Dispersion dependence of linewidth in actively mode-locked ring lasers

Area of Science:

• Optics and Photonics
• Laser Physics
• Fiber Optics

Background:

• Actively mode-locked ring lasers (AMLLs) are crucial for tunable light sources.
• Spectral linewidth is a key parameter determining laser performance and applications.
• Controlling linewidth in wavelength-swept lasers is an active research area.

Purpose of the Study:

• To investigate the effect of anomalous dispersion on spectral linewidth narrowing in wavelength-swept AMLLs.
• To experimentally validate numerical predictions regarding dispersion's role in linewidth reduction.
• To achieve narrow linewidths in a tunable, wavelength-swept laser system.

Main Methods:

• Numerical calculations of static-state AMLLs to predict dispersion effects.
• Experimental setup of AMLLs with both normal and anomalous dispersive cavities.
• Utilizing a linearly chirped fiber Bragg grating to control cavity dispersion (normal or anomalous).
• Characterizing spectral linewidth under various wavelength sweeping conditions.

Main Results:

• Numerical studies predicted linewidth narrowing with anomalous dispersion.
• Experimental results consistently showed narrower linewidths in anomalous dispersion cavities compared to normal dispersion.
• A minimum linewidth of 0.08 nm was achieved with anomalous dispersion.
• Instantaneous linewidth broadening was observed during rapid wavelength sweeping.
• Narrow-linewidth lasing was maintained within a single roundtrip even at high sweep rates.

Conclusions:

• Anomalous dispersion is an effective strategy for spectral linewidth narrowing in wavelength-swept AMLLs.
• The study demonstrates a practical approach to achieving narrow linewidths (0.08 nm) in a tunable laser system.
• Short-length anomalous dispersion cavities enable high-speed wavelength sweeping (150 kHz, 40 nm range) with narrow linewidths.