On Explaining and Reasoning About Optical Fiber Link Problems

DOI: 10.1049/ic:20070402 ISBN: 978-3-8007-3042-1 Location: Berlin, Germany Conference date: 16-20 Sept. 2007 Format: PDF We demonstrate theoretically and experimentally that the differential group delay and differential attenuation slope of an optical fiber with polarization-dependent loss can be explicitly obtained by reflectometric measurements in optical frequency and fiber length domains. (2 pages) Inspec keywords: polarisation; optical links; optical dispersion; reflectometry Subjects: Optical variables measurement; Optical fibre networks

We report results of the extensive development work done on the 270-m optical fiber link for the GRACES project and a preliminary investigations into a high numerical aperture fiber for astronomy. The Gemini Remote Access CFHT ESPaDOnS Spectrograph (GRACES) is an instrumentation experiment to link ESPaDOnS, a bench-mounted highresolution optical spectrograph at CFHT, to the Gemini-North telescope with an optical fiber link. A 270-m fiber link with less than 14% Focal Ratio Degradation (FRD) has been developed jointly by HIA and FiberTech Optica for the experiment. A preliminary study has been conducted by HIA into a high numerical aperture fiber (0.26 numerical aperture) with the intended application of wide field optical spectrographs fiber fed from the telescope prime focus. The Laboratory test results of FRD, transmission, and stability for the GRACES fiber link and preliminary FRD measurements of the high numerical aperture fiber tests are reported.

De-multiplexing the required spectrum in a traffic demand into multiple non-adjacent granular spectrums for dynamic traffic grooming in EON

A mobile base station (MBS) comprises a central unit (CU), a distributed unit (DU), and a remote unit (RU) for efficient deployment in 5th-generation mobile communications system and beyond. The link between a DU and an RU is established using an optical fiber and is renowned as fronthaul (FH). Our study aims to reduce the cost of this FH link. Networking and wirelessly connecting the FH link have been studied to suppress the FH link cost and to flexibly deploy RU1s. Wireless fronthauling is an important solution because it can dispense with optical fiber laying. The location of MBSs is also an important issue. RUs are densely deployed to gain high wireless throughput per area. A DU should be located at a place where RUs can perform cooperative operation and the latency requirement is satisfied. To reduce the optical fiber deployment cost, we studied the DU placement design using wireless multihop links and point-to-point (PtP) optical links. It determines the DU locations to increase the wireless links for reducing the deployment cost. The problem of this scheme is that most of the RUs are still connected to the DU via PtP optical links. This paper proposes a novel FH designing scheme that employs a passive optical network (PON) and wireless multi-hop as well as PtP optical links. We propose a separate designing algorithm for the wireless and PON links. The optical fiber link cost is observed to reduce when compared with that of the previously proposed scheme using a simulation.

Optical fibre links for distributing optical frequencies and time stamps were researched and experimentally tested in the past fifteen years. They have been used mainly for stability comparison of experimental optical clocks. But recent development puts demands on a technology transfer from laboratory experiments to the real industry. The remote calibration of interrogators of Fibre Bragg Grating strain sensory networks is one of important examples. The first step of the adoption the time and frequency broadcasting should be the drop-out free long-term operation of this technology between research laboratories connected via long-haul fibre links. We present a 306 km long-haul optical fibre link between the cities of Prague and Brno in the Czech Republic where a coherent transfer of stable optical frequency and a stable time signal has been firstly demonstrated. The link between ISI CAS Brno and CESNET Prague uses an internet communication fibre where a window of 1540-1546 nm is dedicated for the coherent transfer and 1PPS signal. The link is equipped with 6 bidirectional EDFA amplifiers. The optical frequency standard based on the highly-coherent laser Koheras Adjustik working at 1540.5 nm and stabilized with a saturation absorption spectroscopy technique was used for the coherent wave transfer. The suppression of the Doppler shift induced by the optical fibre was based on an accoustooptical modulator with a servo-loop including a fast PID controller processing the beat-note frequency given by mixing of the Adjustik laser (Brno) and the reflected frequency of this laser from the far end of 306 km long-haul fibre link (Prague). We verified the Doppler shift suppression for the coherent wave with a measuring method analysing the transport delay of the 1PPS signal.

