The GATE Grind

GATE 2026 IN – Question 28

Communication and Optical Instrumentation · Optical sources and detectors · 1 mark · Multiple choice

The slope efficiency of a laser diode is $0.5\ \mathrm{W/A}$, and the output optical power at a current of 100 mA is 30 mW. Assuming piecewise-linear characteristics of the laser diode, the threshold current of the laser is ________ mA.

  1. 0
  2. 20
  3. 40
  4. 60

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Correct answer: (C) 40

Explanation

Above threshold, optical power is $P=\eta_s(I-I_{th})$. Therefore $I-I_{th}=P/\eta_s=0.030/0.5=0.060\ \mathrm A$.

With $I=0.100\ \mathrm A$, $I_{th}=0.100-0.060=0.040\ \mathrm A=40\ \mathrm{mA}$. The correct option is **C**.