What are Fiber Optic Receivers?
In an optical fiber communication system, the data is transmitted in the form of light signals. The light generated using an LED or laser diode from a fiber optic transmitter is sent through an optical fiber cable and reaches the receiver. A fiber optic receiver is a device that converts an optical signal into an electrical signal. It is a crucial component in a fiber optic communication system, as it allows the transmission of data over long distances through optical fibers. This fiber optic receiver consists of an optical detector or a photodetector, and a low-noise amplifier. The incoming light signal from the single-mode or multi-mode optical cable reaches the fiber optic receiver. The optical detector decodes this binary information and converts it into an electrical signal. The low noise amplifier inside the receiver increases the signal level suitable for processing.
Construction and working
The incoming light received by the fiber optic receiver is converted into electronic pulses by the photodetectors. These electrical signals from the detector are then processed by the limiting amplifier in the receiver. Limiting amplifiers amplify the received signal to a level that can be detected and processed by the electronic circuits in the receiver while limiting its amplitude to a specific range. They generate a suitable square wave which can then be processed in any logic circuitry. The received signal may undergo further signal processing before the data from the fiber optic receiver is passed on. The block diagram of a fiber optic receiver is shown in figure 1.
Components of Fiber Optic Receiver
Figure 1: Block diagram of fiber optic receiver
An optical detector in a fiber optic receiver is a device responsible for converting the optical signal propagated through the fiber into an electrical signal capable of being processed by the electronic circuits within the receiver. The generated electrical signal is proportional to the intensity of the incident optical light. The optical detector is typically a photodiode, which is a semiconductor device that generates an electrical current when light is incident upon it.
When the optical signal arrives at the detector, it causes the electrons in the photodiode to be excited, which in turn creates a flow of electrical current. The amount of current generated is proportional to the intensity of the optical signal, allowing the photodiode to convert the optical signal into an electrical signal with an amplitude that reflects the original signal's strength.
The electrical signal generated by the photodiode is typically very weak and requires additional amplification by an electronic circuit, such as a trans-impedance amplifier, before it can be processed further. The electronic circuitry also may include filters, amplifiers, and other components that are designed to enhance the performance of the receiver.
In fiber optic receivers, a variety of optical detectors or semiconductor photodetectors are used such as positive-negative junctions (p-n) photodiode, a positive-intrinsic negative (p-i-n) photodiode, an avalanche photodiode, or metal-semiconductor-metal (MSM) photodetectors.
The amplifier in a fiber optic receiver is responsible for amplifying the electrical signal generated by the photodetector, which converts the optical signal to an electrical signal. Fiber optic receiver uses low-impedance or trans-impedance amplifiers. When low-impedance amplifiers are used, bandwidth and receiver noise decreases with resistance. For trans-impedance amplifiers, the gain of the amplifier affects the bandwidth of the receiver. Photodiode and other sensor current outputs can be converted into a usable signal output in the form of voltage by using transimpedance amplifiers. It provides simple linear signal processing using an operational amplifier and a resistor for dissipating current.
Figure 2: Limiting amplifier
A limiting amplifier is another type of amplifier used in fiber optic receivers. The primary function of a limiting amplifier is to limit the amplitude of the electrical signal to a specific range, making it easier to process and reducing the effects of noise and distortion. A limiting amplifier is shown in figure 2.
The output signal processing and control circuitry in a fiber optic receiver are essential components that work together to convert the amplified electrical signal from the amplifier into a usable output signal.
This generally consists of a number of electronic circuits that are designed to filter, equalize, and amplify the electrical signal from the optical detector. The goal of this block is to remove any distortions or noise that might have been introduced during transmission through the fiber or amplification and to produce a high-quality electrical signal that accurately represents the original data.
The output signal processing block may include equalizers to compensate for the frequency-dependent loss in the fiber, filters to remove noise and interference, and amplifiers to increase the strength of the electrical signal. It may also include a clock recovery circuit that extracts the timing information from the electrical signal to synchronize the receiver with the transmitted data.
The control circuitry is responsible for controlling and monitoring the various components of the receiver to ensure that it operates correctly. This includes a microcontroller or a digital signal processor that communicates with the other blocks of the receiver to perform tasks such as gain control, temperature compensation, and fault detection.
The control circuitry block may also contain a feedback loop that adjusts the performance of the receiver based on the quality of the received signal. For example, if the signal is weak, the control circuitry may increase the gain of the amplifier to boost the signal strength. And if the signal is too strong, the control circuitry may reduce the gain to prevent the signal from becoming distorted.
Applications
Fiber optic receivers are mostly used in telecommunications. They are used in long-distance communication systems to transmit data over vast distances with minimal loss of signal strength. This is achieved by converting the electrical signals into optical signals, which are then transmitted through a fiber optic cable. At the receiving end, the optical signals are converted back into electrical signals, which can then be decoded and understood by the receiving device.
Fiber optic receivers are also used in the internet and cable TV industries. They are used to transmit high-definition video and audio signals over long distances without any loss of quality. This is particularly useful in the case of cable TV as it allows for the transmission of multiple channels of high-definition television without the need for multiple cables.
Fiber optic receivers are also used in the medical field. They are used in endoscopy, which is a medical procedure that allows doctors to view the inside of the body without the need for surgery. The optical signals are transmitted through a small fiber optic cable, which is inserted into the body. At the receiving end, the signals are converted into images that can be viewed by the doctor.
