Emilio Vezzoni

Performance and limitations of laser diodes stabilized to the sides of molecular absorption lines of ammonia

Performance and limitations of laser frequency stabilization based on the locking to the side of an absorption line are discussed: experimental results of spectroscopic measurements and of the stabilization of 1543‐nm DFB laser diodes to ammonia absorption lines are presented.

Impact of DFB-LD FM response distortion, in amplitude and phase, on FSK coherent systems

The influence of amplitude and phase distortions of the FM response of DFB (distributed feedback) lasers on coherent FSK (frequency shift keying) transmission systems has been investigated. It was found that pattern effects are mainly due to phase distortions, while the IF (intermediate frequency) spectrum broadening is due to both amplitude and phase distortions. Possible remedies are described, including device selection, line coding, and phase and/or amplitude equalization of the FM response. A comparison between observed spectra and computer analysis has been performed, showing a good agreement. Transmission experiments at 140 Mb/s have been carried out, showing the predicted effects on the eye diagram.<<ETX>>

Laser frequency stabilization for high bit-rate FSK multichannel coherent systems

After reviewing some previously proposed laser frequency stabilization techniques for multichannel coherent systems, the authors present a simple, alternative method, making use of a scanning Fabry-Perot resonator. This technique, which proved effective in a preliminary experiment stabilizing a single DFB (distributed feedback) laser diode within less than 500 kHz, was successfully applied to the collective stabilization of a two-channel transmitter and can be easily extended to a multichannel configuration with a high number of channels. The same principle can operate, with some modification, in the presence of FSK (frequency shift keying) direct modulation. This has been experimentally demonstrated for a DFB laser that was directly modulated up to 2 Gb/s.<<ETX>>

Demonstration of polarisation independent coherent transmission by synchronous intra-bit polarisation spreading

Polarization-independent heterodyne detection of an FSK (frequency shift keying) signal has been demonstrated by the synchronous intrabit polarization spreading technique, which consists in the switching or proper modulation of the state of polarization of the laser output field inside the bit period. This approach is particularly suitable for distribution networks, as the polarization diversity function is shifted to the transmitter, thus resulting in a simpler and cheaper heterodyne receiver. A transmission experiment, making use of a 1.55 mu m DFB (distributed feedback) laser diode, directly FSK modulated at 155 Mb/s, a properly developed high-speed polarization modulator, and single filter heterodyne detection, has been successfully carried out. The penalty resulting from preliminary bit-error-rate measurements ranges from 4 to 6.5 dB, including the 3 dB intrinsic loss common to all polarization spreading techniques.<<ETX>>

Characterization Of Laser Diode Intensity Noise At Microwave Frequencies With High Sensitivity

A measurement technique has been developed, that allows the accurate characterization of laser diode intensity noise up to microwave frequencies, with high sensitivity. Relative intensity noise levels lower than 10-15 Hz-1 can be measured, with good accuracy, up to 10 GHz. Several examples are presented, showing the effects of reflections on laser noise.

Optical Fibre System and Related Sources

Optical fibres are finding increasingly widespread use in all telecommunication applications. Industrially developed countries have plans for installation of thousands of kilometers of optical cables in the near future. Technical advantages of optical fibres are the justification for this success: low attenuation, high bandwidth, low weight and small size, insensitiveness to e.m. disturbances, suitability for a large range of applications, easy upgradability. Applications include intra building and intra vehicle communications, local area distribution (links between computers), subscriber loop, links between telephone exchanges, inter city, long distance communication, submarine cables.

Measurement of Chromatic Dispersion in Single Mode Fibres by Incoherent Sources

We have demonstrated for the first time that accurate chromatic dispersion measurements are feasible in single mode fibres, making use of sinusoidally modulated incoherent sources. This technique has been extended in the long wavelength range, thus allowing the accurate determination of the zero chromatic dispersion wavelength in an easy way and with simple instrumentation.