Abstract: [Consistent with the present disclosure an apparatus and related method are provided for controlling the leaf-receiver local oscillator laser and leaf-transmitter laser for cases where separate transmit and receive local oscillator lasers are included in a transceiver. As a result, full capacity in bidirectional transmission can be realized on a single fiber. The leaf local oscillator frequency is controlled using a feedback signal generated based on an output from the leaf-digital signal processor (DSP), and the leaf transmit laser is controlled using a feedback signal based on an output of the remote hub-DSP, which is carried from the hub to the leaf nodes by a general communication channel (GCC) as part of a data signal, or a separate subcarrier also referred to as an auxiliary channel or out-of-band channel. This ensures that the frequencies transmitted subcarriers from the leaf nodes do not collide or overlap with one another in frequency.

Abstract: Consistent with the present disclosure, multiple forward error correction (FEC) encoders are provided for encoding a respective one of a plurality of data streams. A mechanism is provided to mix or interleave portions of the encoded data such that each subcarrier carries information associated with each data stream, as opposed to each subcarrier carrying information associated with only a corresponding one of the data streams. As a result, both higher SNR and low SNR optical subcarriers carry such information, such that errors occurring during transmission are distributed and not concentrated or limited to information associated with a single data stream. Accordingly, at the receive end, each FEC decoder decodes information having a similar overall error rate. By balancing the error rates across each FEC encoder/decoder pair, the overall ability to correct errors improves compared to a system in which mixing or interleaving is not carried out.

Abstract: Techniques are described for implementing an out-of-band communication channel used to exchange control channel information in sub-carrier-based optical communication systems. In an example implementation, a transmitter includes a laser operable to supply an optical signal, a digital signal processor operable to supply first electrical signals based on first data input to the digital signal processor and second data input to the digital signal processor, digital-to-analog conversion circuitry operable to output second electrical signals based on the first electrical signals, modulator driver circuitry is operable to output third electrical signals based on the second electrical signals, and an optical modulator operable to supply first and second modulated optical signals based on the third electrical signals. The first modulated optical signal includes a plurality of optical subcarriers carrying user data. The plurality of optical subcarriers also being amplitude modulated to carry control information.

Abstract: Systems and methods for performing impairment compensation in point-to-multi-point communication systems are described. In a data snapshot mode, a hub node can send instructions to each communication node connected to the hub node to send a data snapshot of data being received and processed by the communication nodes at a particular time. In a trench line mode, a hub node sends instructions to each communication node to send trench line data back to the hub node. The hub node uses the data snapshot or trench line data to determine how to tune filter coefficients in the hub node to perform impairment compensation and improve performance of the communication system.

Abstract: Consistent with the present disclosure, multiple forward error correction (FEC) encoders are provided for encoding a respective one of a plurality of data streams. A mechanism is provided to mix or interleave portions of the encoded data such that each subcarrier carries information associated with each data stream, as opposed to each subcarrier carrying information associated with only a corresponding one of the data streams. As a result, both higher SNR and low SNR optical subcarriers carry such information, such that errors occurring during transmission are distributed and not concentrated or limited to information associated with a single data stream. Accordingly, at the receive end, each FEC decoder decodes information having a similar overall error rate. By balancing the error rates across each FEC encoder/decoder pair, the overall ability to correct errors improves compared to a system in which mixing or interleaving is not carried out.

Abstract: Techniques are described for implementing an out-of-band communication channel used to exchange control channel information in sub-carrier-based optical communication systems. In an example implementation, a transmitter includes a laser operable to supply an optical signal, a digital signal processor operable to supply first electrical signals based on first data input to the digital signal processor and second data input to the digital signal processor, digital-to-analog conversion circuitry operable to output second electrical signals based on the first electrical signals, modulator driver circuitry is operable to output third electrical signals based on the second electrical signals, and an optical modulator operable to supply first and second modulated optical signals based on the third electrical signals. The first modulated optical signal includes a plurality of optical subcarriers carrying user data. The plurality of optical subcarriers also being amplitude modulated to carry control information.

Abstract: Systems and methods for performing impairment compensation in point-to-multi-point communication systems are described. In a data snapshot mode, a hub node can send instructions to each communication node connected to the hub node to send a data snapshot of data being received and processed by the communication nodes at a particular time. In a trench line mode, a hub node sends instructions to each communication node to send trench line data back to the hub node. The hub node uses the data snapshot or trench line data to determine how to tune filter coefficients in the hub node to perform impairment compensation and improve performance of the communication system.

Abstract: Consistent with the present disclosure independent phase and frequency clock recovery on each SC. Both leaf and hub perform digital clock recovery on each SC by increasing the Rx-ADC sampling rate by a few ppm (˜16 ppm), and using a delay compensating element, together with gapped clocks. The gaps and delay compensating elements are independent on each SC. The delay element is performed using the frequency domain DSP engine, where the frequency domain equalizer coefficients are modified with a delay compensating element Thus, each SC can have its own fine timing frequency and timing phase tuning, and fine tracking of its own jitter. When the delay compensating element, which, for example, may include a finite impulse response (FIR) filter, reaches the end of its range, a clock gap equal to an integer number of symbols is performed. The delay element can be reset by the same number of symbols providing continuous phase interpolation.

Abstract: A receiver for an optical communications system which corrects distortion of a received signal. A clock recovery system utilizing a feedback and feedforward system are provided. The feedback loop comprises a phase detector and a clock source, while the feedforward loop comprises the phase detector and a delay element for delaying the output of distortion correction system. The feedback loop has a significantly lower bandwidth than the feedforward path. There are also provided methods of optimizing tap weights and of acquiring initial tap weights.

