Analysis of Changes in Modulation Parameters on Optical Communication System

This study aims to analyze the NRZ-OOK modulation format in free-space optical communication. Simulations were conducted to examine the influence of the SpS parameter and optical signal power at the input modulator on the average power of the modulated optical signal. The research employed a computer simulation approach, where the optical signal was transmitted through an optical path without cables, enabling fast data transmission and longer distances compared to wired media. The method used specifically uses a machine learning-based application, Python. The simulation results indicate that higher SpS values result in a more accurate and smoother optical signal representation. Furthermore, an increase in the optical signal power at the input modulator increases the average power of the modulated optical signal. However, negative optical power values do not hold any relevant physical meaning. The magnitude of the Pi_dBm value also affects the optical signal spectrum, with higher optical power generating more robust frequency components. Graphs with negative optical power exhibit significant noise due to distortion and non-linearity. The findings of this study provide a better understanding of the influence of these parameters in the NRZ-OOK modulation format for free-space optical communication.


Introduction
NRZ modulation is a modulation format that uses constant levels representing bit 0 and bit 1 [1].The high level (e.g., +1) represents bit 1, while the low level (e.g., -1) represents bit 0 [2].There is no level change within each bit period; hence, it is called "Non-Return-to-Zero" (NRZ) [3].On the other hand, OOK modulation is a modulation scheme that discretely changes the optical intensity between two levels, namely "on" (optical signal present) and "off" (optical signal absent) [4,5].There are several transmission parameters in the NRZ-OOK (On-Off Keying) modulation format, including SpS (Samples per Symbol), Rs (Symbol rate), Pi_dBm (optical signal power at modulator input in dBm), Vπ, and Vb (MZM parameters).In this simulation, the focus will be on two parameters: SpS and Pi_dBm.
SpS refers to the number of samples representing one symbol in the modulated optical signal.Higher SpS values result in a smoother and more accurate optical signal representation.A higher SpS value allows for a better approximation of rapid changes in the optical signal.The optical signal power at the input modulator (Pi) is the optical power input to the electro-optic modulator, such as a Mach-Zehnder modulator (MZM) [6].
The magnitude of this optical power affects the average power of the modulated optical signal produced.A higher Pi inputted to the modulator leads to a higher average power of the modulated optical signal.This result is related to the linear response of the modulator and the resulting optical intensity [7].
The magnitude of the optical signal power influences the frequency components in the optical signal spectrum [8].The Higher optical power results show that stronger frequency components are being generated.Higher optical power also produces a larger amplitude in the modulated optical signal.This result means that the amplitude of the optical signal waveform will increase with an increase in optical power.Additionally, using negative power at the input modulator leads to significant distortion [9] and non-linearity [10] in the resulting optical signal.Negative power at the input modulator causes undesired changes in the intensity pattern of the optical signal, resulting in significant noise and reduced optical signal quality [11].This research applies the NRZ-OOK modulation principle in optical communication, a modulation format that influences transmission quality, thus aiding in designing more efficient systems [12].

Method
This simulation process describes machine learning using the Python application.First, The process sets aside the variables that will be used and run the NRZ-OOK modulation format for free-space optical communication within a simulation box.The research uses a pre-designed optical signal to implement the NRZ-OOK modulation format for free-space optical communication within a simulation box.Based on previous research, the simulation is conducted to test the feasibility [13].NRZ refers to a modulation format where the amplitude level of the signal remains constant at the same level throughout one bit.At the same time, OOK is a modulation method where the optical signal is turned on or off to represent data bits.In free-space optical communication, optical signals are transmitted through an optical path without cables, enabling faster data transmission and longer distances than wired media.The known digital information is transmitted via optical signals using two different amplitude levels.

Results and Discussion
The NRZ-OOK modulation format for free-space optical communication has been designed, simulated, and tested within a simulation box.In a research study, the generated NRZ-OOK modulation indicated that changes in the optical signal can result in significant noise [14][15][16].Based on the simulations, the impact of the SpS parameter and the optical signal power at the input modulator on the average power of the modulated optical signal can be observed.Higher SpS values result in more accurate representation and smoother optical signal modulation.Additionally, increasing the Pi will immediately increase the average power of the modulated optical signal.This result can be observed in Table 1.In contrast, when the optical signal power parameter at the modulator input is negative, it does not hold any relevant physical meaning.This condition is due to the code provided, where the value of Pi_dBm is used to calculate the optical power at the input modulator using the formula Pi = 10**(Pi_dBm/10)*1e-3, where Pi_dBm represents power in decibel-milliwatt (dBm) units.Pi represents the optical power in watts (W) to be input into the electro-optic MZM.Therefore, if the value of Pi_dBm is negative (Pi_dBm = -15), the calculation will result in a positive value for Pi.However, negative Pi_dBm values will affect the power spectral density in the graph of the optical signal spectrum compared to positive Pi_dBm values.This spectrum can be observed from the differences in the graphs in Figure 1 and Figure 2 in the simulation using an SpS value of 10.The magnitude of the Pi_dBm value influences the resulting graph of the optical signal spectrum because higher optical power generates stronger frequency components within the spectrum.Additionally, the magnitude of the Pi_dBm value also affects the waveform generated.Higher optical power leads to a larger amplitude in the modulated optical signal.
Graphs with negative power values inputted into the modulator exhibit significant noise due to distortion and non-linearity in the optical signal.When the optical power is negative, the resulting optical signal from the modulator experiences significant distortions and non-linearities.The optical signal with negative power at the modulator input causes undesired changes in intensity patterns, resulting in significant noise.This signal from the simulation can be observed more clearly in Figure 3 and Figure 4 using an SpS value of 10.

Conclusion
This study found that the SpS parameter significantly influences the accurate and smooth representation of the modulated optical signal.Higher values of SpS result in a more precise representation of the modulated optical signal.Furthermore, an increase in the Pi leads to an increase in the average power of the modulated optical signal.The higher the optical power inputted into the modulator, the higher the average power generated by the modulated optical signal.However, using negative values for Pi_dBm in the optical power calculation yields positive optical power values.Nonetheless, negative values of Pi_dBm hold no relevant physical meaning in the context of optical communication systems.The magnitude of the Pi_dBm value affects the resulting graph of the optical signal spectrum.Higher optical power generates stronger frequency components in the spectrum of the modulated optical signal.Graphs with negative optical power inputted into the modulator exhibit significant noise.Negative optical power leads to distortions and non-linearities in the optical signal, resulting in substantial noise.The average value of optical modulation obtained is approximately 12,000 dBm if the Pi_dbm input is positive.Meanwhile, the average modulated optical value obtained is around 18,000 dBm if the Pi_dbm input is negative.