Understanding Optical Responsivity

Let’s delve into the concept of optical responsivity, a crucial parameter in the realm of fiber optics.

What is Optical Responsivity?

Optical responsivity essentially quantifies how efficiently an optical system converts light input into a usable output signal. Think of it as the gain of the system, but specifically for light.

• Definition: It’s the measure of the output signal relative to the input optical signal.
• Application: For a photodetector, it tells us the ratio of the electrical output (current or voltage) to the incident optical power.
• Units: Responsivity is typically expressed in amperes per watt (A/W) or volts per watt (V/W).
• Dependence: Importantly, optical responsivity is not a constant. It’s influenced by the wavelength of light being used and other characteristics of the sensor itself.

Here’s an example of a stressed eye diagram, showcasing how optical signals are evaluated, and where responsivity comes into play:

The Optical Responsivity Formula

The relationship between input light and output signal is expressed by the following formula:

Where:

• IP-P represents the peak-to-peak current at the input of the Transimpedance Amplifier (TIA).
• ρ (rho) signifies the optical responsivity we are trying to calculate.
• OMA refers to the Optical Modulation Amplitude. OMA represents the difference between the two power levels seen in the stressed eye diagram (refer to “What is OMA” for a deeper explanation).

In essence, the formula demonstrates that optical responsivity is the ratio of the peak-to-peak current output to the optical modulation amplitude of the input signal. This ratio tells us how well the system converts light into an electrical signal.