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The Concept of Free Spectral Range in Optical Resonators

The free spectral range of an optical resonator, also known as the axial mode spacing, refers to the frequency spacing between its axial resonator modes. This parameter plays a crucial role in the performance of optical resonators.

Calculating Free Spectral Range

The free spectral range for an empty standing-wave resonator can be calculated using the formula:

Δ = c / 2L

Where c is the speed of light and L is the length of the resonator. For a ring resonator, the factor of 2 is removed since there is no double pass.

When the resonator is filled with a dispersive medium, the free spectral range is determined by the group index:

Δ = c / (2ngL)

Here, ng is the group index, which accounts for the frequency dependence of the refractive index due to chromatic dispersion.

Significance of Free Spectral Range

The free spectral range impacts the performance of devices like Fabry–Pérot interferometers and wavelength-tunable single-frequency lasers. A larger free spectral range allows for a broader optical frequency range but may compromise spectral resolution.

Optimizing Resonators

To enhance spectral resolution, one can increase the finesse of the resonator by minimizing round-trip power losses. Additionally, adjusting the resonator length or using different mirror configurations can influence the free spectral range and overall performance of the device.

Conclusion

Understanding the free spectral range is essential for designing and optimizing optical resonators for various applications in photonics and laser technologies.

Source: www.etalons-galop.com
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