Abstract: A system and method for applying a time-varying phase shift to an optical signal is described. Such a phase shift results in a frequency shift of the optical signal, which can be useful for instance in sensing applications. The design uses cross phase modulation (XPM) in a nonlinear medium such as optical fiber. The pump producing the XPM experiences a change in energy along the medium, for instance due to loss. The pump and signal have mismatched group velocities such that they walk-off each other in time, and the pump pulse repetition rate is chosen so that it has a specific relationship with respect to the walk-off. The design is compatible with very low signal loss and does not require high fidelity electrical control signals. It is capable of high-efficiency one-directional serrodyne frequency shifts, as well as producing symmetric frequency shifts. It can also be made polarization independent.

Abstract: A method for spectral filtering an input optical signal is described. The filtering system uses Brillouin gain inside an interferometer. The interferometer is biased to null signals that propagate without Brillouin gain, but due to asymmetric levels of gain in the interferometer arms the interference null is disturbed for signals of an optical frequency that experience Brillouin gain. The filter thereby preferentially passes signal frequencies that are inside the Brillouin gain bandwidth creating a high extinction ratio filter. The narrow bandwidth of the Brillouin gain effect can allow for spectrally narrow filters such as 30 MHz. The filter can be realized in a stable Sagnac loop configuration even if the pump and signal are combined prior to the Sagnac loop by using a power control device such as an attenuator placed asymmetrically inside the loop. The filter can achieve net gain and can be designed to have a tunable bandwidth.

Abstract: A method for controlling the splitting ratio of an input optical signal to one or more output ports is described. The splitting ratio of a fiber-coupled signal in the communications band is controlled using cross phase modulation from a pump signal in the 980-1090 nm band. This design allows the nonlinear fiber in which the cross phase modulation occurs to be standard single mode fiber having a zero dispersion wavelength between 1250 and 1350 nm, such as SMF-28e fiber, which helps to maintain the lowest possible loss and a low cost while still allowing for power efficient interactions with signal wavelengths in the technologically important 1520-1610 nm band. The design is compatible with low insertion loss, narrow switching windows, and low added noise. The location of the pump pulse can be controlled allowing for the location of an input pulse to be determined.

Abstract: A method for down-converting an RF signal is described that optically phase modulates an RF signal onto an optical carrier then applies an RF local oscillator (LO) phase modulation which down-converts the RF signal to an intermediate frequency after appropriate optical signal processing and optical-to-electrical photo-detection. The LO phase modulator is constructed such that a common hot electrode is shared among more than one optical mode, where an optical mode can be separate waveguides or optical wavelengths. The relative phase of the LO frequency applied to each optical mode can be different between the different optical modes. The resulting down-converted photo-detected signals of different LO-phase can be processed to reduce noise. A single LO phase modulator can down-convert multiple RF signals carried by multiple optical wavelengths, and a harmonic generation stage with multi-phase-matching peaks can be used to linearize each RF signal.

Abstract: A method for transmitting and receiving a radio frequency (RF) signal over an optical channel is described, where a nonlinear optical harmonic generation device is used to increase the system performance. The RF signal is phase modulated onto an optical carrier. The received optical carrier propagates through a nonlinear optical harmonic generation device, which increases a phase modulation depth at the harmonic wavelength. This larger modulation depth can be used to achieve larger gain. By photo-detecting both the fundamental and the harmonic optical wavelengths, then properly scaling and subtracting the two photo-detected signals, the enhanced modulation depth at the harmonic optical wavelength can be used to cancel out unwanted nonlinear distortions thereby linearizing the measured RF signal.

Abstract: A system and method for measuring the temporal delay an optical signal experiences along a path is provided it uses single photon sensitive detectors and multiple optical pulse rates. The multiple optical pulse rates are chosen to allow each to be isolated in post-processing even if only a single detector is employed. The detectors can be time-gated at a repetition rate synchronized but different from the optical pulse rates, including the use of a pulsed-pump up-conversion detector. The pulse rate choice allows improved performance, including an extension of the unambiguous temporal delay range. The ability to isolate the pulse rates can also be used for measuring multiple path delays simultaneously or for spectrally resolving path characteristics without requiring the use of spectral filtering. The post-processing function can be segmented to include an initial signal quality estimation step so further processing can be aborted if it is unlikely to succeed.

