Depending on the compatibility of material used and desired device and circuit characteristics, there are other possible combinations of photodetectors and amplifiers that can form a monolithically integrated front-end photoreceiver circuit. The important point to realize is that the integration involved a single step epitaxy of theHBT, from which the pin modulator was selectively defined by processing. The collector region of the HBT also serves as the i-region of the diode. The two devices were then monolithically integrated with the addition of the required passive elements. The same scheme can be conveniently used to realize a front-end photoreceiver, and the HBT is the preamplifier. Another compelling reason for using a PIN-HBT combination stems from noise and sensitivity considerations.
The sensitivity of the HBT based photoreceiver is better than a FET-based photoreceiver at high frequencies. This is because the sensitivity of a PIN-FET receiver is proportional to B3, where B is
the bit rate while the sensitivity of a PIN-HBT is proportional to B2.
The epitaxial layer structure, the equivalent circuit of a trans-impedance amplifier design, and the photomicrograph of a fabricated circuit with the active and passive element. A valuable technique to enhance photoreceiver response at high frequencies is inductive peaking, where an inductor is placed in series after the photodiode at the input of the amplifier or in other circuit locations. Digital optical communication systems require optical time-domain measurements to characterize system pulse performance. These measurements include both single and multi-valued waveforms. Multi-valued waveform measurements commonly called eye diagrams include pulse parameter masks, extinction ratio, and jitter. Eye diagrams are formed by overlaying multiple single-valued pseudo-random binary sequence (PRBS) waveforms on the displayed oscilloscope or eye diagram analyzer. In general,
the more open the eye is, the lower the likelihood that the system may mistake a “1” bit for a “0” bit or vice versa. The eye diagram opening width, the time between the zero-to-one/one-to-zero crossings, shows the time interval the signal may be sampled without error due to intersymbol interference. The eye diagram measurement is illustrated schematically. The central open region of the eye diagram is a measure of the BER
of the circuit. It gives a probability of the circuit in making correct decisions regarding incoming zeros and ones. In conclusion, the PIN-HBT photoreceiver is a simple and versatile OEIC capable of demonstrating that the dominant noise sources are the base current and the feedback resistor. Front-end photoreceivers. In the design of an optical fiber communication system, whether for use in long-distance.
A key element is a receiver in communication or for bussing data over short distances and operating at low or high data rates.The basic purpose of the receiver is to detect the incident light and convert it to an electrical signal containing the information impressed on the light at the transmitting end. Sensitivity plays a vital role in deciding the number of repeaters needed in a long-haul communication system. The receiver sensitivity is defined as the minimum amount of optical power level
needed at the receiver input so that the signal–to–noise ratio is greater than a given value.
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