Abstract: Return-to-zero (RZ) formatted data is recovered and transmitted using non-return-to-zero (NRZ) devices. A NRZ clock and data recovery device (CDR) interprets the clock rate of a RZ formatted signal as twice its actual clock rate. Due to this interpretation, extra zeroes will be inserted in the data stream. The extra zeroes introduced by the NRZ interpretation of the data are discarded, and the interpreted clock rate is divided resulting in preserving the values of the original data stream of the RZ formatted signal. A NRZ encoded data stream at a specific clock rate is processed so that when the data stream is transmitted to a recipient expecting RZ formatted data, the recipient interprets the correct data.

Abstract: A frequency division multiplexing (FDM) node used in optical communications networks provides add-drop multiplexing (ADM) functionality between optical high-speed channels and electrical low-speed channels. The FDM node includes a high-speed system and an ADM crosspoint. The high-speed system converts between an optical high-speed channel and its constituent electrical low-speed channels through the use of frequency division multiplexing and preferably also QAM modulation. The ADM crosspoint couples incoming low-speed channels to outgoing low-speed channels, thus implementing the ADM functionality for the FDM node.

Abstract: A device is used to wavelength lock two optical signals to some frequency offset. A photomixer section produces a frequency test signal from the beat component of the two optical signals. The frequency of the frequency test signal reflects whether the actual frequency offset of the two optical signals equals the desired offset. A frequency filter with a monotonically varying transfer function is used to filter the frequency test signal. Thus, different gains are applied to different frequencies. Comparison circuitry uses the filtered signal to determine whether the frequency filter applied the gain which corresponds to the desired frequency offset and generates a corresponding error signal.

Abstract: A frequency division multiplexing (FDM) node used in optical communications networks provides add-drop multiplexing (ADM) functionality between optical high-speed channels, and low-speed tributaries. The FDM node includes a high-speed system and an ADM crosspoint. The high-speed system converts between an optical high-speed channel and its constituent electrical, low-speed channels through the use of frequency division multiplexing. The ADM crosspoint couples any incoming low-speed channels and any incoming tributaries to any outgoing low-speed channels and tributaries, thus implementing the ADM functionality for the FDM node.

Abstract: A system transmits digital data over an optical fiber at high aggregate data rates and high bandwidth efficiencies. The system includes a modulation stage, a frequency division multiplexer, and an optical modulator. The modulation stage QAM-modulates a plurality of incoming digital data channels. The frequency division multiplexer combines the QAM-modulated signals by frequency division multiplexing them into an RF signal. The optical modulator uses the RF signal to modulate an optical carrier for transmission over an optical fiber.

Abstract: The bias point of an electro-optic modulator, such as an Mach-Zender modulator, is automatically controlled. A pilot signal, preferably two pilot tones at different frequencies, is applied to the modulator. The output of the modulator then contains various components resulting from the pilot signal. An error signal is generated, preferably coherently, based on one of these components, termed the pilot component, which preferably is located at the difference frequency of the two pilot tones. A bias signal which controls the bias point of the modulator is adjusted based on the error signal. The coherent generation of the error signal facilitates feedback loops based on phase and/or amplitude control and also supports locking the electro-optic modulator to a bias point with a preselected slope (either negatively or positively sloping).