(Aktu Btech) Optical Communication Important Unit-3 Optical Sources

In B.Tech AKTU Quantum Book, you will discover the realm of Optical Communication. Learn how to understand this cutting-edge technology by accessing important notes, frequently asked questions, and helpful insights. Unit-3 Optical Sources

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Q1. Discuss the advantages and disadvantages of LED. 

Ans. Advantages:

• 1. Since there are no mirror facets and, in some configurations, no striped geometry, production is straightforward.
• 2. The low cost of LEDs is a result of their straightforward manufacture.
• 3. It is trustworthy since its decline is not catastrophic.
• 4. Its drive circuitry is straightforward since it typically uses smaller drive currents and is less temperature-dependent.
• 5. The LED’s light output is linear with respect to current characteristics.

Disadvantages:

• 1. Lower optical power coupled into a fiber. 
• 2. Usually lower modulation bandwidth.  
• 3. Harmonic distortion. 

Q2. Explain the structure of double heterojunction LED. 

Ans.

• 1. The principle of operation of the DH LED is shown in Fig.
• 2. The device consists of ap-type GaAs layer sandwiched between ap-type AIGaAs and an n-type AlGaAs layer. 
• 3. When a forward bias is applied, electrons from the n-type layer are injected into the p-type GaAs layer through the p-n-junction and transform into minority carriers.
• 4. When they spread from the junction, these carriers rejoin the majority carriers. As a result, photons are generated with energy that is equal to the bandgap energy of the p-type GaAs layers.
• 5. The potential barrier posed by the p-p heterojunction prevents the injected electrons from diffusing into the p-type AlGaAs layer.
• 6. As a result, only the GaAs junction layer experiences electroluminescence, which offers good internal quantum efficiency and high radiance emission.

Q3. What types of materials are used for optical sources? What are the advantages of double Hetro structure ? Compare surface emitting and edge emitting LED structures.

Ans. A. Types of materials are used for optical sources: 

• i. Types of materials used for LED (Optical source) are:
  • 1. Gallium Arsenide (GaAs)  
  • 2. Gallium Phosphide (GaP) 
• ii. Types of materials used for edge emitting semiconductor laser diode (Optical source) are:  
  • 1. Gallium Arsenide (GaAs)  
  • 2. Indium Phosphide (InP)  
  • 3. Gallium Antimonide (GaSb) 
  • 4. Gallium Nitride (GaN) 

B. Advantages:

• 1. It offers higher efficiency with low to high radiance compare to single homojunction (p – n +) LED type.  
• 2. Emitting wavelength of GaAs/AJGaAs based DH LEDs range approx. between 0.8 to 0.9 𝛍m.

C. Comparison: 

Q4. Explain the principle of semiconductor lasers and draw the emission characteristics.

Ans. A. Principle of semiconductor laser: The general operation of laser are as follows:

i. Absorption:

• 1. Quanta or photons, which are distinct energy packages, are used when light interacts with matter.
• 2. According to the quantum theory, atoms may only exist in discrete energy states; as a result, when light is absorbed or emitted, they move from one discrete energy state to another.
• 3. The frequency of the absorbed or emitted radiation f is related as the difference in energy between the higher energy state E₂ and lower energy state E₁ by the expression: 

• 4. Fig. illustrates a two energy state where an atom is initially in lower energy state E₁.
• 5. When a photon with energy (E₂ – E₁) is incident on the atom it may be excited into the higher energy state E₂, through absorption of the photon. This process is referred as stimulated absorption of photon. 

ii. Emission:

• 1. When the atom is initially in the higher energy state E₂ it can make a transition to the lower energy state E₁ providing the emission of a photon.      
• 2. The emission process can occur in two ways: 
  • a. Spontaneous emission 
  • b. Stimulated emission 

• 3. From Fig., it can be seen that, in contrast to stimulated emission, where a photon with energy equal to (E₂ – E₁) interacts with the atom in the higher energy state to cause it to return to the lower state with the creation of a second photon, spontaneous emission occurs when an atom from a higher energy state simply randomly returns to a lower energy state.  

B. Emission characteristics: 

Q5. Explain distributed feedback (DFB) laser diode.

Ans.

• 1. A typical DFB laser configuration is shown in Fig.(a). A DFB laser diode has the Bragg grating incorporated into its heterostructure in the vicinity of an active region.
• 2. The Bragg grating works like a mirror, selectively reflecting only one wavelength, 𝜆_B. This wavelength can be found from the Bragg condition given as:

where ∧ is the period of grating and n is the refractive index of the medium. 

• 3. In a distributed feedback laser diode, the word “distributed” means that reflection occurs at numerous points scattered along the active region rather than at a single point (as in Fabry Perot LDs), and the word “feedback” emphasizes that we have the ability to send stimulated photons back into an active medium.
• 4. To do this, a portion of the light is reflected at each grating slope as depicted for one beam in Fig (b). If Bragg’s condition is met, all of the portions of light reflected at each slope of this corrugated structure combine to reflect back the majority of the light.
• 5. The final output of the DFB laser diode is the wave of wavelength 𝜆_B containing the radiation of only single longitudinal mode and as a result, the spectral width of this radiation is extremely narrow, as shown in Fig.

Q6. Write down the difference between semiconductor diode and LED.

Ans. 

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