Light Sources

Abstract

In this chapter, nine sections unfold in front of the reader the physics and details of the main types of light sources that can be found in optical laboratory. The first section summarizes specifications of the tungsten halogen lamp—the most common but not very simple device. Standard dimensions, clamping requirements, marking abbreviations, filament geometry, spectral radiometry, electrical characteristics, installation requirements and other most practical parameters help the user to quickly choose what he needs. Light emitting diodes (LEDs) are the sources of incoherent radiation in wide spectral interval determined by the energy gap of semiconductor material they use. Simple theoretical estimates explain the shape of the spectrum, while practical packaging considerations determine angular distribution. Physics of white-light and resonant-cavity LEDs is explained. Understanding of current–voltage and flux-current characteristics is necessary for efficient use of these devices. Practical rules and electrical schemes of connecting LEDs are essential to avoid typical mistakes. Laser diodes (LDs)—coherently emitting devices—are explained next. Divergence, spectral and polarization properties of LDs are compared to those of LEDs. Coherence of LD radiation results in speckle pattern that may be a serious hindrance in applications. To reduce it, the superluminescent diodes (SLD) may be used. However, both LDs and SLDs show significant astigmatism that can be corrected by means of miniature aspheric lens packages or anamorphic prisms. Electrical drivers for LDs are more complicated than those for LEDs and usually require feedback signals from embedded photodiodes. When utmost beam quality and coherence are needed, helium–neon laser must be the choice, particularly stabilized versions. Principles and practical schemes of stabilization are explained. A special type of stabilized He–Ne lasers widely used in heterodyning and interferometry is the Zeeman laser—two frequency highly stabilized source of coherent radiation. Much more complicated physics, laying in the background of this type of lasers, requires deeper physical explanation, including the Zeeman splitting and Kramers–Kronig relation. Tunable argon and helium-cadmium lasers are explained next. Among the pulsed sources, the most important is the neodymium laser. Narrow and powerful pulses of 1.06 μ radiation are bound to triggering pulses, so that understanding of the pulse sequence is important to organize measurements. Whereas lasers are the sources of coherent radiation, compact and rugged xenon flash lamps are inexpensive sources of broadband incoherent radiation of microsecond duration.

What you will face in the market: lamps, lasers, LEDs, and gas-discharge sources. Your cost-effective choice.