In the context of the key components contained within the system, the optical fibre communication system is not markedly different to any other kind of communication system, the optical fibre communication system works through a light source that moves through a fibre channel before arriving at an optical receiver. Once received by the optical receiver, an electric signal is generated out of the modulated light.
7.3.2.1 Optical Transmitter
(Senior & Jamro, 2009) and (M. J. N. Sibley, 1995) explain that the optical transmitter is a source of light where the transmitter circuit’s drive current is altered to result in on-off keying, which is
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then used to modulate the optical transmitter’s output. The optical output power of the transmitter is then altered to varying degrees. The core component within the transmitter is the semiconductor diode. The semiconductor diode can be either a laser diode or a light-emitting diode (LED). Senior and Jamro (2009) and Sibley (1995) add that the semiconductor diode is a forward-biased diode where an optical fibre is used to align the intensity of the output light with the diameter of the semiconductor.
In order to send data across the optical channel, the optical communication system depends upon a transmitting component which contains an LED photon source. This is the primary component of the transmitting block. LEDs are categorised based on the time it takes for them to respond as well as on their wavelength. As Komine and Nakagawa (Komine, Lee, Haruyama, & Nakagawa, 2009) explain, in most cases, LEDs have a longer response time than the rate at which data is transmitted. In other cases, the rate of data transmission is often equal to the response time of the LEDs.
The optical transmitter used in this project is the HFBR-15X7Z (Appendix 9.2). The HFBR-15X7Z transmitter is a high power 650 nm LED in a low cost plastic housing designed to efficiently couple power into 1 mm diameter plastic optical fibre and 200 μm Hard Clad Silica (HCS®) fibre. With the recommended drive circuit, the LED operates at speeds from 1-125 MB/s. Some of HFBR- 15X7Z features are the data transmission at signal rates of 1 to 125 MB/s over distances of 100 meters, compatible with inexpensive, easily terminated plastic, optical fibre, and with large core silica fibre and high voltage isolation. These features allowed it to work perfectly on different applications like intra-system links, board-to-board, rack-to-rack, telecommunications switching systems, computer-to-peripheral data links, PC bus extension, industrial control, and medical instruments.
The optical transmitter system designed as part of this project is illustrated in figure (7.4), figure (7.5) and figure (7.6). In order to ensure that the system was able to achieve strong optical power able to manage a long POF cable, the author used the HFRB-15X7Z transmitter. The HFRB-15X7Z
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transmitter achieves a signal rate of between 1 MB/s and 125 MB/s through either a plastic optical fibre with a diameter of 1mm or a hard clad silica glass optical fibre with a diameter of 200 µm.
Figure 7-4 Optical transmitter layouts (Appendix 9.2)
Figure 7-5 Optical Transmitter Circuit
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The key component in all optical communication systems is the optical receiver. This is because the optical receiver is responsible for detecting the level of optical power and converting this into an electrical current that is in line with the degree of change in optical power. As Sibley (1995) points out, it is common for the light close to the receiver to be weak. This is because of signal distortions and link loss. Therefore, it is important that the photodetector chosen is appropriate to the communication system being used. This includes considerations such as high performance quality, affordability, good reliability, minimum error / low noise, a rapid response time based on the data rate needed, high sensitivity at the wavelength needed, and good efficiency when converting photons (optical power) to electrons (electrical power).
(Brundage, 2010) adds that in most cases, optical receivers represent the final destination for data sent via an optical link channel. The optical receiver contains a photodetector, preamplifier and current-to-voltage circuitry. This being said, the optical energy received is converted by the optical receiver to an electrical signal that is strong enough for other electronic components to process the signal.
Senior and Jamro (2009) and Sibley (1995) explain that the photo detector serves as a demodulator that is able to convert the optical signal into an electrical signal. The photo detector is the core component of the optical receiver and must therefore meet a certain level of performance. Photo detectors come in a wide variety of different forms, constructed with a range of different materials, and they perform various functions. However, it is the nature of the system and the needs associated with it that should determine which photo detector is used.
The HFBR-25X6Z (Appendix 9.2) is used as an optical receiver in this project is a high bandwidth analogue receiver containing a PIN photodiode and internal transimpedance amplifier. With the recommended application circuit for 125 MB/s operation, the performance of the complete data link
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is specified for 0-25 metres with plastic fibre and 0-100 meters with 200 μm HCS® fibre. A wide variety of other digitizing circuits can be combined with the HFBR- 25X6Z Series to optimize performance and cost at higher and lower data rates.
The HFBR-25X6Z receivers contain a PIN photodiode and transimpedance pre-amplifier circuit in vertical (HFBR-2536Z) blue housing, and are designed to interface to 1mm diameter plastic optical fibre or 200 μm hard clad silica glass optical fibres. The receivers convert a received optical signal to an analogue output voltage. Follow-on circuitry can optimize link performance for a variety of distance and data rate requirements. Electrical bandwidth greater than 65 MHz allows design of high speed data links with plastic or hard clad silica optical fibre.
The optical receiver system designed as part of this project is illustrated in figure (7.7), figure (7.8) and figure (7.9). The tests performed in the current study were carried out with the HFBR-2526Z optical receiver, which is appropriate for a POF cable.
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Figure 7-8 Power supply Circuit for Optical Receive (Appendix 9.2)
Figure 7-9 Optical receiver part
7.3.2.3 Plastic Optical Fibre (POF)
The core and cladding of the plastic optical fibre are made from organic polymers, which make manufacturing the optical fibre inexpensive and allow the product to be used without any difficulty. POF has been constructed out of Polymethyl Methacrylate and Fluorinated Acrylic (PMMA) more recently. This kind experiences 110 dB/km losses in the visible wavelength. As Senior and Jamro
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(2009) and Sibley (1995) explain, due to absorption and Rayleigh scattering (linked to polymers’ anisotropic structure and density fluctuations) its loss type is similar to that of glass fibre.
The HFBR (Appendix 9.3) fibre optic cable has been used in this project to transfer data between the optical (transmitter/receiver). The HFBR-R of plastic fibre optic cables is constructed of a single step-index fibre sheathed in a black polyethylene jacket. The HFBR fibre optic cable is compatible with Avago Versatile Link Family of connectors and fibre optic components. It has 1 mm diameter with 0.22 dB/m typical attenuation.
For the purposes of data transfer in this project, the author used 10 metres long cable in order to send data from the optical transmitter to the receiver. This is illustrated in figure (7.10).
Figure 7-10 HFBR-R optical fibre (Appendix 9.3)