Precise time and frequency signal transfer over a long distance is one of the key activity in time and frequency metrology as precise time plays critical roles in many of the advanced technology as well as in scientific research. Since, last few decades optical fibre-based time and frequency signal transfer techniques has attracted immense importance as optical fibre-based methods show very low instability over long distances in various configurations. This article presents the time synchronization of a remotely located clock with a reference atomic clock over a 10 km long optical fibre link within an uncertainty of 100 ps. Effect of ambient conditions mainly temperature variations on the White Rabbit (WR) based optical fibre link has been studied and a simple method to mitigate the effect of ambient variation on time synchronization accuracy has been presented. Stability of the phase compensated optical fibre link in terms of modified Allan deviation is reaching ~ 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">–16</sup> within one day of integration time and time deviation goes down to 2.2ps at 256 sec integration time.

Optical fiber links offer the best performance to transfer time and frequency. They are beneficial to primary metrology, and there is a strong effort to develop a European network of T&F fiber links. In Italy, we realized a coherent optical fiber link from the National Metrological Institute INRIM to the INAF radio-antennas in Medicina, Bologna and the physics laboratories in Sesto Fiorentino, for a total haul of 642 km. We demonstrated the transfer of primary frequency references to the radio-antenna in Medicina, with potential impact for future development in radioastronomy and VLBI. The extension of this link targets to arrive to Matera, at the Space Geodesy Center of ASI. We describe here the space metrology interests of the optical fiber link, the present set-up of the experiment the preliminary results and the future developments.

With the progress in the development of optical frequency standards grows the demand for the dissemination of stable optical frequencies. To date, optical fiber links constitute the most promising medium to bridge large geographical distances while still maintaining a high degree of frequency stability and accuracy. We investigated the transfer of an optical frequency along different fiber links during the past years and achieved a fractional instability and uncertainty at a level lower than <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>19</mml:mn> </mml:mrow> </mml:msup> </mml:math> using fiber links with lengths of up to almost 2000 km. We give an overview of different techniques and methods that can be used in combination with optical fiber links to achieve a stable frequency transfer. The results of different fiber links are summarized and an outlook of future links is given.

Free-space optical measurement systems can have a direct impact on evaluation systems operational in propagation paths. During propagation via optical fibers, light suffers scattering or interference, causing some output signal loss with an uncertainty outcome. Therefore, this study aims to explore the instant decisions related to the use of single- and multi-mode fiber optics and how they affect the gathering of data from high-speed optical measurement instrument links. The study also seeks to address a number of design methodology aspects and the empirical outcomes related to a surface topography measurement sensor based on fiber optics capable of surface roughness or step-height measurement. The study suggests that the Fourier transform profilometry method (FTP) can overcome the disadvantages of optical metrology sensors (e.g., bulkiness, challenging set-up, high costs, and low speed). However, despite eliminating vertical height problems, the Fourier transform profilometry (FTP) did have some shortcomings for every outcome related to core variables, including the dispersive optical fiber link sensor. The synthetic wavelength method enabled the dispersive optical fiber link sensor to calculate the vertical step height of the selected sample (1 µm). There was improved step-height repeatability, with satisfactory from 20 to 18 nm outcome improvement range. Additional investigations are necessary to establish the compatibility of single- or multi-mode optical fiber sensors with particular instruments, especially those currently preferred for embedded metrology applications.

This article examines the requirements for the design of optical fiber communication networks used in telecommunication, data center communication, and analog applications. Figures of merit for different applications are discussed, including signal‐to‐noise ratio, bit error rate, modulation error ratio, and spurious‐free dynamic range. Optical link budgets are reviewed in detail, including transmission loss, wavelength‐dependent attenuation, splice loss, and connector loss. Factors affecting optical link performance are presented, including modal and chromatic dispersion, mode partition noise, relative intensity noise, and other link design parameters. Examples of a simple approach to optical ink design are also discussed.

