Laser Stabilization

Victor Zhao
Stony Brook Laser Teaching Center
September 2008
Advisors: Dr. Sam Goldwasser and Dr. John Noe

Motivation

Background

Lasers are useful for their properties of coherent, monochromatic light. Because of their monochromaticity, the operating frequencies of many lasers are restricted to frequency ranges around the operating atomic transition of a particular laser type. Yet it is useful to stabilize a laser's operating frequency to a even smaller variation through a variety of techniques.

In particular, a gas laser consists of a resonant cavity--an enclosed tube with highly reflective mirrors at both ends--in which a gas gain medium is enclosed and light emitted by excited atoms travels and builds in between the two mirrors. For the light traveling in between the two mirrors inside the resonant optical cavity to amplify properly, the wavelength of the resonating light needs to be a divisible half-wavelength of total length of the laser tube. The possible resonating wavelengths, which are called resonant modes, in the tube are thus affected by the actual lenght of the laser tube. How much light is produced from a resonant mode is dependent on how much gain the medium inside the tube can provide, which is centered around an atomic transition, but is wider due to effects such as doppler-broadening, the shifting in wavelength produced by transitioning atoms because of their gaseous motion. The formula describing the wavelength of a resonant mode thus follows:

		[lambda] = (2*L) / n

Lambda is the wavelength of the resonant mode, L is the length of the resonant optical cavity, and n is the mode number and is very large to produce a wavelength within the gain curve, which indicates how much a frequency of light is amplified by a particular atomic transition. The individual resonant modes that travel within the laser tube and are output have wavelengths much smaller than the width of the gain curve, about [..]. Since the length of a laser tube is the determining factor for the frequency of a resonant mode and therefore the frequency of light output by the laser, many laser stabilization techniques seek to control the length of the laser tube and therefore the wavelength of the resonant modes.

One utilized method of controlling laser tube length is through feedback-controlled heating of the tube, which prevents minute changes in laser tube length due to temperature changes which nonetheless cause significant variations in laser output frequency.

There are many applications of the laser which depend on a precise operating frequency much narrower that of a normal laser. Such lasers have uses in metrology, the science of measurement, where they are used to measure precise lengths, define units of measurement, and measure minute gravitational effects.

Setup

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Measurements

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