We present the latest progress on digital coherent optical transmission technologies and their prospects for deployment for broadband radio access, ultimately realizing a full-coherent integrated network. Recent advances in digital coherent optical transmission technologies have made it possible to utilize both the intensity and phase of an optical field, thus allowing seamless convergence with a wireless network. Such a system, which we term a full-coherent system, can offer substantial advantages in terms of transparency, cost, and bandwidth scalability for broadband radio access networks.
We have described a frequency-stabilized, polarization-maintained erbium fiber ring laser. This laser has no frequency
modulation at the output beam. A tunable single-mode laser has also been newly developed by simultaneously
controlling a tunable FBG with a 1.5 GHz bandwidth and a PZT in the cavity. The frequency stability reached as high as
1.3 x 10-11 for an integration time of 1 s and the linewidth was as narrow as 4 kHz. Using this coherent laser as a light
source, we successfully transmitted a 20 Msymbol/s coherent quadrature amplitude modulation (QAM) signal over 525
km and achieved error free transmission.
Polarization mode dispersion (PMD) is a serious limiting factor for high bit rate optical telecommunication systems. It is important and necessary for network managers to obtain PMD information at single end of the fiber cable. In this paper, a novel measurement method of PMD in optical fibers and devices is proposed, which is based on coherent optical frequency domain reflectometry technique (OFDR). In the method, a frequency-shifted feedback (FSF) fiber laser is developed as light source; therefore a frequency chirped probe light with good linearity, wide chirp range, and high chirp rate is realized. The PMD is determined by self-delayed heterodyne detection from the beat frequency generated by interference between lights from the Fresnel reflection at the far end of the device under test, which makes the measurement at single end of the device possible. Experiment is demonstrated on both polarization maintaining fibers (PMF) and single mode fibers (SMF), and results are in agreement with interferometric method.
A Frequency-Shifted Feedback (FSF) laser has an intracavity acousto-optic modulator (AOM) and the spectral output consists of a chirped frequency comb evenly spaced at the cavity free spectral range (FSR). An FSF laser is a useful source for optical frequency domain reflectometry (OFDR). We present a new average atmospheric temperature sensor by OFDR using an FSF laser for the first time. The beat signal, which is detected through the self-delayed heterodyne detection of an FSF laser, is proportional to the path difference, and measurements can be done within the frequency bandwidth of a cavity FSR. Furthermore, the beat frequency characteristics are unrelated to the beat order. Therefore, the path measurement resolution is consist and unrelated to the path difference. Changes in atmospheric refractive index primarily depend on variation of temperature and pressure. Observing variation in path difference with an FSF laser should allow calculation of the average atmospheric temperature along the path if the change in pressure is known. As the path difference increases, the temperature resolution improves. This paper outlines the principle of the average atmospheric temperature measurement using an FSF laser and presents preliminary experimental result.
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