OPTICAL SOURCES

  • LIGHT EMITTING DIODES(LED)
  • LASERS

The development of efficient semiconductor optical sources along with low-loss optical

fibers, led to substantial improvements in fiber optic communications. Semiconductor optical

sources have the physical characteristics and performance properties necessary for successful

implementations of fiber optic systems. It is desirable that optical sources must be,

1. Compatible in size to low-loss optical fibers by having a small light emitting which are

capable of launching light into fiber.

2. Launch sufficient optical power into the optical fiber to overcome fiber attenuation and

connection losses allowing for signal detection at the receiver.

3. Emit light at wavelengths that minimize optical fiber loss and dispersion.

4. Optical sources should have a narrow spectral width to minimize dispersion.

5. Allow for direct modulation of optical output power.

Maintain stable operation in changing environmental conditions (such as temperature).

Cost less and be more reliable than electrical devices, permitting fiber optic communication

systems to compete with conventional systems. Semiconductor optical sources suitable for fiber

optic systems range from inexpensive Light Emitting Diodes (LEDs) to more expensive

semiconductor lasers. Semiconductor LEDs and laser diodes are the principle light sources used

in fiber optics.

Semiconductor sources are designed to operate at wavelengths (i.e., 850 nm, 1300 nm and 1500

nm) that minimize optical fiber absorption and maximum system bandwidth. By designing an

optical source to operate at specific wavelengths, absorption from impurities in the optical fiber

such as hydroxyl ions (OH-) can be minimized. Maximizing system bandwidth involves designing fibers and sources that minimize chromatic and inter modal dispersion at the intended

operational wavelength.

Basic LED configurations used for optical communication are

  • Surface Emitter or Front Emitter orBurrus
  • Edge Emitters

Surface Emitting LED:

High radiance is obtained by restricting the emission to a small active region within the device. A well is etched in a substrate (GaAs) to avoid the heavy absorption of the emitter radiation and to accommodate the fiber. These structures have a low thermal impedance in the active region and hence radiance emission into the fiber. Double hetero structures are used to get increased efficiency and less optical absorption. The structure of a high radiance etched well DH(Double Hetero structure) surface emitter which is also known as burrus type LED is as shown in figure

This structure emits light in band of 0.8 to 0.9 um wavelength. The plane of the active lightemitting region is made perpendicular to the fiber axis. The fiber is cemented in a well matched through the substrate of the fiber so that maximum emitted light is coupled to the fiber. Due to large band gap conjoining area, the internal absorption is less and the reflection coefficient at the back crystal face is high, hence forward radiance is good. The active area in circle is of 50μm in diameter and up to 2.5μm thick. The emission from this active area is isotropic with 120° halfpower beam width is used for practical purpose. Isotropic pattern from a surface emitter is lambertian pattern.

The source is equally bright when viewed from any direction but power diminishes as cosΦ where Φ is the angle between viewing direction and to the normal to the surface. Power is down to 50%, when Φ = 60°, so that the total half power beam width is 120°. The power coupled into multimode step index fiber may be estimated from the relationship.

PC =П(1-r)ARD (NA)2

Where, PC = Power coupled into fiber

r = Fresnel reflection coefficient

A = Emission area of source

RD= Radiance of the source

NA = Numerical aperture

Power coupled into the fiber depends on

(i) Distance and alignment between emission area and the fiber.

(ii) Medium between the emitting area and the fiber.

(iii) Emission pattern of SLED

Addition of Epoxy resin in the etched well reduces the refractive index mismatch and increases the external power efficiency of the device. Hence the power coupled in the double hetero structure surface emitters are more than Pc(optical power) that is given by equation (1), For

graded index fiber-direct coupling requires the source diameter of about one half the fiber core diameter.

Edge Emitter LED:

This LED consists of Active junction region which is the source of the incoherent light and two guiding layers.To match the typical core diameters (50-100 μm),the contact strips are 50-70μm wide. The emission pattern of Edge emitter is more directional than surface emitter.In the plane parallel to the junction the emitted beam is lambertian with a half power width of 120°.In the plane perpendicular to the junction ,the half power beam width is 25-35°.

The Edge emitter LED is as shown is fig: