Continuously Tunable Comb Filter Based on a High-Birefringence Fiber Loop Mirror With a Polarization Controller

A spectrum tunable method is presented for a high-birefringence fiber loop mirror (HiBi-FLM) comb filter with a polarization controller (PC) in the loop, where not only the wavelength of the comb filter can be continuously tuned with peak transmission remaining at unity, but the peak amplitude can be also continuously tuned without wavelength shift. By using the Jones matrix, we theoretically analyze the comb filter. The analytical expressions of the reflectivity and transmissivity are derived in terms of the orientation angles of three waveplates of the PC. The optimum conditions for wavelength and peak amplitude tunable operations are obtained. Our theoretical results show that when the two quarter-waveplate (QWP) angles are optimally set, the PC could act as a phase shifter or a polarization rotator, which empowers the comb filter to be wavelength continuously tunable or to be peak amplitude variable from 0 to 1 by only rotating the half-waveplate (HWP) of the PC. Moreover, the wavelength shift has a linear relationship with the HWP angle. When the two QWP angles are not optimized, PC has a combined effect of a phase shifter and a polarization rotator, leading to a wavelength shift accompanied by a change of the peak amplitude when tuning the HWP. Our theoretical prediction has been verified by experimental results.