A second-order optical Butterworth Fabry-Pérot filter.

Area of Science:

• * Optics
• * Photonics
• * Signal Processing

Background:

• * Flattop passband filters are crucial for signal processing, enhancing detection sensitivity and power efficiency.
• * Optical filters typically use dielectric stacks with passband widths over 100 GHz, limiting narrow bandwidth applications.
• * Fabry-Pérot cavities are single-pole resonators suitable for efficient single-frequency transmission but constructing multi-pole filters is challenging in the optical domain.

Purpose of the Study:

• * To propose and implement a second-order Butterworth-type optical filter with a narrow flattop passband.
• * To bridge the gap between existing wideband optical filters and the need for narrower bandwidth solutions.
• * To demonstrate a novel method for achieving narrow bandwidth flattop filters in the optical domain.

Main Methods:

• * Implemented a second-order Butterworth-type optical filter within a single two-mirror Fabry-Pérot cavity.
• * Utilized the coupling of two polarization modes to achieve the flattop response.
• * Characterized the filter's performance, including passband width, stopband suppression, and insertion loss.

Main Results:

• * Demonstrated a narrow flattop passband width of 2.68(1) GHz.
• * Achieved a maximum stopband suppression of 43 dB.
• * Measured a passband insertion loss of 2.2(1) dB with out-of-band power suppression falling as the fourth power of detuning.

Conclusions:

• * The proposed method enables the creation of significantly narrower bandwidth flattop optical filters.
• * This approach is scalable to even narrower filters, offering potential improvements in laser phase noise and LIDAR sensitivity.
• * The developed filter technology provides higher quality narrowband filtering for applications like Raman spectroscopy.