Question

1) Class Reviews

If you sum up light transmission, absorption and scattering do you get unity (1)? If yes why, if not what is missing?

In your owns words describe the difference between phase and group velocity

From Maxwell’s equations derive the wave equation. State explicitly any assumption you

made.

How is the beating length defined of a directional coupler. Explain in your own words why a

symmetric coupler is needed to fully switch a signal from a cross-to-bar output and a

asymmetric one cannot be used?

2) Laser Analysis

a) Calculate the threshold gain of a double heterostructure (DH) edge emitting laser with an intrinsic loss a?i = 20 cm-1. Assume an overlap factor of G? = 20%. The laser is 100 um long, and the gain medium’s index refractive index is comparable to that of Silicon. Assume both facets have an equal reflection of R = 40% and outside is air.

b) Find the threshold carrier concentration N for an optical gain of g(N) = a(N-Ntr), where a = 1500 cm-1, and Ntr = 1016cm-3.

3) Modulator

Draw the bands for a Quantum Confined Stark effect. How does the optical signal modulation created?

State the equation for a phase shift-based modulator. A figure of merit (FOM) is to create a small Vp?L, meaning a small device length and low voltage to get to a p?phase shift. Name 3 design options to improve (reduce) this FOM.

4) Photodetector

In a PN junction where is light absorbed? Meaning a) in the flat-region on the p-side, or b) on the flat region on the n-side, c) neither of those, but in the deleted region where the bands bend?

Use your answer from before to explain why a P-i-N photodetector is a ‘better’ photodetector than a PN junction.

Calculate the photo-current for a detector with an quantum efficiency of 90%, a gain of 100, a responsivity of 0.7 A/W receiving an input power of 5mW

Answer 1