All-Optical Single-Longitudinal-Mode Forward Brillouin Microwave Oscillator with an Unbalanced Fiber Mach-Zehnder...

Area of Science:

• Optics and Photonics
• Microwave Photonics
• Optical Engineering

Background:

• Traditional optoelectronic oscillators (OEOs) require photoelectric conversion and are susceptible to electromagnetic interference.
• Achieving single-longitudinal-mode (SLM) operation with narrow linewidth and high side-mode suppression is crucial for microwave photonics (MWP).

Purpose of the Study:

• To propose and experimentally demonstrate an all-optical single-longitudinal-mode forward Brillouin microwave oscillator (FB-MO).
• To utilize an unbalanced Fiber Mach-Zehnder interferometer (UF-MZI) for enhanced MWP generation.

Main Methods:

• The proposed FB-MO incorporates a UF-MZI with asymmetric fiber lengths (5 km and 500 m) to create unbalanced feedback rings.
• A spectral Vernier effect is employed to improve the effective free spectral range (FSR) and maintain SLM operation.
• The design eliminates the need for photoelectric conversion devices and external modulation.

Main Results:

• The all-optical SLM FB-MO achieved a 3-dB linewidth of approximately 140 Hz.
• Acoustic-mode and side-mode suppression ratios were measured at 26 dB and 31 dB, respectively.
• Power and frequency stability showed fluctuations of ±1 dB and ±100 Hz over 60 minutes, with phase noise as low as -120 dBc/Hz at 100 kHz offset.

Conclusions:

• The all-optical SLM FB-MO with UF-MZI offers a robust and high-performance solution for MWP generation.
• The design's inherent advantages include immunity to electromagnetic interference and favorable spectral characteristics.
• This technology shows significant potential for applications in radar monitoring and wireless communication systems.