Automatic turbulence mitigation for coherent free-space optical links using crystal-based phase conjugation and...

Area of Science:

• Optical Communications
• Atmospheric Optics
• Nonlinear Optics

Background:

• Free-space optical (FSO) links offer high bandwidth but are susceptible to atmospheric turbulence.
• Turbulence degrades performance by causing mode coupling, reducing signal quality in coherent detection systems.
• Existing turbulence mitigation techniques are often limited to low data rates.

Purpose of the Study:

• To demonstrate automatic turbulence mitigation for high-speed coherent FSO links.
• To implement a novel phase conjugation technique for enhanced signal integrity.
• To evaluate the performance of a 2-Gbit/s quadrature-phase-shift-keying (QPSK) FSO system with mitigation.

Main Methods:

• Utilized degenerate four-wave-mixing (DFWM) based phase conjugation for turbulence compensation.
• Employed a counter-propagating Gaussian probe beam from receiver to transmitter through the turbulent channel.
• Generated QPSK data using a fiber-coupled phase modulator and created a phase conjugate data beam via DFWM in a photorefractive crystal.

Main Results:

• Achieved up to ~14-dB higher local oscillator (LO)-data mixing efficiency compared to unmitigated links.
• Demonstrated error vector magnitude (EVM) performance below 16% under various turbulence conditions.
• Successfully mitigated turbulence effects in a 2-Gbit/s QPSK coherent FSO link.

Conclusions:

• Degenerate four-wave-mixing (DFWM)-based phase conjugation effectively mitigates atmospheric turbulence in high-speed coherent FSO systems.
• The proposed method enables robust data transmission at 2 Gbit/s with QPSK modulation.
• This approach offers a promising solution for reliable FSO communication in turbulent environments.