Synthesis of Nano-Structured Ge as Transmissive or Reflective Saturable Absorber for Mode-Locked Fiber Laser

Area of Science:

• Materials Science
• Optics and Photonics
• Nanotechnology

Background:

• Passively mode-locked fiber lasers are crucial for ultrafast optics.
• Developing efficient and cost-effective saturable absorbers is key to advancing laser technology.
• Germanium-based materials offer unique optical properties for laser applications.

Purpose of the Study:

• To investigate the potential of amorphous germanium (α-Ge) films and germanium nanoparticles (Ge NPs) as saturable absorbers.
• To evaluate their performance in initiating and sustaining passive mode-locking in erbium-doped fiber lasers (EDFLs).
• To compare the effectiveness of transmissive (α-Ge) and reflective (Ge NP/Au) configurations.

Main Methods:

• Synthesis of α-Ge films and Ge NPs using hydrogen-free plasma-enhanced chemical vapor deposition (PECVD).
• Integration of α-Ge as a transmissive saturable absorber.
• Fabrication of Ge NP-on-Au (Ge-NP/Au) films as a reflective saturable absorber.
• Characterization of laser performance, including threshold power, pulsewidth, and spectral linewidth.

Main Results:

• Transmissive α-Ge films achieved self-starting mode-locking with a modulation depth of 52-58% and pulsewidths around 700 fs, compressible to 290 fs.
• Reflective Ge-NP/Au films enabled mode-locking with broadened pulsewidths of 3.7-3.9 ps.
• α-Ge demonstrated efficient saturable absorption with soliton compression effects.
• Ge-NP/Au exhibited limitations as a reflective mode-locker due to surface scattering.

Conclusions:

• Ultra-thin α-Ge films are effective transmissive saturable absorbers for EDFLs.
• Free-standing Ge NPs show potential as reflective saturable absorbers.
• Both α-Ge and Ge NPs are promising materials for developing ultrafast fiber lasers.