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 radiation characteristics. Those gas lasers of particular value in photobiology are described to give a survey of their design features and emission properties.

1.2 Basic principles

Before the invention of the laser, available light sources emitted light from thermal excitation such as a tungsten filament lamp with a spectrum corresponding to a quasi-black body emitter at the temperature of the emitter, or by spontaneous emission from atoms or molecules as in a gas discharge arc or fluorescent tube. Their brightness is limited by the temperature of the emitter. In the laser the essential process of stimulated emission was introduced, so that, in conjunction with population inversion, a net optical amplification (or gain) could be created. A brief description of the principles of operation of lasers is given here; extensive discussion can be found in various texts [5–8].

Stimulated emission was first used in 1953 to create a high brightness source from transitions between the two lowest levels of ammonia molecules, giving a very narrow emission line at a wavelength of 12.6 mm; this was termed the maser since the wavelength was in the microwave region.

The basic elements of a laser are an active medium with suitable energy levels, a means of injecting energy into the medium (a process known as pumping) and a resonator in which the amplification process can occur. In a 2-level system interacting with light radiation with which it is in resonance, the processes of absorption, spontaneous emission and stimulated emission can occur. The light wave can be absorbed or be amplified depending on the population densities N1 and N2 of the two levels. For a wave travelling in the z direction the change in irradiance of the wave over a length dz is dI = γIdz. For an initial irradiance I0 at z = 0, the beam intensity varies along the propagation distance z exponentially as I(z) = I0eγz. The unsaturated gain or loss coefficient γ of the medium at frequency v is

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

Here g1and g22 are the degeneracies of the two levels and A21 is the Einstein A-coefficient, related to the lifetime τ of the upper level as A21 = 1/τ. The quantity g(v) is the normalized spectral function (or lineshape function) for the transition, such that

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

The gain coefficient γ(v) can be > 0 if N2 >g2/g1N1. In this case the irradiance grows exponentially. The gain coefficient varies as v-2, making it easier to achieve higher gain in the infra-red than in the ultra-violet, although some cw UV lasers and pulsed UV lasers are available. Spontaneous emission which scales as v3 competes with stimulated emission.

A laser species may be characterized by its stimulated emission cross-section σ0. This is related to the gain coefficient γ(v) as

γ(v) = (N2 - g2/g1 N1)σ0

This leads to

σ0 = c2A21g(v)/8πv2

i.e. the stimulated emission cross-section depends on the characteristics of the emitter.