Analysis of Effect of Input Power and RF Optical Signal Rate on Optical Communication System with OOK-NRZ Modulation

This study analyzed optical and RF signals in optical communication systems using the On-Off Keying Non-Return to Zero (OOK-NRZ) modulation method and the direct-detection receiver model with PIN photodiodes. In this study, a simulation was carried out using Python software. Changes in optical modulation input power and Symbol rate (Rs) were explored to understand their effect on signal characteristics. Models of ideal photodiodes (noiseless) and noisy photodiodes (considering thermal noise, shot noise, and bandwidth constraints) were used to compare receiver signals. The eye diagram analysis was employed to visualize receiver signal variation in a given time span. The results of this analysis provide a better understanding of the influence of changes in input power and symbol rate on optical communication systems through the OOK-NRZ method and insight into the characteristics of the receiving signal under realistic conditions. This study is able to provide guidance for the design and evaluation of optical communication systems using OOK-NRZ modulation and direct detection receivers.


Introduction
The modulation process is necessary for fulfilling signal transmission needs [1,2].The modulation process is a process of converting information signals or laying on data signals into specific forms.Optical modulation is a technique that utilizes light beams as pulses of light waves as a carrier signal [1] [3].The choice of modulation type used can be determined by the expected application, such as the characteristics of the channel used, available bandwidth, and channel susceptibility to changes (fading) [4].Optical modulation techniques have several advantages over conventional modulation techniques that use electrical signals as information carrier signals [4,5].Among them is that the resistance to noise is high; this is because the signal cannot be influenced by electromagnetic fields.Another advantage is the signal transmission or bit rate speed, which reaches hundreds of gigabits per second [6].
The method in this study employed a scheme of OOK modulation, where it seems to be a switch that has two conditions at the time of on and off [1,3].Bit 1 is interpreted as "on" or representing the optical pull that occupies either all or part of the bit duration.In contrast, a bit value of 0 is interpreted as "off" or representing the absence or disappearance of the optical pulse [2,7].OOK modulation consists of two sub-systems, namely OOK-NRZ and OOK-RZ.On the type of Non-Return to Zero (NRZ) OOK modulation, it indicates when bit 1 occupies the entire bit interval and bit 0 has no pulse [8,9].
Meanwhile, the OOK Return to Zero (RZ) modulation type can be interpreted at a time when bit 1 occupies only part or half of the bit interval, and bit 0 is not expressed by a pulse signal [10].
Measurement and analysis of optical signals are essential for signal quality assessment, optical performance monitoring (OPM), fault diagnosis, and fault detection [10].In intensity-based modulationdirect detection (IM-DD) systems, the eye diagram serves as the primary analysis tool for comprehensively modeling the quality of the optical signal and intuitively displaying the characteristics of various impairments in optical communication systems [11].Eye diagram analysis focuses on signals by methods of On-Off Keying (OOK) and pulse amplitude modulation (PAM), which are widely used in optical access and optical network data centers [12,13].A lot of information can be drawn from eye diagrams, including important characteristic parameters (such as high and low levels, cross percentage, and eye height and width), overall signal performance indicators (including modulation format, Qfactor, and OSNR), link properties and information (such as fiber transmission distance), and interference features caused by device imperfections [1,12] Previous research by A. M. Zaiton compared modulation performance evaluation with OOK NRZ and Carrier-Less Amplitude Phase (CAP), which produces an eye diagram with input variations.This study was performed by varying the input modulator's value and the symbol's rate.This aimed to obtain the visualization results of signal characteristics in the form of radio frequency and optical signal spectrum and the form of an eye diagram to determine the transmission of optical signals.

Methods
This study was started with a literature study on fiber optic modulation related to the OOK-NRZ method.Exploration in this research was carried out thoroughly using Python software.This study is OOK-NRZ modulation, which is a modulation with the condition that bit 1 will occupy the entire bit interval and bit 0 does not have a pulse.Followed by data acquisition in Python software, the following steps are followed: 1) Manipulating the modulator input by giving an increase starting from -30 dBm to 10 dBm.2) Manipulating symbol rate (Rs) manipulation at each modulator input.3) Performing optical modulation on RF signal and optical signal spectrum.4) Applying the eye diagram on the spectrum of the optical signal and the RF signal obtained to visualize the signal characteristics.5) Conducting visual analysis of the eye diagram, RF signal, and optical signal spectrum.Figure 1 is an electronic photodetector design with a trans-impedance connection of the photodiodes with an OPA847 low-noise operational amplifier of large bandwidth [1].

