Optical and Semiconductor Laser Fundamentals: Threshold, Spectrum, and Photodetectors

Is the photon energy of the laser emission larger than the bandgap energy?

Yes, it is larger.

How are the partial reflectors for optical feedback obtained in a semiconductor laser diode?

By cleaving the crystal along crystallographic planes.

What happens to the optical spectrum before and after the threshold condition is met?

Before threshold: very broad spectrum; After threshold: very narrow spectrum.

What happens to the magnitude of the optical gain to externally injected photons when operating above threshold?

It remains constant, known as gain clamping.

What is the slope efficiency of a laser diode?

The ratio of the increase in optical output power to the increase in injection current above the threshold point.

Is a small or large threshold current beneficial for a battery-operated DVD player?

A small threshold current is beneficial.

What happens to output power if a laser diode heats up while operating at a constant current?

Output power decreases.

What happens to the laser wavelength when it heats up at a constant current?

Wavelength increases.

What is the shape of the far-field beam if a waveguide is 10 times wider than it is tall?

The beam will be elliptical and 'flipped'.

Do you get a higher modulation frequency if the input current is much larger than the threshold?

Yes, much larger input current results in a higher modulation frequency.

Do you see a reading on an ammeter connected to a photoconductor in bright sunlight?

No, without a battery, the created electrons and holes recombine.

Why is it advantageous for the metal contacts of a photoconductor to be very close to each other?

It decreases the transit time of electrons, leading to larger photoconductive gain.

Which is better for measuring a high-frequency optical signal: a photoconductor or a photodiode?

A photodiode is better.

Which resistor would you use to measure low-frequency room lighting for the highest sensitivity?

The largest resistor (e.g., 100k Ohm).

Why is it beneficial to connect a battery to a photodiode and resistor circuit when measuring optical power?

It ensures the photodiode operates in photoconductive mode for a linear relationship.