Investigation of Various Modulation Techniques for Optical Systems Concerning Different Channel Spacing Values

The demand for high bandwidths, enabled by the high data rate applications in use today, for example, highdefinition video broadcast over the Internet, has led to the need for belt-tightening fiber optic communication systems of very high capacity. Optical fiber has become the backbone of modern telecom communications, complemented by its irreplaceable features such as extreme bandwidth, low attenuation, low distortion, lower power consumption, and high data rates. These properties make them more suitable for transmitting data over long connections, where high speed is required. The explosion in cloud computing, machine learning, IoT, and 5G - not to mention between-chip and on-chip communication - has only magnified the demand for more compact, faster, and energy-efficient systems. Perhaps the biggest innovation was density wavelength division multiplexing (DWDM), which enables multiple optical signals to be transmitted on the same fiber, each at different wavelengths. Therefore, this study investigates various NRZ, RZ, and QAM modulation schemes along with several channel spacings to compare the performance degradation of the systems via Quality Factor (QF) and Bit Error Rate (BER). Results reveal that QAM is best in all cases, with an average QF close to 25 in the low BER regime for large channel spacings ranging from <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$100-150 \text{GHz}$</tex>. NRZ was the most susceptible to interference, especially with low spacings, while RZ showed an average performance. These outcomes provide a comparative perspective on the best modulation formats and channel separation to enhance efficiency and reliability in a dense wavelength division multiplexing (DWDM) context.