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Optical Spectral Characteristics of Laser Diodes.
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Semiconductor Materials
 
 

A basic fiber optic communication system consists of essentially three components, viz. an optical source, an optical fiber of the required length, and a photodetector. The role of an optical source is to convert the input electrical signal into an optical signal. The major requirements of the optical sources are: ability to directly modulate the light intensity, compatibility to optical fibers - in terms of the operating wavelengths and power coupling, smaller spectral widths, and reasonable optical power outputs. Because of these requirements the most suitable optical sources are semiconductor Light Emitting Diodes (LEDs) and Laser Diodes (LDs).

Optical fiber communication systems have several advantages over other electrical communication systems, such as coaxial cable communication systems. The major advantages are higher bandwidth, lower signal attenuation leading to larger repeaterless spans, immunity to electromagnetic interference. Optical fibers have a few transmission windows (wavelengths) where the fiber attenuation is lower compared to others. These wavelength bands used in fiber optics are, the 850, 1300 and 1550nm bands. Today most of the telecommunication and data communication applications use the 1260 to 1675nm band (i.e. the O, E, S, C, L and U bands). The 850nm band is used mostly for short data communication links and other modest applications.


LEDs are simple, inexpensive as well as rugged sources. LEDs find use in most of the modest short length links, as well as in LANs. Laser diodes are the sources of choice for all telecommunication applications because of their higher optical powers, small spectral width, and high data rate modulation capability.

 

Optical Sources: Basic Concepts
 

Semiconductors are special materials whose conduction properties lie between those of metals and insulators. Silicon (Si) is the most commonly used semiconductor. A Si atom has four electrons in its outer shell, which are called the valance electrons. The conduction properties of a semiconductor can be explained with the help of energy band diagrams. The valence electrons occupy a band of energy levels called the Valance band, which is the lowest band of allowed states. The next higher band of allowed energy levels for electrons is called the Conduction band. The valance band and the conduction band are separated by an energy gap (or band gap Eg), where no energy levels exist. One can think of an energy band as a band consisting of allowed or possible energy levels. At absolute zero temperature with no applied electric field, all the electrons will be in the valance band. When some external energy is provided to the electrons at the valence band, some of them (the excited electrons) acquire enough energy to jump over the energy gap, leaving holes in the valance band. When an excited electron falls from the conduction band (energy level E2) to the valance band (energy level E1), it releases a quantum of energy called a photon. If ΔE = E2 – E1, then Δ = hf = hc/λ, where h = 6.626 x 10-34 J s is the Planck’s constant, c is the speed of light, and λ is the wavelength. Note that Δ could be greater than or equal to the bandgap Eg. Since many energy levels in the conduction and valence bands can participate in the radiation process many close wavelengths can be radiated. Because of this multi wavelength radiation, the light emitted by the semiconductor will have a wide spectral width, Δλ. In order to have radiation it is necessary to excite many electrons to the conduction band. This needs to be done in the form of an external energy. The most suitable way of doing this is by passing an electric current through the semiconductor.

Intrinsic semiconductors have no impurities added to them and consist entirely of atoms of one material. In an intrinsic semiconductor the number of electrons and holes are both equal to the intrinsic carrier density. When atoms of another material (dopant) are added to an intrinsic semiconductor such that the majority of the carriers are now electrons, such a material is called an n-type material. A p-type can be made by adding atoms of another material such that the majority carriers are holes. Semiconductors with the impurities (dopants) added are termed extrinsic semiconductors.

When a p-type material is brought in contact with a n-type material (in a single, continuous crystal structure) a pn junction is formed. When the pn junction is created, the majority carriers migrate across the junction resulting in recombination between electrons and holes. As a result an electric field or barrier potential develops across the junction which prevents further movement of charges. The junction area now has no mobile carriers and the region is called the depletion region or the space-charge region

 

LEDs: Principle of Operation

Semiconductors are classified as direct-band gap or indirect-band gap based on the shape of the band gap as a function of the crystal momentum k. In direct-band gap materials the maxima of the valence band and the minima of the conduction band occur at the same value of k. However, in indirect-band gap semiconductors, the maxima of valence band and the minima of the conduction band occur at different values of k. In the indirect bandgap semiconductors for electron-hole recombination to take place it is essential that the electron loses momentum such that it has a value of momentum corresponding to the maximum energy of the valence band. The conservation of momentum requires the emission or absorption of third particle, a phonon. This three particle recombination process is far less probable than the two particle process exhibited by the direct-band gap semiconductors. Thus radiative recombination is the simplest and most probable in direct-band gap semiconductors. Examples of indirect bandgap semiconductors are Ge and Si which are elemental semiconductors. GaAs, GaN, AlN, InAs, InP, etc are examples of direct bandgap semiconductors which are III-V compound semiconductors. LEDs and laser diodes are made out of direct-band gap semiconductors. A list of some LED and laser diode material mixtures together with their operating wavelength range and approximate band gap energies are given in Table 1.

