Continuously wavelength-tunable passband-flattened fiber comb filter based on polarization-diversified loop structure

Area of Science:

• Optical Engineering
• Photonics
• Signal Processing

Background:

• Continuous wavelength tuning of optical comb filters is crucial for flexible signal processing in reconfigurable optical systems.
• Existing high-order filter structures with multiple birefringent elements (BEs) present challenges in tuning and orientation determination.

Purpose of the Study:

• To propose and demonstrate a continuously tunable, polarization-independent, passband-flattened fiber comb filter.
• To overcome the complexity associated with tuning and element orientation in previous filter designs.

Main Methods:

• A novel polarization-diversified loop structure incorporating two BEs, a polarization beam splitter, and waveplates (half-wave and quarter-wave) with polarization-maintaining fiber (PMF).
• Utilizing Jones matrix formulation to derive filter transmittance and determine waveplate orientation angles for arbitrary phase shifts.
• Theoretical spectral analysis and experimental verification of continuous tuning and passband flattening.

Main Results:

• Successfully designed and demonstrated a continuously tunable, polarization-independent, passband-flattened fiber comb filter.
• Theoretical analysis confirmed the feasibility of continuous tuning of comb spectra.
• Experimental results validated the theoretical predictions.

Conclusions:

• The proposed filter design offers a significant advancement for reconfigurable optical systems by enabling simple and continuous wavelength tuning.
• The polarization-diversified loop structure effectively addresses the complexities of previous designs.
• Further investigation into the wavelength-dependent evolution of the output state of polarization (SOP) provides deeper insights into filter performance.