The broad bandwidth and long storage lifetimes of Nd3+: glass and ruby lasers have made possible the generation of picosecond laser pulses having peak powers it excess of one gigawatt and repetition rates in the microwave range. The numerous application areas of these pulses include research in nonlinear optics, transient response of atomic and molecular systems, optically generated plasmas, spectroscopy, ranging, optical information processing, and high-speed photography. This paper reviews several experimental techniques for generating, measuring, and utilizing these ultrashort laser pulses.

Picosecond laser pulses

Picosecond Laser Pulses

The Nd:glass laser has become one of the most useful sources of light pulses a few picoseconds in duration. In this paper, we review the results of an extensive study of the time and spectral structure of the Nd:glass laser radiation. The time structure was studied by means of two-photon fluorescence (TPF) patterns: these were scanned by a very thin (28-μ) cell containing the fluorescent dye, the fluorescence being monitored by a photomultiplier. When the Nd:glass laser is Q switched by a rotating mirror or when it is free running, we find TPF patterns fully consistent with a model where the laser emission has the character of Gaussian noise (thermal light), i.e., a model where the modes are randomly phased. When the laser was simultaneously Q switched and mode locked we made two observations : 1) the TPF patterns show that the ultra-short pulses observed previously have an overall duration of ∼8 ps, but also possess an internal substructure containing peaks 0.4-0.8 ps in duration; 2) the spectral width of these pulses is Q -switched train and expands to ∼80 cm-1in the middle of the train. This rapid spectral broadening during pulse buildup is attributed to self-phase modulation in the laser glass matrix due to a nonlinear index n 2 which we evaluate as n{2}= (2 \pm 1) \times 10^{-22} m2/V2(or 1.8 \times 10^{-13} esu). Gain limiting due to this effect and self-focusing become very important at power densities above 1 GW/cm2, presenting a serious limitation on the energy density (J/cm2), which one may hope to extract from Nd: glass laser systems.

Study of the Nd: Glass laser radiation

This paper is a tutorial and review paper on the subject of mode-locking of lasers. It is intended as an introduction to the subject for the general reader as well as an up-to-date overview for specialists in the field. Emphasis has been placed on giving physical understanding of the phenomena and processes involved, rather than on providing details of specific theories and devices.

Mode-locking of lasers,

In this paper we describe a mode-locked laser in terms of traveling pulses of light. We show that the energy absorbed by a saturable absorber is a minimum if the pulselength is a minimum, and that two pulses are essentially as favorable as one if they meet at the position of the dye cell. Under steady-state pulsing conditions, however, we show that the pulses will have a width which depends on their energy. We find that for parameters appropriate to present Nd: glass experiments, the expected length is about 10-11seconds, in agreement with observations. Finally, we demonstrate the rather surprising result that a linearly dispersive medium does not broaden the mode-locked laser pulses to first order.

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Laser mode-locking with saturable absorbers

We investigated phase‐locking effects between longitudinal modes in lasers. In order to show the general trend to be expected for a large number of oscillating modes, we treat three‐, four‐, and five‐mode oscillations. The expected phases depend in a complicated manner upon the relaxation times T1 and T2 of the medium, on the degree of inhomogeneous broadening, the mode separation and location of the medium in the cavity. Simple formation of sharp output spikes at the fundamental frequency are expected where crystals like ruby or YAG are placed near the edges of the cavity. Sharp spikes at twice the fundamental frequency are expected when these solids are placed in the center of a cavity. Certain filters, when placed near the edge or center of the cavity are expected to cause similar locking effects. Gases and solids are expected to act quite differently. The calculations are based on the maximum‐emission principle. This principle will be discussed in a later publication. Some experimental results are als...

G. A. DeMars

Papers

Self‐Locking of Modes in Lasers