# Simultaneous Transmission of Photonic Services over One Fiber with an ITU 100 GHz Grid
## 2 Related Works  
Accurate time, timing, or stable frequency are traditionally important in the fields of metrology, navigation, fundamental physics, and legal time keeping. Currently, they are also important for a significantly broader range of fields, e.g., sensing, Earth sciences, geodesy, astronomy, and seismology. Advances in atomic, optical, and quantum science have led to a significant improvement in the accuracy of atomic clocks. Research in the field of atomic clocks has helped to push the boundaries of basic and applied research. The concept of atomic optical time is presented in [28]. Moreover, the authors of [29] presented research focused on increasing the accuracy of the atomic clock up to 6.4 \(\times\) 10\({}^{-18}\). This resulted in the improvement of the International System of Units (SI) [30], the search for time variation of fundamental constants [31], the clock-based geodesy [32], and other accuracy tests of the fundamental laws of nature [29]. Traditional radio frequency methods of precise time, transmission and dissemination use satellite methods. The more accurate two-way satellite time and frequency transfer (TWSTFT) method [33] achieves subnanosecond stability; however, it requires a pair of dedicated channels on a geostationary satellite, and thus, it tends to be very expensive. The so-called common view method is considerably affordable, using one or more global navigation satellite systems such as GPS, Galileo, GLONASS, etc. [34]. However optical methods are able to deliver significant improvements in terms of stability, mainly regarding the utilization of an optical carrier with a frequency of hundreds of THz (typically 200-600 THz). They can be used for satellite communication [35] or deployed on the ground; however, the performance of the ground methods is compromised by the limited reach and stability due to weather induced attenuation or turbulence [36]. Methods using optical fibers benefit from a significantly shorter path than those of satellite-based methods and afford the possibility of active transmission stabilization. Optical time transfer methods deliver accuracy on the order of picoseconds (or even better), which is significantly better than that of standard network methods, i.e., according to the IEEE Std. 1588. Another advantage over satellite transfer is the elimination of possible jamming and spoofing attacks, antennas, and the necessity of having a clear sky view, which might be challenging to achieve in underground facilities or very large buildings. Recently, there has been increased number of theoretical studies, laboratory trials, and even field implementations of time and frequency transfer, e.g., [15; 37; 38; 39; 40; 41; 42; 43; 44]. Many works [45; 46; 47; 48; 49] have addressed simultaneous transmissions of accurate time and stable frequency. By transferring the exact time and a stable frequency, simultaneous transmission of two photonic services is realized. Next, [50] has presented a simultaneous transmission, where the timestamptransmission is implemented by phase modulation with a spread spectrum pseudorandom modulation at 20 Mchip/s. A significant portion of works report overcoming large distances using fiber shared with data transmissions [15; 37; 40; 41; 49]. At present, there are only a limited number of publications addressing the parallel involvement of further photonic services, e.g., optical fiber sensing. The works reported in [51; 52; 53] describe the idea of the involvement and realization of parallel optical fiber sensing; however, the discussion is limited to different types of optical fiber and the sensing system pulse duration only. The main contribution of this work is the presentation of a practical measurement of accurate time transmission, a sensing system, and the concept of data flow in a single optical fiber or a combination of different types of optical fibers with different pulse widths for the sensor system. Importantly, the system for the transmission of accurate time was built by our team; it is not a commercial solution.