This simulator inspects a magnified PIN photodiode, follows electron and hole separation, and then the transimpedance amplifier and threshold decision that recover an optical bit stream. Adjust optical power, background light, responsivity, feedback resistance, bandwidth, bit rate and threshold to find when bits are recovered, lost or clipped.
• A 3D receiver with a fiber input and coupling optics, a magnified PIN photodiode cutaway, separated electron and hole flow, a transimpedance amplifier with feedback resistor, a comparator and sampling clock, recovered-bit indicators and an electrical waveform monitor. • Eleven controls: data pattern, signal power, zero/one ratio, background light, responsivity, transimpedance, bandwidth, bit rate, decision threshold, an optical-signal enable and a marker toggle. • Twelve readouts including photocurrent, settled zero and one output levels, comparator state, last sampled decision, mid-bit samples, sample mismatches and rise time. • Four guided experiments, a verification bench, Curves & measurements and a Learn & assess tab.
Detector power is the signal plus background, and photocurrent is responsivity times that power plus a 0.002 µA dark current. The amplifier's target output is Rf times the current, Vtarget = Rf (Iphoto + Idark), and the first-order bandwidth sets τ = 1000 / (2πB) ns so the output follows Vraw(t+Δt) = Vtarget + [Vraw(t) − Vtarget] exp(−Δt/τ). The output is clipped at 1.2 V, and a bit is decided as one when it is at or above the threshold at the bit center.
Losing bandwidth makes the envelope too slow to follow individual bits so mid-bit mismatches appear. Background light lifts both levels, and with enough background they clip at 1.2 V so even zero bits read as ones.
This is a representative PIN receiver with fixed responsivity and dark current, a first-order envelope response, output clipping and ideal mid-bit sampling. There is no shot or thermal noise, avalanche gain, clock recovery or BER prediction, and bandwidth and resistance are independently specified teaching parameters. Because there is no noise, lowering the threshold can restore perfect decisions here, whereas a real receiver also requires a noise margin.
Photons absorbed in the intrinsic region create electron-hole pairs, and the electric field sweeps them apart as a photocurrent proportional to optical power. The scene shows the carriers separating.
It converts the small photocurrent into a voltage using its feedback resistor. A higher resistance gives a larger swing for the same current, while bandwidth independently limits how fast the voltage can follow the bits.
The output behaves like a first-order low-pass response. If its time constant is too long compared with the bit period, the signal cannot reach its levels in each bit and mid-bit samples start to disagree with the source pattern.
It adds photocurrent to both bit levels. With enough of it, both output levels clip at 1.2 V and the comparator reports ones even during zero bits.