chapter7

chapter7 - Chapter 7 Lasers After having derived the...

Info iconThis preview shows pages 1–4. Sign up to view the full content.

View Full Document Right Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: Chapter 7 Lasers After having derived the quantum mechanically correct suszeptibility for an inverted atomic system that can provide gain, we can use the two-level model to study the laser and its dynamics. After discussing the laser concept brie y we will investigate various types of gain media, gas, liquid and solid-state, that can be used to construct lasers and ampli f ers. Then the dynamics of lasers, threshold behavior, steady state behavior and relaxation oscillations are discussed. A short introduction in the generation of high energy and ultrashort laser pulses using Q-switching and mode locking will be given at the end. 7.1 The Laser (Oscillator) Concept Since the invention of the vacuum ampli f er tube by Robert von Lieben and Lee de Forest in 1905/06 it was known how to amplify electromagnetic waves over a broad wavelength range and how to build oscillator with which such waves could be generated. This was extended into the millimeter wave re- gion with advances in ampli f er tubes and later solid-state devices such as transistors. Until the 1950s thermal radiation sources were mostly used to generate electromagnetic waves in the optical frequency range. The gener- ation of coherent optical waves was only made possible by the Laser. The f rst ampli f er based on discrete energy levels (quantum ampli f er) was the MASER (Microwave Ampli f cation by Stimulated Emission of Radiation), which was invented by Gordon, Townes and Zeiger 1954. In 1958 Schawlow and Townes proposed to extend the MASER principle to the optical regime. 293 294 CHAPTER 7. LASERS The ampli f cation should arise from stimulated emission between discrete en- ergy levels that must be inverted, as discussed in the last section. Ampli f ers and oscillators based on this principle are called LASER (Light Ampli f cation by Stimulated Emission of Radiation). Maiman was the f rst to demonstrate a laser based on the solid-state laser material Ruby. Figure 7.1: Theodore Maiman with the f rst Ruby Laser in 1960 and a cross sectional view of the f rst device [4]. The f rst HeNe-Laser, a gas laser followed in 1961. It is a gas laser built by Ali Javan at MIT, with a wavelength of 632.8 nm and a linewidth of only 10kHz. The basic principle of an oscillator is a feedback circuit that is unstable, i.e. there is positive feedback at certain frequencies or certain frequency ranges, see Figure 7.2. It is the feedback circuit that determines the frequency of oscillation. Once the oscillation starts, the optical f eld will build up to an intensity approaching, or even surpassing, the saturation intensity of the ampli f er medium by many times, until the ampli f er gain is reduced to a value equal to the losses that the signal experiences after one roundtrip in the feedback loop, see Figure 7.3 295 7.1. THE LASER (OSCILLATOR) CONCEPT Image removed for copyright purposes....
View Full Document

This note was uploaded on 12/02/2010 for the course ECE 6.641 taught by Professor Zahn during the Spring '09 term at MIT.

Page1 / 50

chapter7 - Chapter 7 Lasers After having derived the...

This preview shows document pages 1 - 4. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online