AN INTRODUCTION TO LASER DIODES

The Technical Fundamentals and Basic Physics

semiconductor-laser

Technical Introduction to Laser Diodes

Author: Dr. Matthias Pospiech

Author: Sha Liu

University of Hannover, Germany, Department of Physics

Publish Date: May, 2004

  1. 1.1 Band structure of the semiconductor
  2. 1.2 Recombination
  3. 1.3 State density
  4. 1.4 Direct band gap and indirect band gap
  5. 1.5 Optical feedback mechanism
  1. 2.1 Heterostructures
  2. 2.2 Threshold current
  3. 2.3 Oscillation modes
  1. 3.1 Real lasers
    1. 3.1.1 Gain-guided lasers
    2. 3.1.2 Index-guided lasers
    3. 3.1.3 Gain vs. index-guided structure

  2. 3.2 DBR and DFB Lasers
    1. 3.2.1 Distributed Bragg Reflector (DBR) Lasers
    2. 3.2.2 Distributed Feedback (DFB) Lasers
    3. 3.2.3 DFB vs. DBR Lasers

  3. 3.3 Laser arrays
  4. 3.4 VCSEL (Vertical Cavity Surface Emitting Lasers)
405nm-diode-laser

Introduction

Laser diodes have grown to be a key component in modern photonics technology. This article provides a general introduction in the physics and technology of laser diodes. First, the physical concepts behind laser diodes are explained. In the following an example of an AlGaAs laser is given and analyzed in detail. The last part introduces technological approaches to build competitive laser diodes. The focus is thereby on single mode laser diodes. We consider ways to introduce a waveguide in the laser diode and concepts to make the laser diode wavelength selective. Briefly, we take a look at laser diode arrays for use in high power applications and finally we introduce VCSEL’s (Vertical Cavity Surface Emitting Lasers).

Comparison with other laser systems

The following features distinguish laser diodes from other lasers:

  • Compact. They are built on one chip that contains everything necessary for a laser. This enables semiconductor lasers to be easily inserted in other instruments.

  • High efficiency up to 50%. This enables them to be driven by low electrical power compared to other lasers.

  • Direct excitation with small electric currents, so that conventional transistor based circuits can supply the laser.

  • Possibility of direct modulation with applied current.

  • Small beam waist.
Low costs due to mass production and high reliability made them a key component in various applications. However, they also have some disadvantages. They are highly sensitive to temperature. Although this allows wide wavelength tunability it is unwanted for most applications. Another important unwanted feature is their highly divergent beam. [1]