# Simultaneous Transmission of Photonic Services over One Fiber with an ITU 100 GHz Grid
## 1 Introduction
The two most accurate methods for time and frequency transfer do not utilize standard data packets in the network:  
1. Technology based on amplitude modulation by RF signal--For this purpose, various methods have been developed. The characteristic feature of these methods is the radio-frequency (RF) modulation signal for encoding transmitted time labels. These methods assume a symmetrical transfer path in both directions, i.e., bidirectional transmission using a single fiber, and possibly suitable methods for compensating delay variations due to external influences, especially temperature. In terms of the required bandwidth, a single DWDM channel for each direction is sufficient. Standard transceivers or fiber lasers are used as the source of the optical signal. 2. Transmission of unmodulated optical signal--The subject of transmission is the unmodulated carrier signal from a stabilized laser, which is controlled by the source of a stable optical signal (e.g., an optical clock). Therefore, such a method is used only for the frequency transmission and is not utilized for the time transmission. The standard uncompensated transmission path provides stability on the order of 1 \(\times\) 10\({}^{-14}\) to 1 \(\times\) 10\({}^{-15}\). To increase the stability to the order of 1 \(\times\) \(e^{-19}\), stabilizing the optical fiber noise is required, i.e., to eliminate low-frequency interference caused mainly by vibration and shaking of the optical fiber. This method has low optical channel width requirements, such that a 50 GHz DWDM channel is sufficient. Last but not least, the use of optical fibers as a sensor [23; 24; 25] has also achieved ever-increasing popularity due to falling prices of components and optical fibers. The basic monitoring of perimeters is accomplished by stretching the optical fiber along a given location and by tracking changes in the local refractive index on the fiber when pressure or another force is exerted on it. Early detection of anomalies, perimeter intrusion, and localization are crucial for critical infrastructure security. With imminent sensory systems, it is necessary to ensure mutual coexistence between sensory applications and current data networks [26; 27]. There are several different ways to measure acoustic vibrations near the fiber. The easiest way is analyzing polarization states in the fiber, which provides basic information that something is happening but without any other details, such as the location or the type of the event (temperature or strain changes, acoustic vibrations, etc.). Another option is to use interferometers. Their considerable disadvantage, however, is their high sensitivity; therefore, high noise levels caused by air conditioning, active elements, temperature, etc. are problematic. Moreover, in a basic configuration, two fibers must be used, and localization is not possible. The last option for evaluating changes near the fiber is to use backscattering. Reflectometry systems' advantages are good sensitivity, the possibility of direct localization and classification, and the use of only one fiber. On the other hand, the development of reflectometry systems is complicated, and systems require sophisticated signal processing; because of the low level of the backscattered signal, it is necessary to generate high-power input pulses. Currently, reflectometric systems are the most widely used, but for measurement, dedicated optical fiber is always used, which is inefficient and uneconomical. The rest of this paper is structured as follows. Section 2 provides an overview of the related works. Section 3 provides an overview of our measurement setup, such as the home-developed accurate time transmission system and the \(\Phi\)-OTDR sensor system. Section 4 presents our measurements results. Finally, Section 5 concludes the paper.