In optical simulation, the pseudo random bit sequence is used in lieu of a digital signal to be transmitted in the network. Pseudo random bit sequences are used to test digital systems with the understanding that if random digital sequences can be transmitted through the system without errors, then a deterministic digital sequence can also be transmitted through the same system without errors. However, the PRBS generates a random sequence of binary 1s and 0s. Like the user defined bit sequence generator, the PRBS is used to form binary sequences of 1s and 0s.
Pseudo Random bit sequence (PRBS) generator
One key input to this tool is the bit rate, which can be set locally in the component or as a global parameter affecting the entire project. The user can either manually enter a binary sequence or select a predefined function to generate the preferred sequence. The user defined bit sequence generator is used to form deterministic binary sequences of 1s and 0s. The tools used in these simulations include the user defined bit sequence generator, pseudo random bit sequence (PRBS) generator, NRZ pulse generator, directly modulated laser diode, CW (Continuous Wave) laser, a Mach-Zander modulator, an oscilloscope, an optical time domain visualizer and an optical spectrum analyzer. In this article, only a small subset of the simulation tools relevant to the spectral characteristics of modulated light are reviewed. The Optisystem optical simulation software, described in our simulation video, is loaded with tools to simulate optical components, subsystems and networks. Advanced modulation formats for higher data rates are beyond the scope of this article and are covered in great detail in our OTT optical networking courses. NRZ modulation format is mostly used in low data rate communication systems, typically up to 10Gb/s and rarely used for higher data rates up to 100Gb/s. RZ modulation format has rarely been used in optical communications as it has been determined to be less efficient. This is distinct from RZ (Return to Zero) modulation format in which a transition from a binary 1 to another binary 1 is preceded by a transition from 1 to 0 and back to 1. In NRZ modulation, a transition from a binary 1 to another binary 1 is achieved by maintaining the light in the “on” state. In this modulation format, the “on” state represents a binary 1 and the “off” state represents a binary 0.
Turning laser light on and off or On Off Keying (OOK) is the simplest modulation format used to encode digital signals in optical communications.
The focus here is on the modulation of the intensity or power of the light. The light properties that can be modulated include intensity, phase, frequency, or a combination of two or more of these properties. Modulation controls one or more properties of light in order to encode signals on optical carrier waves that will then transmit the signal across a network. NRZ (Non-Return to Zero) intensity modulation In this article, Optisystem simulation tools are used to demonstrate the effect of direct and external NRZ intensity modulation on optical pulses and spectra of laser diodes. Simulation software, such as Optisystem, is a cost-effective way of testing and studying optical properties under different modulation conditions. Moreover, the modulation required to encode an electrical signal on an optical carrier wave has an impact on the spectral characteristics of the light.
For high performance networks, the spectral width should be as narrow as possible, and the center wavelength should be stable and not drift over time. The spectrum of a light source has a profound impact on the performance of an optical communication network.