We propose and demonstrate a novel dynamically tunable fiber-based Lyot filter for the realization of a dual-wavelength mode-locked fiber laser, operating at center wavelengths of 1535 nm and 1564 nm. The same laser cavity can also be operated in a single-wavelength mode-locked regime with a wavelength tuning range of 30 nm, from 1532 nm to 1562 nm. The proposed dynamically tunable Lyot-filter provides a simple setup for laser mode-locking using a single laser cavity design to generate dual-wavelength pulses, with the flexibility to also allow the generation of single-wavelength pulses with a continuously-tunable center wavelength.
In this paper, we report our results on the use of patterned graphene on SiN waveguides inside an erbium doped fiber ring laser cavity for passive mode locking. We confirm the dominant influence of graphene over non-linear polarization rotation (NPR), arising from polarizing fiber to chip grating couplers, by observing the limits of NPR in high cavity loss fiber lasers. We fabricated waveguides, transferred and patterned graphene to three different lengths, and observed different types of pulses based on the length of graphene, pump power and polarization state of the cavity. We confirmed the success of graphene processing through Raman scattering and saturable absorption measurements, as well graphene absorption simulation. High chip-coupling loss was overcome by using multiple erbium-doped fiber amplifiers. We believe this research will stimulate further interest in combining integrated photonics and fiber laser technologies, and the miniaturization of pulsed laser cavities.
We optimized parameters of a dispersion-tuned wavelength-swept fiber laser by numerically analyzing dynamic characteristics. The optimized laser is experimentally demonstrated and applied to the swept-source optical coherence tomography (SS-OCT) system. The dispersion-tuned wavelength-swept laser (DT-WSL) is a unique tunable fiber laser, whose lasing wavelength can be tuned rapidly without any mechanical tunable filters. Although the wavelength of a DT-WSL can be swept rapidly and widely, the broadening of the instantaneous spectral width at a high sweep rate has been a critical drawback for SS-OCT applications. Numerical simulations have shown that higher modulation frequencies for active mode-locking lead to narrower instantaneous spectral widths. However, a lower modulation frequency is needed to achieve a wider wavelength tuning range. Pulse modulation is employed to solve the trade-off between instantaneous spectral width and wavelength tuning range. In this paper, the characteristics of a sinusoidally modulated and a pulse-modulated DT-WSL are compared numerically and experimentally. The numerical simulation results show that a pulse-modulated laser can achieve spectral widths as narrow as that of the sinusoidally modulated laser with >5 GHz modulation frequency, even when the pulse modulation frequency is as low as 500 MHz. We also study the difference in the laser characteristics with different sweep directions and discover that a positive wavelength sweep leads to a narrower instantaneous spectral width. We also experimentally confirmed that pulse modulation can indeed achieve a narrower spectral width, as expected from our numerical simulation results. The pulse-modulated DT-WSL is then used in an SS-OCT system and successfully achieves a coherence length of 1.3 mm, whereas that of a sinusoidally modulated DT-WSL is limited to only 0.7 mm. Furthermore, we experimentally compare the performance difference in OCT imaging with different wavelength sweep directions, and the results proved that it is advantageous to apply a positive wavelength sweep, as predicted by our numerical simulation.
This paper presents the first implementation of Coherent Doppler LiDAR using an Optical Single Sideband Modulator. Experiments show that range resolution and Doppler shift estimates are greatly improved over the traditionally used AOM.
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