A random laser is a non-conventional laser whose feedback mechanism is based on disorder-induced light scattering. Depending on whether the feedback supplied by scattering is intensity feedback or amplitude feedback, random lasers are classified into two categories: random lasers with incoherent feedback and random lasers with coherent feedback. A brief survey of random lasers with incoherent feedback is presented. It is followed by a review of our recent experimental work on random lasers with coherent feedback, including measurement of the lasing threshold, lasing spectra, emission pattern, dynamical response, photon statistics, speckle pattern and the investigation of relevant length scales. Large disorder leads to spatial confinement of the lasing modes, that is the foundation for the micro random laser. Some theoretical models of random lasers with coherent feedback are briefly introduced. The study of random lasers improves our understanding of the interplay between light localization and c...

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Lasing in random media

Various models of a passively mode-locked quasi-continuous laser are discussed and the evolution of pulses in various cavity configurations is traced. In certain circumstances, the combined action of amplifier and absorber saturation is shown to lead to rapid pulse compression even when the pulse duration is far shorter than the relaxation time of either nonlinear component. The results are in good agreement with recent streak- camera measurements involving the pulsed Rhodamine 6G dye laser and satisfactorily explain the efficient generation of picosecond pulses in CW dye-laser systems. The same pulse-compression mechanism can probably be used to generate ultrashort pulses in other quasi-continuous lasers.

Pulse evolution in mode-locked quasi-continuous lasers

The emergence of new ultrafast optical modulation techniques has opened the way towards a new femtosecond laser technology based on solid-state gain media. The authors address the requirements for stable ultrashort pulse generation in these novel femtosecond sources. The theoretical considerations are backed up by experimental results obtained with a number of different laser systems. The conclusions drawn from the presented theoretical and experimental investigations provide general guidelines for the design and optimization of a wide range of femtosecond solid-state laser oscillators. >

Femtosecond solid-state lasers

The ultrashort pulse-forming properties of lasers are reviewed in terms of the master equation time-domain description of mode locking. The pulse shortening strengths and steady-state operating characteristics of a broad range of modern experimental systems are discussed within the framework of the classic slow-saturable-absorber and fast-saturable-absorber analytical models.

Principles of passive mode locking

The generation of intense ultrashort light pulses in mode-locked laser systems has made possible a wide range of new experiments designed to study the interaction of light with matter. For the quantitative interpretation of the results, accurate measurement of the optical pulse structure is essential, and it is the purpose of this paper to review all the diagnostic techniques currently available. The recent rapid development of the electron-optical streak camera is highlighted, while considerable space is devoted to an extensive description of the many second- and higher order correlation measurements (including the popular two-photon fluorescence method). A discussion of ultrafast shutter techniques is also included, together with a section on pulse chirping and dynamic spectroscopy.

Ultrashort pulse measurements

This paper presents a theoretical treatment of the fluctuation mechanism involved in the generation of picosecond laser pulses with saturable absorbers. The processes responsible for the shortening of the pulsewidth and for selection of the most intense pulse are treated. Some experimental results that confirmed the treatment are presented. The influence of inertia of saturable absorber and nonlinear losses (self-focusing and self-modulation) is discussed.

V. Letokhov

Papers

Fluctuation mechanism of ultrashort pulse generation by laser with saturable absorber

Selective two-step (STS) photoionization of atoms and photodissociation of molecules by laser radiation

Laser action in stellar atmospheres