Result and Discussion
This section presents data from observations in Python simulations for various modulation schemes, with modulator inputs of -30 dBm to 10 dBm and symbol rates of 10 5 and 10 9 .The results for the average power data of the modulated optical signal are as Table 1.  1 represents the average power of the optical signal generated from the OOK-NRZ modulation process by manipulating the input power parameters of the modulator and the symbol rate.When changing the input value of the modulator's optical power, the average power of the optical signal generated after the modulation will change.An increase in input power will increase the average power of the optical signal.At the same time, a decrease in input power also leads to a decrease in the average power of the optical signal.This can have an impact on optical signal strength and optical system performance.The changes in symbol rate or Rs in OOK-NRZ modulation will affect the variation in the speed of electrical pulses within the modulation signal, where the increase in the symbol rate will result in the electrical pulse changing at a higher speed.Changes in the rate of this symbol also affect the width of the optical signal spectrum after modulation.Previous studies have also proved that NRZ has a high transmission frequency, resulting in a higher maximum data capacity transferred in 1 second due to changes in the symbol rate [5,14].Next, it will be discussed about the graph of radio frequency signals and optical signals after being modulated.Figure 2 shows the Radio Frequency signal spectrum, which describes the frequency power distribution of the RF signal with different input values and symbol rates.The y-axis shows the amplitude range, where the range gives an idea of the RF signal power generated.The modulation input power given to the optical signal can affect the amplitude of the RF signal generated after the opticalelectrical modulation process.Thus, if the modulation input power in the optical signal increases, then the amplitude of the RF signal produced also tends to increase.The symbol rate (Rs) also affects the bandwidth and number of frequencies of the RF signal.If the symbol rate value increases, the bandwidth required for the optical signal transmission also increases, leading to an increase in frequencies to cover the signal spectrum.Figure 3 shows an RF signal that has been optically modulated in the time domain.The Label on the chart is "RF binary signals," and the y-axis shows the amplitude of the RF signal in arbitrary units (a.u.).The RF signal reflects the modulation of the optical signal with binary data.
Figure 4 shows a graph of a modulated optical signal by showing the distribution of optical power over a specific frequency range.Modulation input power in optics has a direct influence on the amplitude of the optical signal and its power produced.The greater the input power, the greater the amplitude produced.It also has an impact on the signal strength and noise levels in the modulation system, all of which can affect the quality and performance of the overall modulation system.Figure 5 shows a graph of the modulated optical signal with the amplitude of the optical signal in the time domain.The input modulation power can affect the strength of the resulting optical signal, and the amplitude also gets bigger.In addition, the symbol rate also affects the speed at which optical signals change in the time domain.When there is an increase in the symbol rate, the optical signal changes per period faster.Figure 6 uses the ideal photodiodes model without noise or noiseness, and there is no bandwidth limitation.It is used to produce an ideal receiving signal by applying photodiodes to the amplitude portion of the modulated optical signal.Figure 7 uses a photodiode model with noise or noisy photodiode concerning noise types, such as thermal noise, shot noise, and bandwidth limitations.It is used to generate realistic receiver signals.Through Figures 6 and 7 above, the analysis of the eye diagram was carried out.This function allows you to visualize the variation of the signal in a certain time range.Through analyzing the eye diagram, it can be described the influence of noise on system performance.In addition, it can compare an ideal receiving signal and a noisy receiving signal in terms of waveform, jitter, and distortion that may occur.This result is supported by previous research, namely the modulation of the OOK eye diagram.Figure 7 is significantly distorted due to the magnitude of the input.[6,5].

Figure 1 .
Figure 1.The diagram of the photo-detector connection with the equivalent circuit of a photodiode.

Table 1 .
Average power data of modulated optical signal.