Optical Coherent Detection Systems Research Articles

Performance of free-space optical coherent detection systems under imprecise channels with nonzero boresight pointing errors

Performance of free-space optical coherent detection systems under imprecise channels with nonzero boresight pointing errors

Effects of chromatic dispersion on optical coherent-detection systems

Chromatic dispersion induces to a phase-modulated optical signal a constant phase rotation that was never included in most previous studies. When the constant phase shift is removed by a phase-locked loop, the dispersion tolerance of a coherent-detection system is increased. The dispersion tolerance of phase-shift keying (PSK) signal is compared with differential PSK (DPSK) signals by numerical simulation and experimental measurement. Contrary to conventional belief, PSK signal has larger dispersion tolerance than DPSK signal.

Surface micro-machined optical coherent detection system with ultra-high sensitivity

A surface micro-machined self-homodyne optical coherent detection system is demonstrated. Fabricated on a Silicon chip, such a system comprises a polysilicon membrane as an optical phase modulator, a micro-beamsplitter and two micro-mirrors for beam steering. A three layer polysilicon foundry process provided by MCNC (Mems Center at North Carolina) is used for device fabrication. A signal Max/Min ratio of 22 dB is achieved with this coherent detection system, which in turn offers ultra-high sensitivity in various types of sensing schemes, such as optical refractive index sensing with a minimal detectable index change down to 10 −7, chemical and gas sensing, and laser spectrometry. The optical coherent sensing method is also applicable to the signal readout of inertial sensors, such as in micro-gyroscope technology.

A polarization modulation (PLM)/optical coherent detection system and its phase noise cancelling effect

In coherent optical communication systems, the phase noise of the optical source is the dominant factor in limiting the receiving characteristics and its reduction is required. In this paper, a polarization modulationl optical coherent detection system is proposed and theoretical and experimental studies are carried out for this system. First, it is demonstrated that the polarization modulation signal is a differential phase-modulated optical signal between the two orthogonal polarization components. The fundamental principle for the optical coherent detection system is presented. Next, by taking into account the characteristics of the modulator, a general representation of the polarization modulation optical signals is derived and the inherent characteristics of the present system is shown. Further, for a polarization modulation/optical heterodyne detection system for a long-distance transmission, the detection and demodulation methods are presented. The phase noise cancellation effect is found as well as the fact that the polarization axis matching between the transmitter and the receiver is not necessary if a circular polarization modulation is used. The bit error rate of the polarization modulation/optical heterodyne detection system is analyzed theoretically. By comparing this system with other coherent optical communication systems, the receiving characteristics are evaluated. Finally, fundamental experiments are carried out for the optical heterodyne detection of the polarization modulation signals and the phase noise cancellation effect of the present system is confirmed.