
Applications of Ultrafast Optics
Ultrafast optics has been a very rich research field, and today short-pulsed laser systems find different applications in areas of fundamental research as well as for medical applications. Ultrafast laser systems are used for time-resolved studies in chemistry, optical frequency metrology, terahertz generation, spectroscopy and microscopy, and optical coherence tomography.
The cornerstone of ultrafast optics is the mode-locked lasers, and developments of mode-locked lasers have been a huge research field in itself. In the last few years, mode-locked lasers have moved from just offering a low-cost, rugged, and compact source of ultrashort pulses to offering state-of-the-art ultrashort pulses.
Mode-locked Lasers and Their Prospects
Mode-locked lasers are an extremely promising type of laser.
Historical Background
The history of mode-locked lasers began after the first demonstration of continuous-wave lasing in 1960; the creation of the first mode-locked laser occurred at Bell Laboratories in New Jersey. The term “mode-locking” refers to the requirement of phase locking many different frequency modes of a laser cavity. This locking results in inducing a laser to produce a continuous train of extremely short pulses rather than a continuous wave of light. Mode-locked lasers generate short pulses of intense coherent light. Laser cavities can support many different frequencies or resonant modes. A train of picosecond or femtosecond pulses can be produced by actively or passively controlling the light in the cavity so that those different resonant modes interfere.
The Concept of Mode-locking
The term mode-locking resulted from an interpretation in the frequency domain: in the mode-locked state, axial resonator modes are oscillating with a locked relative phase.
Several types of lasers are particularly attractive for mode-locking:
- Solid-state bulk lasers, based on ion-doped crystals or glasses, are today the dominant type of mode-locked lasers. They allow for very short pulses, very high pulse energies and/or average output powers, high or low pulse repetition rates, and high pulse quality;
- Fiber lasers can also be mode-locked for generating very short pulses with potential setups;
- For applications in optical fiber communications, semiconductor lasers can be built as mode-locked diode lasers;
- Dye lasers have a broad gain bandwidth, allowing for very short pulses. These lasers have been largely replaced with solid-state lasers, as these were able to deliver similar or better performance.