Abstract: A self-calibration procedure for an optical transmitter is provided. During the self-calibration procedure, a phase bias of an optical modulator of the optical transmitter is set so that an in-phase path and a quadrature path of the optical modulator are in phase. Stimulus signals are supplied to the in-phase and quadrature paths of the optical modulator, over a frequency range. Detection, with a photodetector, is made of an optical output of the optical modulator at a plurality of frequency steps over the frequency range. A photodetector converts an optical output of the optical modulator to an electrical signal. First and second measurement values are generated from the electrical signal output from the photodetector. A frequency spectrum and/or time delay is computed from the first and second measurement values for each frequency step value over the frequency range.

Abstract: A system includes a laser configured to generate a tunable optical frequency. The system also includes an optical transmitter to map baseband data to symbols represented in a digital modulation constellation, add a frequency offset to the digital modulation constellation to cause the digital modulation constellation to rotate at a rate equal to the added frequency offset, modulate the optical frequency with the rotating digital modulation constellation, and transmit the resulting modulated optical frequency. The system also includes an optical receiver to receive the transmitted modulated optical frequency and, using the tunable optical frequency, detect the rotating digital modulation constellation conveyed by the received modulated optical frequency.

Abstract: A system includes a laser configured to generate a tunable optical frequency. The system also includes an optical transmitter to map baseband data to symbols represented in a digital modulation constellation, add a frequency offset to the digital modulation constellation to cause the digital modulation constellation to rotate at a rate equal to the added frequency offset, modulate the optical frequency with the rotating digital modulation constellation, and transmit the resulting modulated optical frequency. The system also includes an optical receiver to receive the transmitted modulated optical frequency and, using the tunable optical frequency, detect the rotating digital modulation constellation conveyed by the received modulated optical frequency.

Abstract: In an optical receiver, an optical local oscillator (LO) frequency is generated. A modulated optical frequency is received at the optical receiver. An LO-signal frequency offset between the received modulated optical frequency and the optical LO frequency is determined. A determination is made as to whether the LO-signal frequency offset is in one of multiple predefined non-overlapping target windows that cover respective non-zero LO-signal frequency offsets. If it is determined that the LO-signal frequency offset is not in one of the target windows, the optical LO frequency is tuned to drive the LO-signal frequency offset toward one of the target windows to ensure the LO-signal frequency offset is non-zero.

Abstract: In an optical receiver, an optical local oscillator (LO) frequency is generated. A modulated optical frequency is received at the optical receiver. An LO-signal frequency offset between the received modulated optical frequency and the optical LO frequency is determined. A determination is made as to whether the LO-signal frequency offset is in one of multiple predefined non-overlapping target windows that cover respective non-zero LO-signal frequency offsets. If it is determined that the LO-signal frequency offset is not in one of the target windows, the optical LO frequency is tuned to drive the LO-signal frequency offset toward one of the target windows to ensure the LO-signal frequency offset is non-zero.

Abstract: A receiver for an optical communications system which corrects distortion of a received signal. A clock recovery system utilising a feedback and feedforward system are provided. The feedback loop comprises a phase detector and a clock source, while the feedforward loop comprises the phase detector and a delay element for delaying the output of distortion correction system. The feedback loop has a significantly lower bandwidth than the feedforward path. There are also provided methods of optimising tap weights and of acquiring initial tap weights.

Abstract: A receiver for an optical communications system which corrects distortion of a received signal. A clock recovery system utilizing a feedback and feedforward system are provided. The feedback loop comprises a phase detector and a clock source, while the feedforward loop comprises the phase detector and a delay element for delaying the output of distortion correction system. The feedback loop has a significantly lower bandwidth than the feedforward path. There are also provided methods of optimizing tap weights and of acquiring initial tap weights.

Abstract: This invention relates to a phase control circuit for an optical receiver (1). The phase control circuit (9, 19) comprises a non-linear element (22) and a power detector (24). The non-linear element (22) has a rectifying characteristic, inputs the received electrical signal (7, 17) and provides a rectified signal at its output. The power detector (24) provides an error signal which is used to obtain a phase control signal (5) which is output by the phase control circuit. The invention further relates to a corresponding method for phase control of an optical receiver (1).

Abstract: A receiver for an optical communications system which corrects distortion of a received signal. A clock recovery system utilising a feedback and feedforward system are provided. The feedback loop comprises a phase detector and a clock source, while the feedforward loop comprises the phase detector and a delay element for delaying the output of distortion correction system. The feedback loop has a significantly lower bandwidth than the feedforward path. There are also provided methods of optimising tap weights and of acquiring initial tap weights.

Abstract: This invention relates to a phase control circuit for an optical receiver (1). The phase control circuit (9, 19) comprises a non-linear element (22) and a power detector (24). The non-linear element (22) has a rectifying characteristic, inputs the received electrical signal (7, 17) and provides a rectified signal at its output. The power detector (24) provides an error signal which is used to obtain a phase control signal (5) which is output by the phase control circuit. The invention further relates to a corresponding method for phase control of an optical receiver (1).

Abstract: A method of synthesizing a modulated optical signal using a laser cascaded with an amplitude modulator. A phase drive signal VF(t) is derived for driving the laser to generate an optical carrier signal with a desired phase ?(t). An amplitude drive signal VS(t) is then derived for driving the amplitude modulator to impose a desired amplitude S(t) onto the optical carrier signal generated by the laser. The amplitude drive signal VS(t) is compensated for amplitude modulation of the optical carrier signal generated by the laser.