Table 1

  

Material

Wavelength range, nm

Bandgap energies, eV

GaAs

900

1.4

GaAIAs

800-900

1.4-1.55

InGaAs

1000-1300

0.95-1.24

InGaAsP

900-1700

0.73-1.35

 

Alloys consisting of three elements are called ternary compounds, and four-element alloys are known as quaternary compounds. A specific operating wavelength can be selected for AIGaAs, InGaAsP, and InGaAs devices by varying the proportions of the constituent atoms. Thus devices can be tailored to emit at a selected wavelength in the 780-to 850-nm band or in any of the other transmission bands ranging from 1280 to 1675nm for glass fibers.

When an external current is passed through a pn junction made of a direct band gap material the excess minority carriers injected into the higher energy levels recombine either radiatively or non-radiatively. Radiative recombination gives rise to emission of light, each photon releasing energy of . In non-radiative recombination the released energy is dissipated in the form of heat.

 

 

 

 

 

 

Edge Emitting LED

 

The structure of an edge emitting LED consists of an active junction region and two guiding regions. The refractive index of the guiding layers is lower than that of the active region, but higher than the index of the surrounding material. This type of a structure forms a waveguide channel for the optical radiation. The radiation emits to air from the active area edge of the device. Contact stripes are made to match the edge-emitter active area compatible to optical fibers. The beam emitting from an edge emitting LED is more directional than a surface emitter. The beam is generally elliptical. This can be explained as follows. Due to waveguiding effect the beam is narrower in the plane perpendicular to the pn junction - the full width at half maximum (FWHM) is typically 25 to 35 degrees. However, since there is no waveguiding in the lateral direction, i.e. in the plane parallel to the junction, the beam in this plane is lambertian with a FWHM of 120 degrees. Please see Figure 2 for a schematic of an edge emitting LED.

 

 

 
Internal Quantum Efficiency

 

The internal quantum efficiency ηint is an important parameter of an LED. It is defined as the fraction of the electron-hole pairs that recombine radiatively. If the radiative recombination rate is Rr and the non-radiative recombination rate isRnr, then the internal quantum efficiency is the ratio is the ratio of the radaitive recombination rate to the total recombination rate. ηint is typically 50% in homojunction LEDs, but ranges from 60 to 80% in double-heterostructure LEDs.

 
Optical Power

 

If the current injected into the LED is I, then the total number of recombinations per second is I/q, where q is the electron charge. Total number of radaiative recombinations is equal to int I/q). Since each photon has an energy , the optical power generated internally by the LED is: Pint = (ηint I/q)(hν).

 

 
External Quantum Efficiency

 

The external quantum efficiency ext)of a LED is defined as the ratio of the photons emitted from the LED to the number of internally generated photons. Due to reflection effects at the surface of the LED typical values of ηout are < 10%.

 
LED Characteristics

 

Two important characteristics of a LED are its Light intensity vs. Current and Junction Voltage vs. Current characteristics. These are described briefly below.

 
i) Light Intensity (Optical Power) vs. Current

 

This is a very important characteristic of an LED. It was shown earlier that the optical power generated by an LED is directly proportional to the injected current I (current through the LED). However, in practice the characteristic is generally non-linear, especially at higher currents. The near-linear light output characteristic of an LED is exploited in small length fiber optic analog communication links, such as fiber optic closed-circuit TV.

 
ii) Junction Voltage vs. Current

 

The junction voltage vs. current characteristic of an LED is similar to the V-I characteristics of diodes. However, there is one major difference. The knee voltage of a diode is related to the barrier potential of the material used in the device. Silicon diodes and bipolar junction transistors are very commonly used whose knee voltage or junction voltage is about 0.7 V. Very often it is wrongly assumed that other diodes also have the same junction voltage. In an LED, depending on the material used its junction voltage can be anywhere between 1.5 to 2.2 Volts.

 
LED Intensity Pattern

 

So far we were concerned only about the LED as a device. For it to be used as an optical source in fiber optics communication, we need to couple the output light of the LED to the fiber in consideration. This issue was briefly discussed along with surface emitters and edge emitters. In order to couple maximum amount of light on to an optical fiber, the intensity pattern of an LED must be directional and the area of light emission should be compatible with the fiber dimensions. For short length telecommunication applications as well as LAN applications, multimode silica optical fibers with core diameters of 50 to 100 μm are commonly used. For these applications because of the small fiber dimensions, LEDs with highly directional beams are required. For simpler applications, or for short data communications (say, computer to computer within a hall) large core plastic fibers are in common use. These fibers have core diameters of 1mm or more which makes the power coupling much simpler. These fibers can accept LEDs with lesser directionality. It is important to note that in fiber optics what is important is the amount of power coupled by a source into a fiber and not its total output power.

 
LED to Fiber Light Coupling

 

LED to fiber coupling is a very major issue in optical fiber communication applications employing LEDs. Generally precision, metallic or plastic connectors, such as SMA, ST, FC/PC connectors, are used for proper alignment between the LED and the optical fiber so as to maximize the power coupled on to the fiber. In this experiment a short length of a plastic fiber having a core diameter of 1mm is used. Because of the comparatively larger size of the fiber no special fiber optic connector is required. The LED used had a cap with 1.2mm hole, centered around the emitting area. The plastic fiber is easily aligned by just pushing it through the cap and positioning it about an mm from the LED emitting area.

 
 
 
 
 

 

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