Abstract: A mirror drive mechanism for a tilting mirror is controlled using feedback from one or more interferometric angular sensors. The wavelength of an optical beam is varied as it is fed into an interferometric angular sensor. The wavelength at which the resulting interference pattern is measured to be at a minimum intensity is determined. This wavelength is used to determine a distance quantity representative of the angular position of the mirror.

Abstract: A heterodyne communication system uses coherent data modulation that is resistant to phase noise. In particular, a pilot tone and reference clock signal are transmitted along with the modulated data to form the basis of an electrical demodulation local oscillator at the receiver. The pilot tone and/or reference clock signal carry phase noise which is correlated with the phase noise in the data signal. At the receiver, the local oscillator is generated from the pilot tone and reference clock signal in a manner so that the local oscillator also has phase noise which is correlated with the phase noise in the data signal. Thus, the two noise components can be used to cancel each other during demodulation of the data signal using the local oscillator.

Abstract: A transmitter subsystem generates an optical signal which contains multiple subbands of information. The subbands have different polarizations. For example, in one approach, two or more optical transmitters generate optical signals which have different polarizations. An optical combiner optically combines the optical signals into a composite optical signal for transmission across an optical fiber. In another aspect, each optical transmitter generates an optical signal containing both a lower optical sideband and an upper optical sideband (i.e., a double sideband optical signal). An optical filter selects the upper optical sideband of one optical signal and the lower optical sideband of another optical signal to produce a composite optical signal.

Abstract: An optical wavelength converter receives an input optical signal, and in response, generates an output optical signal having a wavelength that is different from the wavelength of the received input signal. An optical amplifier amplifies the received input signal and delivers the amplified signal to an optical splitter adapted to split the amplified signal into two identical optical signals. An optical multiplexer receives one of the split signals as well as a continuous wave optical signal to generate a combined signal. An semiconductor optical amplifier receives the combined signal at its first side, and the other one of the split signals at its second side. The output signal is generated as a result of a counter-propagating four-wave mixing occurring within the semiconductor optical amplifier. This output signal has the same wavelength as the continuous-wave signal and has an amplitude modulation that is identical to the amplitude modulation of the input signal.

Abstract: A heterodyne communication system uses coherent data modulation that is resistant to phase noise. In particular, a pilot tone and reference clock signal are transmitted along with the modulated data to form the basis of an electrical demodulation local oscillator at the receiver. The pilot tone and/or reference clock signal carry phase noise which is correlated with the phase noise in the data signal. At the receiver, the local oscillator is generated from the pilot tone and reference clock signal in a manner so that the local oscillator also has phase noise which is correlated with the phase noise in the data signal. Thus, the two noise components can be used to cancel each other during demodulation of the data signal using the local oscillator.

Abstract: A heterodyne communication system uses coherent data modulation that is resistant to phase noise. In particular, a pilot tone and reference clock signal are transmitted along with the modulated data to form the basis of an electrical demodulation local oscillator at the receiver. The pilot tone and/or reference clock signal carry phase noise which is correlated with the phase noise in the data signal. At the receiver, the local oscillator is generated from the pilot tone and reference clock signal in a manner so that the local oscillator also has phase noise which is correlated with the phase noise in the data signal. Thus, the two noise components can be used to cancel each other during demodulation of the data signal using the local oscillator.

Abstract: An optical communications system includes a receiver subsystem with at least two heterodyne receivers. The receiver subsystem receives a composite optical signal having two or more subbands of information and corresponding tones. An optical splitter splits the composite optical signal into optical signals. Each optical signal includes a subband(s) and corresponding tone. Each heterodyne receiver receives an optical signal. The receiver includes a heterodyne detector coupled to a signal extractor. The heterodyne detector mixes the optical signal with an optical local oscillator to produce an electrical signal which includes a frequency down-shifted version of the subband and the tone of the optical signal. The signal extractor mixes the frequency down-shifted subband with the frequency down-shifted tone to produce a frequency component containing the information.

Abstract: A transmitter subsystem generates an optical signal which contains multiple subbands of information. The subbands have different polarization. For example, in one approach, two or more optical transmitters generate optical signals which have different polarization. An optical combiner optically combines the optical signals into a composite optical signal for transmission across an optical fiber. In another aspect, each optical transmitter generates an optical signal containing both a lower optical sideband and an upper optical sideband (i.e., a double sideband optical signal). An optical filter selects the upper optical sideband of one optical signal and the lower optical sideband of another optical signal to produce a composite optical 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 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 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 transmitter subsystem generates an optical signal which contains multiple subbands of information. The subbands have different polarizations. For example, in one approach, two or more optical transmitters generate optical signals which have different polarizations. An optical combiner optically combines the optical signals into a composite optical signal for transmission across an optical fiber. In another approach, a single optical transmitter generates an optical signal with multiple subbands. The polarization of the subbands is varied, for example by using a birefringent crystal. In another aspect of the invention, each optical transmitter generates an optical signal containing both a lower optical sideband and an upper optical sideband (i.e., a double sideband optical signal). An optical filter selects the upper optical sideband of one optical signal and the lower optical sideband of another optical signal to produce a composite optical signal.

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: An electro-optic modulator includes a splitting section, at least three transmission legs, an RF phase-shifting section, a DC-phase shifting section, and a combining section. The splitting section splits a received optical signal into sub-signals, one for each transmission leg. The RF phase-shifting section phase shifts at least two of the sub-signals by an amount proportional to a received RF signal; while the DC phase-shifting section phase shifts at least two of the sub-signals by a DC phase. The combining section combines the phase-shifted sub-signals into a modulated optical signal. In a preferred embodiment, the modulator is characterized by design parameters, such as splitting ratio, DC phase shift, RF coupling efficiency, and combining ratio, and these design parameters are selected to ensure that the modulator meets predetermined performance characteristics, such as maximum harmonic levels or minimum signal to noise ratios.

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).

Abstract: A Mach-Zehnder modulator (MZM), which divides a CW laser beam into two optical portions, is biased at a 180.degree. phase difference between the two optical portions. An RF signal and an LO signal are simultaneously applied to one of the optical portions of the laser beam to produce phase changes between the two optical portions. The two optical portions of the laser beam are then recombined into an optical output beam, which is detected by a photodetector. The photodetector generates a photocurrent, which contains a component at a beat frequency--i.e., the frequency difference between the RF and LO frequencies. The waveform of the photocurrent component at the beat frequency is substantially the same (except for amplitude and a fixed phase shift) as the waveform of the RF signal.