Ultraflat optical frequency comb generated based on cascaded polarization modulators

Abstract

A novel approach to generating an ultraflat and stable optical frequency comb with tunable frequency spacing is proposed and experimentally demonstrated. The proposed generator consists of a polarization modulator (PolM) and a polarizer. The joint operation of a PolM and a polarizer is equivalent to intensity modulation, but with a third controllable parameter in addition to the two controllable parameters in conventional intensity modulation. By tuning the three parameters, an ultraflat optical frequency comb with five comb lines is generated. By cascading the PolM with a second PolM, an ultra-flat optical frequency comb with 25 lines is generated. An experiment using two cascaded PolMs is performed. A 25- line frequency comb with the comb flatness within 1 dB is generated.

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“…When the polarization modulated signal is passed through a polarizer, after adjusting the polarization state with the principle axis of the polarizer, polarization-modulation to intensity modulation takes place. The optical field after the polarizer is expressed as [137]:…”

“…The nonlinearity of PolM causes the generation of harmonics that make up the tones of the OFC. The optical field of each line can be found using [137]: [125].…”

“…The output comb provided eleven lines spaced at 42.6 GHz with <2 dB spectral flatness. Chao He et al [137] proposed a similar scheme, here the output OFC had <1 dB spectral flatness and was able to provide 5 lines spaced at 17.5 GHz, 20 GHz and 25 GHz as well as 25 lines spaced at 3.5 GHz, 4 GHz and 5 GHz. The PolM based OFCs do not require DC bias voltages and as a result do not suffer from bias drift issues.…”

“…16 (c). The solution avoided the use of multiple RF sources [137] and frequency dividers or multipliers. The OFC provided 13, 11 and 9 lines, with amplitude variations of 2.1 dB, 1.3 dB and 1 dB respectively, for a FSR of 10 GHz.…”

“…16 (e). Wang and Yao [140] proposed an OFC similar to [137], but using two PolMs. The OEO was implemented using the first PolM, phase shifted fiber Bragg grating (PS-FBG) and a photo-detector (PD).…”