Laser diodes

LASER DIODESBY SHAHNEEEL SIDDIQUI

CONTENTS

INTRODUCTION SPONTANEOUS V/S STIMULATED EMISSION LASER PRINCIPLE LASER DIODE OPERATION LASING ACTION FABRY-PERROT CAVITY MATERIALS USED LASER CHARACTERISTICS OUTPUT SPECTRUM APPLICATION OF LASERS

INTRODUCTION

LASER is short for LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION A laser diode, also known as an injection laser or diode laser, is a semiconductor device

that produces light (electromagnetic radiation) through a process of optical amplification. These radiation have very special properties that makes them used in a wide variety of applications. 

It produces coherent radiation (in which the waves are all at the same frequency and phase) in the visible or infrared (IR) spectrum when current passes through it.

SPONTANEOUS V/S STIMULATED EMISSION

E1

E2

h

(a) Absorption

h

(b) Spontaneous emission

h

(c) Stimulated emission

In hOut

h

E2 E2

E1 E1

Absorption, spontaneous (random photon) emission and stimulatedemission.© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

In stimulated emission, an incoming photon with energy h stimulates the emission process by inducing electrons in E2 to transit down to E1.

While moving down to E1, photon of the same energy h will be emitted

Resulting in 2 photons coming out of the system

Photons are amplified – one incoming photon resulting in two photons coming out.

LASER PRINCIPLE

In actual case, excite atoms from E1 to E3. Exciting atoms from E1 to E3 optical pumping Atoms from E3 decays rapidly to E2 emitting h3

If E2 is a long lived state, atoms from E2 will not decay to E1 rapidly Condition where there are a lot of atoms in E2 population inversion achieved! i.e. between

E2 and E1.

LASER PRINCIPLE(CONTINUED)

When one atom in E2 decays spontaneously, a random photon resulted which will induce stimulated photon from the neighbouring atoms

The photons from the neighbouring atoms will stimulate their neighbours and form avalanche of photons.

Large collection of coherent photons resulted.

LASER DIODE OPERATION

Consider a p-n junction In order to design a laser diode, the p-n junction must be heavily doped. In other word, the p and n materials must be degenerately doped By degenerated doping, the Fermi level of the n-side will lies in the conduction band

whereas the Fermi level in the p-region will lie in the valance band.

LASER DIODE OPERATION(CONTINUED)

P-n junction must be degenerately doped.

Fermi level in valance band (p) and conduction band (n).

No bias, built n potential; eVo barrier to stop electron and holes movement

LASING ACTION

The population inversion region is a layer along the junction also call inversion layer or active region

Now consider a photon with E = Eg Obviously this photon can not excite electrons from EV since there is NO electrons there However the photon CAN STIMULATE electron to fall down from CB to VB. The active region is then said to have ‘optical gain’ since the incoming photon has the

ability to cause emission rather than being absorbed.

FOR SUCCESSFUL LASING ACTION

1. Optical Gain (not absorb) Achieved by population inversion

2. Optical Feedback Achieved by device configuration Needed to increase the total optical amplification by making photons pass through the gain region multiple

times Insert 2 mirrors at each end of laser This is termed as oscillator cavity or Fabry Perot cavity Mirrors are partly transmitted and party reflected

FABRY-PARROT CAVITY

An optical cavity, resonating cavity or optical resonator is an arrangement of mirrors that forms a standing wave cavity resonator for light waves.

Optical cavities are a major component of lasers, that provide feedback of the laser light. Light confined in the cavity reflects multiple times producing standing waves for certain 

frequencies.

FABRY-PARROT CAVITY (CONTINUED)

MATERIALS FOR LED AND LASER DIODES

LASER CHARACTERISTICS

Nanosecond & even picosecond response time (GHz BW) Spectral width of the order of nm or less High output power (tens of mW) Narrow beam (good coupling to single mode fibers)

OUTPUT SPECTRUM

APPLICATIONS OF LASERS

Optical Fiber Communication CDs & Optical Discs Laser nuclear fusion Barcode scanners Laser printing Heat treatment Garment industries

Thank you !Questions are very much welcomed