A method and apparatus for monitoring in-line signal quality and a system. The method for monitoring in-line signal quality includes: according to signal to noise ratios (SNRs) of subcarriers obtained in a transmission initialization period, setting a subcarrier with a highest SNR to be a pilot subcarrier and other subcarriers to be data subcarriers; determining bit allocation and power allocation of the pilot subcarrier and bit allocation and power allocation of the data subcarriers; setting data-decision-based SNR measurement thresholds for the data subcarriers according to the bit allocation of the data subcarriers; and comparing the SNRs of the data subcarriers obtained through data-decision-based SNR measurement in a transmission period with the SNR measurement thresholds of the data subcarriers, and when an SNR of a data subcarrier is less than its SNR measurement threshold, trigger pilot-based SNR measurement of the data subcarrier. Hence, not only temporally continuous in-line signal quality monitoring may be provided, but also accuracy of the monitoring result may be guaranteed.

A transmitter includes a pilot loading apparatus, an optical modulator, and a pilot locking apparatus. The pilot locking apparatus is configured to determine a pilot operating point that is currently on a response curve and that is of the optical modulator, and a target initial amplitude of an electrical pilot signal. The pilot locking apparatus is further configured to control the pilot loading apparatus to adjust an initial amplitude of the electrical pilot signal to the target initial amplitude.

An aspect of the disclosure provides methods and systems for encoding a data bit stream onto a pilot tone signal. Another aspect of the disclosure provides method and systems for pilot tone detection. In both, a coded pilot tone signal is encoded using a code sequence m.sub.1 for each bit value of 1 (b.sub.1) and a code sequence m.sub.0 for each bit value of 0 (b.sub.0) of a data bit stream including pilot tone data bit values of 1 (b.sub.1) and bit values of 0 (b.sub.0), with each code sequence having multiple coding bits in the duration of each bit. Pilot tone detection can further include decoding each code sequence of the coded pilot tone signal using a plurality of successive overlapping measurement windows. In some embodiments each measurement window is of the same duration, being of the duration of each code sequence, and detecting each code sequence comprises selecting one of the plurality of measurement windows to represent a complete code sequence.

A method/system described herein addresses the intrinsic nonlinearity of electrooptic modulators and the restrictions placed on the signals dynamic range in applications such as data communication and sensing. Linear electro-optic modulation utilizing ring resonator electrooptic modulators is produced over a dramatically wider range of the input signal amplitude, which improves the dynamic range and the amount of information that is transmitted via laser light. A distributed and subranging design folds the large dynamic range across multiple linear subranges, with each subrange being addressed using a unique optical wavelength, or a unique optical fiber, or a unique free space path. The subrange within the wide dynamic range of the input signal is captured by the linear portion of the transfer function of a single transfer function. This enables the efficient use of optical links for the transmission and processing of analog and multilevel signals, overcoming the limitations that were hindering progress.

The disclosure is directed to a method and system for generating a pilot tone for an optical signal with an optical telecommunications system. The pilot tone is generated in the digital domain by modulating the data to be transmitted to a destination node within the optical telecommunications network. The modulation of the data introduces occurrence modulation to the optical signal.

A receiving device receives a received signal in which a data signal, modulated by using a phase modulation method, and a pilot signal are time-multiplexed. The receiving device includes a synchronizing circuit that synchronizes the phase of the received signal. The synchronizing circuit extracts a pilot signal from the received signal. The synchronizing circuit estimates a phase error by comparing the extracted pilot signal and a predetermined pattern. The synchronizing circuit conducts phase rotation on constellation points of the received signal in accordance with the reference phase, obtained from the phase error, and the phase in the modulation method related to the received signal. The synchronizing circuit estimates a phase estimate value of the received signal in accordance with the constellation points, on which phase rotation has been conducted. The synchronizing circuit compensates for a phase error of the received signal in accordance with the phase estimate value.

A detection apparatus and method for noise intensity and a coherent optical receiver where the detection method includes: preprocessing a received signal to obtain a pilot sequence contained in the received signal; removing a phase noise of a receiving pilot sequence by using a known transmitting pilot signal; calculating a noise power density of the receiving pilot sequence with no (or without) phase noise within a predetermined spectral width near a pilot frequency; and calculating power of a linear noise of the received signal based on a bandwidth of the received signal and the noise power density within the predetermined spectral width. Hence, linear noises and nonlinear noises may be split efficiently without introducing much complexity, and information on intensities of various noises may be obtained.

Methods and devices for IQ time skew and conjugation compensation and calibration of a coherent transmitter or transceiver are described. A pilot tone is combined with a digital data signal such that relative powers of the pilot tone in each of two frequency bands of the transmitted data signal may be detected by a pilot tone detector and used to calculate the time skew between I and Q modulation channels of the transmitter. A transmitter DSP applies IQ time skew bias to the data signal to compensate for any calculated IQ time skew. The pilot tone detector also provides the transmitter DSP with the information necessary to detect phase conjugation of the optical signal, which can be corrected by inverting the polarity of the data signal or changing the phase bias point of the optical modulator.

Described herein is a solution to address the intrinsic nonlinearity of analog signals and the restrictions this places on the signals dynamic range. The subject matter described herein produces linear electro-optic modulation over a dramatically wider range of the input signal amplitude. This is accomplished by a distributed multiwavelength design that folds the large dynamic range across multiple linear subranges, with each subrange being addressed using an optical wavelength. As a result, the subrange within the wide dynamic range of the input signal is captured by the linear portion of the transfer function of a single transfer function. Several physical implementations of this subject are presented herein. This innovation enables the efficient use of optical links for the transmission and processing of analog and multilevel signals, overcoming the limitations that were once hindering progress in this field.

Methods and systems for superchannel subcarrier monitoring using amplitude modulated (AM) tones include modulating an optical subcarrier of a superchannel with a first frequency that is chosen to be substantially smaller than a baud rate for data modulated onto the optical subcarrier. Certain pairs of subcarriers in the superchannel may be modulated with AM tones having a common frequency with complementary phase for power equalization. Then, downstream detection and monitoring of the optical subcarrier may be performed based on demodulation of the first frequency. Each optical subcarrier in the superchannel may be modulated using a respectively unique combination of AM tone and phase.