Signal stability in optical transmission The project offers an opportunity to build our skills in the stability of optical transmission of signals. This includes variations in throughput but also the characteristics of the laser or signal that affect quality. The results are achieved in a number of smaller sub-projects, where several successful results have been published or presented at international conferences.
When a signal is sent through an optical fiber, its delay will vary over days, depending mainly on variations in temperature, but also other mechanical tension that may occur. For time-sensitive signals this gives an unacceptable uncertainty, unless the delay is constantly measured and taken into account. The project evaluates various parameters affecting the optical transmission, and methods to compensate for them.
Transfer of time in the fiber optic communications
In co-operation between the research groups for Time & frequency and Photometry & radiometry, the analysis of how a time signal is affected by transmission in optical fibers was conducted. For accurate transmission of time signals, it is of utmost importance that all of the delays are known or eliminated. The basic technique used is to send the signal in both directions and assume that the delay is symmetric, an assumption that is valid with good accuracy, if not faultless, in today's fiber optic network.
The applied evaluation of time-transfer in fiber optic networks is funded by the National Post and Telecom Agency (PTS), while the basic research is conducted in this project. Measurements apply to both the optical phase and phase of the electrical synchronization signal which modulates the light.
Time delay is measured and evaluated after the effects of temperature variations and external disturbances, but also optical effects such as non-linear phase-shift at high power, and crosstalk from adjacent channels at high intensity in wavelength division multiplexed systems. Work has been carried out by researchers, but also undergraduates, who evaluated the degree of symmetry in a laboratory environment and measured very low spread in the readings.
Dynamic time stabilization of optical signal
Even for short distances may the variable delays in the fiber-optic transmission cause significant uncertainty in the stability. To meet this need, a system has been demonstrated by performing a dynamic compensation of the delay, in counterphase to the events in the fiber. The system is based on a two-way transfer, in which both the transmitted and the received signal is delayed in the compensation setup. Analysis then tries to keep the low-frequency phase stable in the transmitter, and thus obtain a stable signal phase at the receiver. The results of the demonstration has been presented at the Topical Meeting on Precise Time and Time Interval, PTTI'08.
Development of ultra-stable laser
In cooperation with the "low gas flow" developed a fiber-based ring laser whose wavelength is ultimately determined by a gas-filled Fabry-Perot cavity. The aim of the laser is to achieve a stability sufficient to detect the wavelength shift caused by a density change in the cavity.