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

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

View Full Document Right Arrow Icon
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 fi 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 fi 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 fi er tubes and later solid-state devices such as transistors. Until the 1950’s 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 fi rst ampli fi er based on discrete energy levels (quantum ampli fi er) was the MASER (Microwave Ampli fi 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
Image of page 1

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

View Full Document Right Arrow Icon
294 CHAPTER 7. LASERS The ampli fi cation should arise from stimulated emission between discrete en - ergy levels that must be inverted, as discussed in the last section. Ampli fi ers and oscillators based on this principle are called LASER (Light Ampli fi cation by Stimulated Emission of Radiation). Maiman was the fi rst to demonstrate a laser based on the solid-state laser material Ruby. Figure 7.1: Theodore Maiman with the fi rst Ruby Laser in 1960 and a cross sectional view of the fi rst device [4]. The fi 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 fi eld will build up to an intensity approaching, or even surpassing, the saturation intensity of the ampli fi er medium by many times, until the ampli fi 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
Image of page 2
295 7.1. THE LASER (OSCILLATOR) CONCEPT Image removed for copyright purposes.
Image of page 3

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

View Full Document Right Arrow Icon
Image of page 4
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern