Optical Sources

History of Lasers In 1917, Einstein predicted the existence of spontaneous and stimulated emission by which an atom can emit radiation. To make use of the stimulated emission for the construction of coherent optical sources – Townes and Schawlow in the US and Basov and Prochorov in the USSR. In Maiman demonstrated the first laser.

The Einstein Coefficients  N1N1  22  hh

Example 1

Population inversion Light amplification can take place only if R st > R abs or N 2 > N 1. Under thermal equilibrium, it is N 1 > N 2. Therefore, N 2 should be increased by external means. The condition N 2 > N 1 is referred as population inversion.

Review of Semiconductor Physics

Example 2

Direct and Indirect Band Gaps

P-N Junctions p-typen-type

P-N Junctions

Non-radiative recombination

Light emitting diode (LED) LED is a forward-biased p-n homojunction or heterojunction. Radiative recombination of electron-hole pairs in the depletion region generates light. For LEDs, radiative recombination of holes and electrons is dominated by spontaneous emission and stimulated emission is negligible. The emitted light is incoherent with a relatively wide spectral width (30-60 nm) and a relatively large angular spread. For bit rates less than Mb/s together with multimode fibers, LEDs are usually the best light source. LEDs can not be used for long haul WDM systems because of their large spectral width.

LED: Light-Current Characteristics

Example 3

Laser diode (LD) Laser diodes emit light through stimulated emission while LEDs emit light through spontaneous emission. Laser diodes can emit high powers (~100 mW) and also it is coherent. A relatively narrow angular spread of the output beam compared with LEDs permit high coupling efficiency. A relatively narrow spectral width of LD makes it a suitable candidate for wavelength division multiplexing (WDM) applications.

Laser diode (LD) When a photon of energy hf impinges on the system, electrons in the valence band can be excited to conduction band. This is called absorption. Electrons in the conduction band are also stimulated to make a transition to valence band in the presence of a photon of energy hf which is equal to the band gap energy. This is called stimulated emission. Electrons in the conduction band could emit a photon of energy hf without any external stimulation and go to valence band. This is spontaneous emission. The stimulated emission exceeds absorption only if the number of electrons in the excited state exceeds that in ground state. This condition is known as population inversion. Population inversion is achieved by various “pumping” techniques. In semiconductor lasers, population inversion is accomplished by injecting electrons into the semiconductor material.

Light Amplification by Stimulated Emission for Two Level Systems  z)  z+dz) P1P1 P2P2 z z+dz

Light Amplification by Stimulated Emission

Optical Feedback and Laser Threshold

Example 4

Optical Feedback and Laser Threshold

DFB and DBR Semiconductor lasers

Laser Diode Rate Equations

Example 5