Abstract: The invention is related to phase detection in lidar systems using single photon detectors (SPDs). The frequency at which the SPDs are time gated is related to but not an integer multiple of the frequency of the transmitted optical pulses. Each return optical pulse arrives with a particular temporal position with respect to the nearest gate, and thus is detected with a related detection efficiency. The SPD output can be segmented into multiple time-multiplexed signals whose relative detection efficiency reveals the phase of the optical return pulses, and no such phases have negligible detection efficiency for all the time-multiplexed signals. To mitigate the impact of afterpulsing and other saturation effects, when a first optical pulse is detected with high detection efficiency the next optical pulse that is detected with high detection efficiency is separated by a time period about equal to or greater than the detector dead time.

Abstract: An interferometer used for modulating an optical signal with an electrical signal is described, where the optical signal can be subsequently detected so as to sample and digitize the electrical signal. Nonlinear optical elements can be located inside the interferometer to reduce the minimum detectable electrical input signal size. The interferometer can contain more than two arms to improve the tolerable dynamic range of the electrical signal. In some cases some outputs of the interferometer are dependent on the electrical input frequency while others have minimal frequency dependence, thereby allowing the frequency of the input electrical signal to be measured more easily. Ideally the modulator operates in a push-pull mode with a single electrode for the input electrical signal. Such a modulator can be constructed by using appropriate optical delay elements.

Abstract: A secure optical communication system and method are disclosed. Short optical pulses are first modulated with data, then dispersed in time so that they spread out over multiple bit periods, then the desired code is applied to the dispersed pulses. The encoding may include frequency shifts or phase shifts or other. The dispersed optical symbols overlap in time so an applied code chip thus acts on multiple symbols simultaneously. There are generally multiple code chips per dispersed symbol. The coding device does not need to be synchronized to the data rate. Multiple wavelength division multiplexed channels may be encoded simultaneously. The signal propagates to a decoder that is synchronized with encoder to apply a complementary code thereby canceling out the effect of the encoder. The encoder and decoder can be realized by varying the wavelength of an optical pump to a parametric amplifier, allowing for a wide-band frequency shift.

Abstract: An apparatus and method are disclosed to digitize an input electrical signal. A sequence of nonuniformly spaced optical pulses impinges an optical modulator, where the pulses are modulated by the electrical input signal, the optical pulses being detected by one or more photoreceivers, the photoreceiver outputs being electrically sampled in analog-to-digital converters and then processed in a digital signal processor (DSP) in order to measure the electrical input signal. The series of nonuniform pulses are formed by interleaving L uniformly spaced pulse streams each of repetition period T and where fmax, the maximum allowable spectral frequency of the input electrical signal, is greater than 4·L·f/2, where f=1/T.

Abstract: A single photon detection system and method are disclosed which have a control block for helping to monitor and optimize performance, especially at high detection rates. The system is based on photon detectors constructed with avalanche photodiodes (APD) gated in time to operate in the Geiger mode. An electrical reference frequency is generated which is subtracted from the APD output in order to better isolate the breakdown event. The resulting signal is sampled and analyzed to allow the control unit to optimize the magnitude and phase of the electrical reference frequency. The control unit may also change the gate pulse shape and phase, including by the use of a digital-to-analog converter. The gate pulse can be shifted off an input optical pulse so as to estimate dark count rate, or shifted to measure a reference input signal to estimate detection efficiency.

Abstract: A method of digitizing an analog electrical signal combines optical and electronic techniques in order to improve the resolution, sampling rate, input frequency range, or flexibility. It implements an optical interferometric modulator, which modulates an input optical signal by the input electrical signal combined with a calibration signal. A set of two or more photoreceivers receiving the output optical signals from the optical modulator produce output electrical signals, which are digitized and processed in a DSP to produce a digitized version of the electrical input signal, and a digitized calibration signal value is used to optimize the input electrical signal digitization. The method and the device can be used in many fields including instrumentation, communications, and imaging.

Abstract: An optical spectrum generator is disclosed, which is based on generating conjugate optical frequencies via the use of two or more nonlinear interactions, where filters are used to select the conjugate output of the preceding nonlinear interaction as the input to the following nonlinear interaction. The cascaded nonlinear interaction is seeded with an incoming optical beam and pumped with multiple pump beams. The conjugate output of the last nonlinear interaction can be fed-back to the input of the first nonlinear interaction thus creating a recirculating path. An add-drop multiplexer is implemented for efficiently combining the pump beams with conjugate beams. The generated output frequencies are related to the difference in frequency between the pumps which can be changed as a function of time to create a time-varying output frequency. The magnitude of the variation in output frequency can be magnified by recirculating the signal through the system multiple times.