Stanford EE259 I Photodetection principles (direct vs. coherent), lidar system arch. I 2023 I Lec 17

TL;DR

Lidar uses semiconductor photodiodes that convert photons to electrons for efficient and accurate detection. Responsivity and bandwidth are crucial factors for optimizing performance.

Transcript

and let's uh start with a quick review of uh last lecture so last time uh we talked about lasers we learned about lasers specifically uh well the lazing principle and then specifically we looked at solid uh semiconductor laser diodes or solid state lasers that are the type of lasers that are used in lidars and then we started talking about uh the t... Read More

Key Insights

• 📷 Semiconductor photodiodes are the primary photo detection devices used in lidar systems.
• 💿 Responsivity measures the efficiency of converting optical input to electrical output, while bandwidth determines the device's responsiveness to changes in optical input.
• 🕳️ Responsivity depends on quantum efficiency, reflectivity, absorption coefficient, and electron-hole pair recombination efficiency.
• ❓ Different semiconductor materials have varying absorption coefficients, affecting responsivity at specific wavelengths.

Questions & Answers

Q: How do semiconductor photodiodes convert photons to electrical current?

Semiconductor photodiodes absorb incident photons in the depletion region, generating an electron-hole pair. The electric field in the region then separates these charges, producing a photocurrent.

Q: What factors affect the responsivity of a photodiode?

Responsivity is influenced by reflectivity, absorption coefficient, and electron-hole pair recombination efficiency. Higher quantum efficiency and lower reflectivity and recombination lead to higher responsivity.

Q: How does wavelength affect the responsivity of a photodiode?

Responsivity increases linearly with wavelength, meaning higher wavelengths result in higher responsivity. Different materials have different absorption coefficients at specific wavelengths.

Q: What is the relationship between responsivity and Quantum efficiency?

Responsivity is a measure of efficiency and is directly proportional to the Quantum efficiency of the photodiode.

Q: What is dark current, and why is it important?

Dark current is a small current that flows through the photodiode even in the absence of incident light. It acts as a noise source and affects the sensitivity of the device.

Q: How is the minimum detectable optical power determined?

The minimum detectable optical power, or Pmin, is usually set at the power corresponding to the dark current. This ensures that the device can accurately detect low levels of optical power.

Summary & Key Takeaways

• Lidar uses semiconductor photodiodes with a pin diode structure to convert photons to electrons via the photoelectric effect.

• Responsivity is the measure of efficiency in converting optical input to electrical output, while bandwidth determines the device's response to changes in optical input.

• Responsivity depends on the ratio of electron flux to photon flux, while bandwidth needs to be optimized for specific applications.

• Quantum efficiency, reflectivity, absorption coefficient, and electron-hole pair recombination affect responsivity.

• Absorption coefficients vary with semiconductor materials and wavelengths.