Controllable, intense spectral peaking with a spectral filter and optical fiber

206 MHz fully stabilized all-PM dispersion-managed figure-9 fiber laser comb

High-repetition-rate optical frequency combs are useful for precision spectroscopy because of their high power per comb mode, but conventional high-repetition-rate lasers do not have a broad enough spectrum. In this study, a fully stabilized polarization-maintaining figure-9 mode-locked fiber laser with a high repetition rate of 206 MHz and a broad spectrum was demonstrated by employing simultaneous control of cavity dispersion and length. The laser exhibited a 3 dB spectral bandwidth of 88 nm and a compressed pulse width of 66 fs. Additionally, fCEO and frep phase locking were implemented, resulting in low (0.21 rad) in-loop carrier-envelope-offset frequency phase noise. To the best of our knowledge, this is the widest spectrum bandwidth and shortest pulse duration directly obtained from an all-PM figure-9 fiber laser oscillator to date. The combination of high repetition rate and broad spectral range makes this system very useful for a wide range of applications, especially in the field of precision spectroscopy.

Continuous tuning of pulse parameters in a soliton fiber laser by adjusting the effect of nonlinear polarization rotation

We demonstrate that through inserting a short length of highly birefringent small-core photonic crystal fiber (Hi-Bi SC-PCF) into a soliton fiber laser, the nonlinear polarization rotation effect in this laser can be manipulated, leading to continuous tuning of the output pulse parameters. In experiments, we observed that by adjusting the polarization state of light launched into the Hi-Bi SC-PCF and varying the cavity attenuation, the laser spectral width can be continuously tuned from ∼7.1 to ∼1.7 nm, corresponding to a pulse-width-tuning range from ∼350 fs to ∼1.56 ps. During the parameter tuning, the output pulses strictly follow the soliton area theory, giving an almost constant time-bandwidth-product of ∼0.31. This soliton fiber laser, being capable of continuous parameter tuning, could be applied as the seed source in ultrafast laser systems and may find some applications in nonlinear-optics and soliton-dynamics experiments.

Background-reduced spectral peak generation using a nonlinear loop mirror with a gas cell

Spectral peaking in an optical fiber is a useful phenomenon for comb mode filtering and wavelength standards. However, for highly sensitive spectroscopic applications, it is important to suppress the pedestal components. Here we propose and demonstrate pedestal-suppressed spectral peak generation using a nonlinear fiber loop mirror with a molecular gas cell. The physical mechanism and fundamental properties were investigated numerically, and the output characteristics were examined experimentally. Almost background-free spectral peaks were generated successfully in the 1.65-µm wavelength range using a CH4 gas cell. The maximum signal-to-background ratio was more than 30 dB. Stable operation without any feedback control was achieved. It is expected that the proposed method is useful for highly sensitive spectroscopic applications.

Frequency modes filtering of spectral peaks in optical frequency combs through molecular gas absorption and nonlinear polarization rotation

Spectral peak generation is a recently reported phenomenon that narrow spectral dips of the optical spectrum turn into sharp peaks as they propagate through nonlinear optical fibers. We demonstrated the nonlinear polarization rotation-based spectral peak mode filtering to increase the signal-to-background ratio (SBR). The spectral peaks with almost constant frequency separation were generated from the femtosecond pulses absorbed by the CH4 gas through the highly nonlinear fiber. The generated spectral peaks were filtered through the polarizing beam splitter by the nonlinear polarization rotation, and the SBR was improved from 9 dB to ∼20 dB. The spectral peak generation phenomenon and the mode filtering were numerically confirmed by solving the coupled nonlinear Schrödinger equations. The demonstrated method can generate strong comb modes with wide frequency spacing which are useful for highly sensitive environmental gas sensing spectroscopy. The wavelengths of the spectral peaks are fixed by the absorption spectra of the used gas cells. Therefore, this method can generate high quality spectral peaks of any wavelengths with wide spectral ranges through proper combinations of gas cells.