Optical Fibre Communication Experiments PDF

# Chapter 7: Optical Fibre Communication ## Experiment 7(b): Laser Characteristics ### Objective: To study LASER characteristics. ### Theory: - LASER stands for _Light Amplification by Stimulated Emission of Radiation_. - It is a light amplifying device, but it is very difficult to use as...

# Chapter 7: Optical Fibre Communication ## Experiment 7(b): Laser Characteristics ### Objective: To study LASER characteristics. ### Theory: - LASER stands for _Light Amplification by Stimulated Emission of Radiation_. - It is a light amplifying device, but it is very difficult to use as a pure amplifier. - It is mainly used as an optical oscillator. - They are widely used as an optical source in optical communication, especially in long-haul communication. - There are many types of lasers, but semiconductor lasers are the most popular because they are low power, small size and cheaper. - Semiconductor lasers are also known as _injection lasers_ of _laser diodes_. - They are made by using _heavily doped p⁺-n⁺ junction direct band gap materials_. - They usually work in the infrared region. ### Procedure: 1. Understand the two energy levels: E₁ (lower energy level) and E₂ (higher energy level). 2. Absorption: The atom absorbs a photon and moves from E₁ to E₂. 3. Spontaneous Emission: When the excited electron moves from E₂ to E₁, energy is released randomly. 4. Stimulated Emission: An excited atom emits a photon of the same energy as the incoming photons. 5. Population Inversion: For optical light amplification, more atoms should be in the E₂ state than E₁. 6. Optical Feedback and Oscillation: When there is a continuous stimulated emission and the emitted photons have similar phase and polarization, the light amplified is called "laser action". 7. To ensure coherence and maintain laser action, the emitted photons are kept inside the laser medium and an optical cavity is formed using a pair of mirrors. 8. One of the mirrors reflects the light and the other mirror allows some light to come out as the laser beam. ### Semiconductor Laser (Laser Diode LD) - Semiconductor lasers work based on the same principle as the general laser. - The mirror structure is formed by cleaving the semiconductor crystal plane. - The surfaces of the cleaved crystal create a Fabry-Perot resonator. - When semiconductor lasers are forward-biased, both spontaneous emission and stimulated emission occur. - When the current applied exceeds the threshold current, a high coherent beam of light is emitted. - The characteristics of Laser Diode are observed using a power meter and a current meter. - **Spatial Emission Characteristic**: The far-field pattern is obtained by observing the emitted pattern on a diffusing screen 11 cm away from the diode. - The near-field pattern is obtained by placing the screen 2 cm away from the diode. - **Spectral Characteristic**: The power vs current characteristics are obtained by keeping the bias voltage fixed. - Results should be observed and plotted on graphs. ## Experiment 8: Optical Detector ### Objective: To study the characteristics of optical detectors. ### Theory: - Optical detectors are devices that detect light. - They are classified into two types according to the internal gain: - **Without Internal Gain** - p-n photodiode - p-i-n photodiode - **With Internal Gain** - Avalanche Photodiode (APD) - p-i-n photodiodes work in reverse bias. - The current generated in a reverse biased p-i-n photodiode is due to both thermally-generated minority carriers and photo-generated carriers. - The reverse biased current is proportional to the light intensity. - The photodiode circuit is: - A photodiode is connected to the circuit with a resistance and two voltmeters. - The current is measured by the ammeter. - When no light is incident, the current flowing in the circuit is called the dark current. - When light is incident, the current varies with the light intensity. ### Procedure: - The p-i-n photodiode is tested. - Optical power is applied using a He-Ne Laser with 3 mW power and a variable attenuator. - The dark current, reverse biased current, power vs current characterization and the voltage vs current characteristic are measured. - The same procedure is repeated for the APD photodiode, but the power is 10 μW and the voltage is up to 130V. - The results should be observed and plotted on graphs. ## Experiment 13: Study Of Different Connectors ### Objective: - To study different types of optical fiber connectors. ### Theory: - The different connectors are developed to meet different requirements in optical fiber based communication. - Connectors should be easily attached, have low cost and low loss. - Connectors are usually plug-like in structure and are designed to align two optical fibers. - They are typically made of beige, blue or green colours. - Common connector types: - ST - SC - SMA - FCD - FC - D4 - DIN - Biconic - Connectors are usually pre-attached to the fiber end, but some are available separately and need to be attached on site. - The insertion loss of a connector is usually 1 dB per connector. ### Procedure: - Study and observe the different types of connectors mentioned above. - The different connection methods, their advantages and disadvantages should be considered. ### Precautions: - Do not bend the optical fiber to avoid bending losses. - Make sure the circuit is neat and secure. ## Experiment 12: Splice Available Optical Fiber. ### Objective: - To permanently splice the optical fiber. ### Theory: - Splicing is done to join two optical fibers. - There are two types of splicers: - Fusion Splicer: - It permanently fuses two fibers by applying an electric arc that melts the fibers together. - It is used for fiber optic splicing in OSP and premises. - It works with single mode and multi mode fibers. - Two types of core alignment: - _Core alignment_ (uses optical microscope) - _Cladding alignment_ (uses stationary V-grooves) - Mechanical Splicer: - It aligns two fibers by using epoxy resin or V-groove. - It is used for splicing ribbon fibers. ### Procedure: - Choose the appropriate splicer for the type of fiber. - Clean the fiber ends. - Cut the fiber ends at 90° using a cleaver. - Align the fiber core using an optical microscope or V-groove alignment. - Apply the electric arc for a short time. - Reinforce the splice for a stronger connection. ### Precautions: - The fiber should not be bent to avoid bending losses. - The circuit should be neat and secure. ## Result: - The experiment should be tested and the results should be recorded. - The results of different connectors and splicing methods should be compared. **Please note:** This markdown summary was generated from the OCR text of the document. The formatting is not perfect, and there may be some errors in the OCR. I did my best to capture the main ideas and structure of the original document, but you should always check the original document for accuracy. The images were omitted from the output.