Gas state lasers

Table of contents

Abstract 05
1.1 Introduction 05
1.2 Basic principles 06
1.2.1 Line broadening mechanisms in gases 07
1.2.2 Saturation irradiance 09
1.2.3 Threshold operation 09
1.3 Basic structures of gas state lasers 10
1.3.1 Gas laser media 10
1.3.2 Optical resonators 12
1.3.3 Pumping techniques 13
1.3.4 Emission characteristics 13
1.3.4.1 Coherence 13
1.3.4.2 Divergence 13
1.3.4.3 Coherence length 14
1.3.5 Pulsewidth control 14
1.3.6 Wavelength selection and frequency control 15
1.4 Types of gas lasers 16
1.4.1 Helium-neon 16
1.4.2 Ion gas lasers 16
1.4.3 Excimer lasers 20
1.4.4 Molecular lasers 23
1.4.4.1 Carbon dioxide 23
1.4.4.2 Carbon monoxide 25
1.4.4.3 Nitrogen 26
1.4.4.4 F2 27
1.4.5 Metal vapour lasers 27
1.4.5.1 He-Cd 27
1.4.5.2 Copper and gold vapour 27
1.5 Perspectives 29
References 30

Abstract

A brief review of the structures and operation of gas lasers of particular interest in photobiology is given in this chapter along with details of their emission characteristics. Gas lasers provide laser wavelengths over a very broad range from the vacuum UV to the far-IR in continuous wave and pulsed operation and from low to high powers. Many of these wavelengths have found valuable application in photobiology, particularly in the several forms of fluorescence technique, microscopy, imaging and Raman spectroscopy, as well as in such techniques as optical trapping and tweezers, micromanipulation and microdissection. Extensive use has been made of the He-Ne laser (wavelength 632.8 nm), argon ion laser (488.0 and 514.5 nm), krypton ion (647.1 nm), He-Cd laser (441.6 and 325.0 nm), excimer lasers (ArF 193 nm, KrF 248 nm, XeCl 308 nm) and nitrogen laser (337.1 nm). The ion and excimer lasers have application in the pumping of tunable dye and titanium-sapphire lasers. Since the discovery of the first gas laser in 1961 many gas lasers have been devised, a detailed understanding built up of their operation and performance and the commercial technology has reached a highly developed and mature state.

In recent years solid-state alternatives to several of the common gas lasers have been developed based on diode lasers, diode pumped solid-state lasers and fibre lasers which offer advantages of compactness and efficiency and operation from low voltage power supplies. However, solid-state substitutes are not presently available at many of the useful gas laser wavelengths, such as the high power pulsed output from excimer lasers in the UV, high power continuous wave output in the near UV, visible and near IR, and mid-and far-IR wavelengths from molecular gas lasers.

1.1 Introduction

The interaction of laser light with biological samples gives an optical signal which carries information on the composition, structure and function of the sample. The probe laser light may be used in the basic measurements of absorption, fluorescence and phosphorescence emission, and Rayleigh and Raman scattering. In this there is a remarkable variety of techniques which have evolved with important applications in photobiology: spectral microscopy and imaging, scanning confocal microscopy, optical labelling, ultrafast pulse and fluorescence lifetime spectroscopy, fluorescence probe (marker) spectroscopy, fluorescence recovery after photobleaching (FRAP), fluorescence in situ hybridization (FISH), fluorescence resonance energy transfer (FRET) and optical trapping and tweezers [1–4]. Special attention can be drawn to the use of fluorescence in several of these techniques with the use of spectral (continuous), dynamic (pulsed) and imaging methods. These applications include laser scanning confocal fluorescence microscopy, time-correlated single-photon counting lifetime studies, total internal reflection fluorescence, near-field scanning fluorescence optical microscopy (NSOM), fluorescence correlation spectroscopy and multi-photon fluorescence microscopy [4]. The Raman scattering technique in its various forms has widespread application in photobiology through its sensitivity to molecular vibrations. Surface enhanced Raman scattering derives an enhanced Raman signal from molecules attached to surfaces to give molecular structural information. Coherent anti-Stokes Raman scattering microscopy enables the identification of molecular constituents of cells without the use of dye markers.

Gas lasers provide source radiation for many of these techniques. In particular, these include the argon (514.5 and 488 nm) – and krypton (647.1 nm) – ion lasers at medium power levels and which provide UV, visible and near-IR wavelengths; the He-Cd laser (441.6 and 325.0 nm), excimer lasers (ArF 193 nm, KrF 248 nm, XeCl 308 nm), N2 lasers (337.1 nm) and F lasers (157 nm). The ion lasers and excimer lasers are also used as pump sources for the tunable liquid dye and solid-state titanium-sapphire lasers. In addition there are several gas lasers which have more specialised applications in photobiology.

In this chapter the basic principles of laser operation are briefly reviewed and some of the laser...