# Key Concepts¶

There are three distinct layers in the Finesse interaction that will be referred to throughout this documentation:

The Model

a particular optical configuration or layout (see finesse.model)

The Simulation

when a Finesse model is executed to generate a particular output (see finesse.simulations)

The Solution

data generated by a particular simulation (see finesse.solutions)

## Detectors and Probes¶

A detector is defined as a component that converts an optical input into an electrical output, this can be a simple photo diode measuring a light power, or a complex device with multiple (optical) inputs and signal processing such as demodulation. A detector will have input and output nodes, will be computed as part of the matrix representing the model. It can only perform linear operations (for signal processing) and therefor applies only to of AC signals. For example, a detector cannot measure the light power at DC. However, detectors can be useful for modelling signal propagation in mixed systems (optical, electrical, mechanical).

A probe is a user-defined output of any simulation result, for example a specific light field amplitude or an electrical signal. A probe can include specific post-processing of the simulation output, for example, summing up powers in optical field components, or demodulation. A probe is not part of the model itself and does not block the light or signal path and it can therefor include non-linear post-processing. Many probes can be connected to each note without interfering with the model or each other.

Todo

DC and AC signal nomenclature

## The model graph¶

Any Finesse mode is defined as a nodal network, which is stored in a graph.

## The simulation matrix¶

Finesse uses a sparse matrices to perform the main simulation task.

Todo

add description and DC, and signal matrices (and quantum?)