Optical Cavity (F-P Cavity)

In the context of optical filters, "F-P Cavity" stands for Fabry-Perot Cavity, which is a key component of Fabry-Perot filters. Here's a deeper dive into its meaning and impact on filter performance:

What is an F-P Cavity?

An F-P cavity is essentially a resonant optical cavity formed by two highly reflective parallel mirrors facing each other with a precise cavity length in between. Think of it like a chamber where light gets trapped and bounces back and forth between the mirrors.

How does it work?

Light entering the cavity undergoes multiple reflections between the mirrors. However, only specific wavelengths constructively interfere after these reflections and remain within the cavity, resulting in the filter effect. These specific wavelengths resonate with the cavity length, hence the term "resonant cavity."

Impact on filter performance:

The F-P cavity significantly impacts the performance of the filter in several ways:

    • Center wavelength: The cavity length determines the dominant wavelength that resonates within the cavity and gets transmitted, defining the filter's center wavelength.
    • Bandwidth: The reflectivity of the mirrors and the cavity length influence the bandwidth of the transmitted light. Higher reflectivity and precise cavity length result in a narrower bandwidth,meaning only a small range of wavelengths around the center wavelength pass through.
    • Sharpness: The sharpness of the filter edges (transition between passband and stopband) also depends on the cavity design. High reflectivity and precise cavity length contribute to sharper transitions.
    • Sensitivity: F-P filters are inherently sensitive to changes in the cavity length, temperature, and incident light angle, which can affect the filter performance. Careful design and control are crucial.


    • Due to the resonant nature, F-P cavities can achieve very narrow bandpass filtering, making them excellent for applications requiring precise wavelength selection, like laser cleanup or spectroscopy.
    • They offer a relatively simple design compared to other filter types.


    • They are sensitive to environmental changes and require careful control for stable performance.
    • Their performance can be affected by misalignments or imperfections in the cavity.


Understanding the F-P cavity is crucial for comprehending the behavior and performance of Fabry-Perot filters. By optimizing the cavity design and controlling its environment, you can achieve highly selective and precise filtering for various applications.

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