In optical fiber communications, avalanche photodiode (APD) receivers can detect valid data hits when approximately how many photons are received per bit interval?

Introduction / Context:
Avalanche photodiodes (APDs) are semiconductor light detectors used in optical fiber links to sense very weak optical signals. By operating with internal gain (avalanche multiplication), an APD can register the presence of a small number of photons during a bit interval. Understanding rough photon-count sensitivities helps learners compare device classes (PIN vs. APD) and appreciate link budget design.

Given Data / Assumptions:

• The question seeks an order-of-magnitude photon count associated with reliable hit detection in an APD receiver.
• We consider typical telecom wavelengths and realistic bit intervals, not single-photon quantum detectors.
• Receiver includes practical noise sources (dark current, thermal noise) and a decision threshold.

Concept / Approach:
An APD provides internal gain M (for example, 10–100), improving sensitivity compared to a PIN diode. While exact sensitivity depends on data rate, wavelength, noise bandwidth, temperature, and coding, educational references often quote that APD-based receivers can detect on the order of ~100 photons per bit (sometimes lower with coding and optimal conditions). Hence, among the given choices, 100 photons is the best representative threshold for a viable hit at low BER in a simplified exam context.

Step-by-Step Solution:

Verification / Alternative check:
Convert receiver sensitivity (for example, −40 dBm at a certain BER) to photons per bit using energy-per-photon E = h * c / λ and bit time T = 1 / R. This cross-check shows that photon counts on the order of tens to hundreds per bit are consistent for APD-based designs at moderate rates.

Why Other Options Are Wrong:

• 200/300/400 photons: Possible in noisier or higher-rate scenarios, but do not capture the lower limit advantage of APDs emphasized in textbook comparisons.
• None of the above: There is a plausible correct value listed (100 photons).

Common Pitfalls:
Treating the quoted number as universal; in practice, coding gain, temperature, and target BER shift required photon counts. Always refer to datasheets and system link budgets for exact figures.

Final Answer:
100 photons