Intermodal interference induced significant frequency modulation to amplitude modulation conversion in a broadband...

Area of Science:

• Fiber optics
• Laser physics
• Nonlinear optics

Background:

• Intermodal interference in large-mode-area (LMA) fiber lasers can affect pulse characteristics.
• The frequency modulation (FM) to amplitude modulation (AM) conversion is a critical phenomenon in fiber laser systems.
• Understanding this effect is crucial for controlling and optimizing broadband pulse generation.

Purpose of the Study:

• To investigate the conversion of the FM-to-AM effect induced by intermodal interference in broadband LMA fiber lasers.
• To theoretically and experimentally analyze the influence of intermodal interference on output pulse characteristics.

Main Methods:

• Numerical simulations were employed to model the spectrum transfer functions at different positions of the LMA fiber end face.
• Experimental measurements were conducted using a single-mode fiber sampling oscilloscope and a bulk detector.
• Analysis focused on the impact of intermodal interference and eigenmode characteristics on pulse detection.

Main Results:

• Numerical simulations demonstrated that spectrum transfer functions vary with position on the LMA fiber end face, leading to different output broadband pulses.
• Experimental results using a single-mode fiber sampling oscilloscope corroborated the simulation findings, showing variations in output pulses.
• No amplitude modulation was observed when measuring with a bulk detector, attributed to the orthogonal characteristic of the eigenmodes.

Conclusions:

• Intermodal interference in LMA fiber lasers significantly influences the FM-to-AM conversion.
• The method of pulse measurement critically affects the observed output characteristics, highlighting the importance of detection techniques.
• The orthogonal nature of eigenmodes in bulk detection prevents the observation of amplitude modulation.