Subcarrier multiplexed (SCM) optical fibre links provide efficient high capacity microwave links. If these links are to be used reliably it is essential to fully understand their nonlinear dynamics. The laser plays a key role in an optical fibre link. Since the laser diode is inherently nonlinear it is essential to fully understand the dynamics of the laser diode under multi-tone modulation. So far the chaotic dynamics of laser diodes have only been analysed for the case of single-tone modulation. As a step towards understanding the nonlinear dynamics of microwave SCM optical fibre links, we present a theoretical investigation into the nonlinear dynamics of directly modulated laser diodes under two-tone modulation. (4 pages)

This paper presents a new architecture for flexible and secure networking in battlefields. Our architecture is based on the combination of optical (fiber) and wireless links. Our objective is to be able to carry on undeterred communication without the attack/eavesdropping nodes being able to detect the presence of any communications. We combine the advantages of optical links, such as high bandwidth, low delays, low error rates, good security, with the advantages of wireless links such as mobility and flexibility. From security point of view, we also assume presence of red zones, which are the ones controlled by the adversary or where the adversary can trace wireless activities. The advantage of fiber optical links was demonstrated in various military conflicts. On the other hand, in a dynamic environment, such as those of battlefields, it is expected that fiber cabling will be limited or difficult to be extended. Based on these assumptions we develop our hybrid wireless-optical protocol. The wireless communications are also optimized by searching the best tradeoff among coverage, energy use and security, which means that the wireless propagation should avoid overlapping with red areas, controlled by the adversary. In order to achieve the required optimization among the above mentioned factors, in our wireless protocols we change the transmission power (range) and the selection of retransmitting nodes.

Bending effects on bandwidth and loss of a graded-index polymer optical fiber (GI POF) were investigated, and those influences on the optical link performance were discussed simultaneously for the first time. The numerical apertures (NAs) of the GI POFs were deliberately varied from 0.154 to 0.292. A bending radius larger than 10 mm induced little mode coupling and little change in the bandwidth of the GI POF. A bending radius smaller than 10 mm caused degradation in the bandwidths of the higher NA GI POFs. On the other hand, in the lower NA GI POFs, even more severe bending could cause little change in their bandwidths. Thus, the lower NA GI POF seemed preferable in suppressing the bandwidth change. However, the higher NA GI POF exhibited the lower bending loss. The preferable NAs for both characteristics were completely opposite. Moreover, the bending loss under underfilled launch (UFL) could be lower than that under overfilled launch (OFL). On the other hand, the bending loss was equivalent to the bandwidth degradation in view of the link power penalty. Therefore, the bandwidth change and loss caused by the bending were the critical factors to optimize the optical link considering the link power budget. The high-NA GI POF with almost ideal refractive-index profile could provide stable high performance in overgigabit communication even under any bending conditions.

This paper reports an analysis of the performance of wireless broadband communication systems employing fiber-optic links to connect the base station with the control station. Rainfall attenuation on the radio link, and nonlinear distortions arising from the direct modulation of the laser source, besides the noise contributions coming out from both the optical transmitter and receiver, are considered in the analysis. As far as the radio link is concerned, the outage probability, due to the rainfall attenuation, is recounted for relevant cases; while, concerning the optical link, several carrier-to-noise ratio curves versus the optical modulation index of the channels are reported, varying different system parameters. With respect to the previous literature, a complete analysis of the optical link performance, in this type of systems, has been accomplished by taking into account, simultaneously, all the effects influencing the performance.

We present a symmetric physical layer based secret key generation scheme for Point-to-Point Optical Link (PPOL) communication by exploiting Polarization Mode Dispersion (PMD) as a random and inimitable channel characteristic. The randomness and security strength of generated cryptographic keys based on PMD is significantly high. In this paper, we present that random modulation of a probe signal caused by PMD in a high-speed data communication network (40Gb/s and 60Gb/s) is reciprocal with average Pearson correlation coefficient of 0.862, despite the presence of optical nonlinearities, dispersion, and noise in the system. 128-bit symmetric cryptographic key has been successfully generated using the proposed scheme. Moreover, PMD based encryption keys passed the National Institute of Standards and Technology (NIST) tests. We have shown through simulations with a 50km link that, with optimal key generation settings, symmetric keys can be generated with high randomness (high P-values for NIST randomness tests) and with sufficient generation rates (>50%). Furthermore, we considered an attack model of a non-invasive adversary intercepting at 10km into the link and found that the generated keys have high average key bit mismatch rates (>40%).