Sub-cycle QAM modulation for VCSEL-based optical fiber links.

Area of Science:

• Optical communications
• Digital signal processing

Background:

• Quadrature Amplitude Modulation (QAM) is a bandwidth-efficient modulation technique.
• Directly modulated lasers (DMLs) are cost-effective light sources for optical fiber systems.
• Subcarrier modulation below the symbol rate can improve signal performance.

Purpose of the Study:

• To investigate the theoretical and experimental performance of QAM signals using a subcarrier frequency lower than the symbol rate.
• To assess the suitability of this modulation scheme for optical fiber links with DMLs.

Main Methods:

• Theoretical analysis of QAM modulation with subcarrier frequencies below the symbol rate.
• Experimental setup using real-time signal generation.
• Transmission of a 10-Gbps 4-level QAM signal over 20-km single-mode fiber (SMF).
• Utilizing an un-cooled 1.5-µm Vertical Cavity Surface Emitting Laser (VCSEL).

Main Results:

• Successful real-time generation and transmission of a 10-Gbps 4-level QAM signal within a 7.5-GHz bandwidth.
• The signal utilized a subcarrier frequency at half the symbol rate.
• A low fiber transmission power penalty of only 2.5 dB was observed.
• No electronic equalization was required.

Conclusions:

• Sub-cycle QAM modulation is a promising technique for high-spectral-efficiency optical fiber communications.
• This method is particularly attractive for systems employing directly modulated light sources like VCSELs.
• The experimental results validate the theoretical advantages, demonstrating robust transmission over 20 km SMF.