fiber optic light source for optical communication systems
The basic fiber system consists of transmitter, fiber and receiver.
The transmitter has a light source that is modulated by a suitable driving circuit depending on the signal to be transmitted.
The choice of the light source depends on the specific application.
For high-speed optical fiber communication systems with speeds higher than 1 Gbit/s, the choice of light sources is more critical.
Several basic requirements should be met by the source.
The first requirement is that it needs to emit the wavelength corresponding to the loss window of the fused quartz of the most common fiber material, that is, 1. 3um and 1. 5um windows.
This is very critical because fiber-link roads often operate at a distance of dozens of kilometers without a repeater.
For the optical power of a given wavelength, a lower fiber loss causes a larger repeater spacing.
The second requirement is high speed digital modulation.
At present, the speed of optical fiber communication system has reached 40 Gb/s and 100 Gb/s.
This requires modulation of the light source at a speed of more than 2. 5Gb/s.
To meet this requirement, two modulation methods have been developed.
The first is to directly modulated the light source at the desired speed.
The second type of modulation is the use of the LiNbO 3 external modulator.
For the second type, a light source is required to provide a stable power output.
The next very important feature of this light source is the small spectral width of the light source.
This will significantly affect the size of the dispersion proportional to the width of the source.
Splitting in the fiber leads to signal overlap and significantly reduces the bandwidth capacity of the system.
Although there are many different types of light sources, only LED (
Or laser diodes (LD)
Because of the requirements mentioned above.
Led and ld have small size, high power efficiency and many other beneficial features.
> Laser diodes (LD)
Laser is to enlarge the light by the excitation Emission of Radiation.
The laser is highly monochrome and is conceptually similar to an electronic oscillator.
The laser consists of an active medium that can provide light amplification and an optical cavity that provides the necessary light feedback.
The most common laser diode is composed of p-
N junction, powered by the injected current.
It is formed by doping a very thin layer on the surface of the wafer.
Crystals are doped to produce n-type and a p-
Type area, one on top of the other, resulting in p-n junction.
Laser diodes can be used as laser diode modules.
Some manufacturers offer a wide range of laser diode modules from continuous wave, line generator, modular, NIR and more.
The semiconductor laser uses a micro-chip of Gallium.
Arsenide or other foreign semiconductors that produce coherent light in a very small package.
The energy level difference between the guide band and the valence band electrons provides a mechanism for laser action.
High power diode laser is the most effective light transmitter.
They can also be used in laser diode instruments, enabling users to accurately control laser diode current and temperature.
They can be operated in continuous wave mode by selecting the laser drive current, or they can be modulated by using the modulation function on most drives.
To accurately stabilize the wavelength, the laser temperature can be fixed.
The active component is a solid state device that is not completely different from the LED.
In addition to the critical drive requirements, LD does have some drawbacks.
Optical properties are generally not equal to other types of lasers.
In particular, some types of coherence may be lower in length and color.
> LEDs (LED)
LED is p-of forward bias-
Where the e-n knot
H composite through the process of consciously emission led to the generation of light radiation.
The structure of the LED is similar to the structure of the laser diode, but there is no feedback cavity.
The emission of the LED is due to spontaneous restructuring, and the output of the LED is very different from the output of the LD (laser diode).
Led has the advantages of low energy consumption, long life, good robustness, small size and high reliability.
Unlike the laser diode, there is no threshold, and the output power increases smoothly as the current increases.
The output power is saturated at high current.
The total power output from the LEDs can be several milliwatts.
Since spontaneous radiation is random and appears in all directions, the output of the LED is not directional.
The angle of the output beam is usually in the 30 ° range perpendicular to the knot, parallel to the approximately 120 ° range of the knot.
In addition to fiber optic communication, LEDs are also used in many other applications such as aviation lighting, automotive lighting, and traffic signals.