All-normal dispersion ytterbium-doped fiber laser mode locked by nonlinear multimode interference

Area of Science:

• Optics and Photonics
• Laser Physics
• Fiber Optics

Background:

• Mode-locking is crucial for generating ultrashort laser pulses.
• Nonlinear optical effects in fiber structures offer new avenues for laser design.
• All-normal dispersion fiber lasers provide unique pulse characteristics.

Purpose of the Study:

• To investigate the mode-locking capability of a nonlinear multimode interference (MMI) based saturable absorber.
• To explore the use of a single mode-multimode-single mode (SMS) fiber structure as a saturable absorber.
• To demonstrate efficient mode-locking in an ytterbium-doped all-normal dispersion fiber laser.

Main Methods:

• Numerical simulation of intensity and wavelength-dependent transmission in an SMS fiber structure.
• Characterization of nonlinear MMI saturable absorber properties.
• Experimental setup of an ytterbium-doped fiber laser in an all-normal dispersion configuration.
• Analysis of pulse duration, repetition rate, and signal-to-noise ratio.

Main Results:

• The nonlinear MMI in an SMS fiber structure acts as a saturable absorber.
• A narrow-band wavelength-tunable spectral filter is essential for effective mode-locking.
• Single-pulse mode-locking was achieved at a low pump power of ~56.3 mW.
• The laser generated 180 ps pulses at a ~9 MHz repetition rate with a ~50 dB signal-to-noise ratio.
• Multi-pulse and noise-like rectangular pulse regimes were observed at higher pump powers.

Conclusions:

• Nonlinear MMI in SMS fiber structures can effectively function as saturable absorbers for fiber lasers.
• The developed all-fiber laser offers efficient mode-locking at low pump power.
• The laser system demonstrates versatile operation regimes, adaptable to different pumping conditions.