Embodiments of the present invention provide a method and an apparatus for transmitting and receiving a client signal in an optical transport network. In the transmission method, a received client signal is mapped into a variable-rate container OTU-N, wherein a rate of the OTU-N is N times as high as a preset reference rate; and then, the variable-rate container OTU-N is split into N optical stab-channel transport units OTUsubs by column, where a rate of each OTUsub equals to the reference rate; next, the N optical sub-channel transport units OTUsubs are modulated onto one or more optical carriers; at last, the one or more optical carriers is transmitted through a fiber.

A function of detecting an unused path through which actual data is not transmitted in a long-distance redundant network is realized at low cost. In an optical transmission system 20, each of the optical transceivers 21a and 21b that are connected to each other by an optical fiber cable 22 and disposed separately includes a protocol IC unit 35. The protocol IC unit 35 transmits an idle signal A1 with empty data using an optical signal P1 to an unused path of the optical fiber cable 22. At the time of this transmission, the protocol IC unit 35 outputs, to the transmission unit 33, a control signal C1 for performing, at a fixed modulation period, ON/OFF modulation on the optical signal P1 on which the idle signal A1 is superimposed. Also, the protocol IC unit 35 transmits an OAM signal O1 at an OAM period that is a period different from a modulation period, and performs control to turn ON the control signal C1 at the time of this transmission. The protocol IC unit 35 performs control to set the QAM period T2 as a period longer than or equal to a plurality of modulation periods T1. The transmission unit 33 is configured to perform ON/OFF modulation on the optical signal P1 using the control signal C1, and transmits the modulated optical signal P1.

A function of detecting an unused path through which actual data is not transmitted in a long-distance redundant network is realized at low cost. In an optical transmission system 20, each of the optical transceivers 21a and 21b that are connected to each other by an optical fiber cable 22 and disposed separately includes a protocol IC unit 35. The protocol IC unit 35 transmits an idle signal A1 with empty data using an optical signal P1 to an unused path of the optical fiber cable 22. At the time of this transmission, the protocol IC unit 35 outputs, to the transmission unit 33, a control signal C1 for performing, at a fixed modulation period, ON/OFF modulation on the optical signal P1 on which the idle signal A1 is superimposed. Also, the protocol IC unit 35 transmits an OAM signal O1 at an OAM period that is a period different from a modulation period, and performs control to turn ON the control signal C1 at the time of this transmission. The protocol IC unit 35 performs control to set the QAM period T2 as a period longer than or equal to a plurality of modulation periods T1. The transmission unit 33 is configured to perform ON/OFF modulation on the optical signal P1 using the control signal C1, and transmits the modulated optical signal P1.

Probabilistic constellation shaping (PCS) is applied to a desired probability distribution over the 2-D constellation points. Constellation points are partitioned into multiple disjoint sets in which all the constellation points within a subset have the same energy level (i.e., amplitude) or distance from the origin on the complex plane. Each of the sets may be further subdivided into smaller disjoint sets of constellation points to facilitate labeling of the constellation points. The sets may be indexed from 0 to the total number of disjoint sets to form an index set. The desired distribution may then be applied over the index set either using a distribution matcher (DM) or using a lookup table. The desired distribution may be generated before forward error correction (FEC) encoding that preserves the generated amplitude distribution through FEC encoding of data bits. The scheme may map the FEC encoded data bits to the constellation points, such that the probability of occurrence of each signal set (with a specific energy level) follows the desired probability distribution within a fixed codeword length. In addition, PCS can be applied to both square and non-square constellations, which may or may not be arranged on a Cartesian grid.

Probabilistic constellation shaping (PCS) is applied to a desired probability distribution over the 2-D constellation points. Constellation points are partitioned into multiple disjoint sets in which all the constellation points within a subset have the same energy level (i.e., amplitude) or distance from the origin on the complex plane. Each of the sets may be further subdivided into smaller disjoint sets of constellation points to facilitate labeling of the constellation points. The sets may be indexed from 0 to the total number of disjoint sets to form an index set. The desired distribution may then be applied over the index set either using a distribution matcher (DM) or using a lookup table. The desired distribution may be generated before forward error correction (FEC) encoding that preserves the generated amplitude distribution through FEC encoding of data bits. The scheme may map the FEC encoded data bits to the constellation points, such that the probability of occurrence of each signal set (with a specific energy level) follows the desired probability distribution within a fixed codeword length. In addition, PCS can be applied to both square and non-square constellations, which may or may not be arranged on a Cartesian grid.

Processing a received optical signal in an optical communication network includes equalizing a received optical signal to provide an equalized signal, demodulating the equalized signal according to an m-ary modulation format to provide a demodulated signal, decoding the demodulated signal according to an inner code to provide an inner-decoded signal, and decoding the inner-decoded signal according to an outer code. Other aspects include other features such as equalizing an optical channel including storing channel characteristics for the optical channel associated with a client, loading the stored channel characteristics during a waiting period between bursts on the channel, and equalizing a received burst from the client using the loaded channel characteristics.

The Bi-Di coherent transmission system is configured with at least one pair of modules coupled to one another via a single fiber. The modules each are configured with a pair of laser outputting two reference signals at respective different wavelengths λ.sub.1o and λ.sub.2o, photonic transceiver and a wavelength division multiplexer (WDM) coupler. The photonic transceivers each have transmitter and receiver branches integrated in a photonic circuit and receiving the reference signals. The transmitter is configured to modulate the received reference signals λ.sub.1oT and λ.sub.2oT which are further coupled into the WDM coupler. The WDM couplers each sort out one of the modulated signals and transmit the other modulated signal such that the transmitted modulated signal at different wavelengths λ.sub.1oT and λ.sub.2oT are coupled into respective opposite ends of the fiber and propagate towards one another in opposite directions. The transmitted modulated signals arc coupled into respective branches through the WDM couplers with each transmitted modulated signal interfering with the reference signals at wavelengths λ.sub.1oT and λ.sub.2oT. The photodiodes of respective receiving brandies are configured to detect a beat frequency of the interfering signals at the same wavelength.

Provided is a coherent detection implementing apparatus, system and method. The apparatus includes: a first transceiver unit, configured to send an optical signal in a first direction to a second device, wherein the optical signal in the first direction includes a direct current optical signal with a first wavelength and a modulated optical signal with a second wavelength; and configured to receive an optical signal in a second direction from the second device; and a first coherent receiver, connected with the first transceiver unit, and configured to take a part of the direct current optical signal with the first wavelength in the optical signal in the first direction as a Local Oscillator (LO) light for coherent reception, perform coherent frequency mixing between the LO light and the optical signal in the second direction, and demodulate the optical signal in the second direction.

Provided is a coherent detection implementing apparatus, system and method. The apparatus includes: a first transceiver unit, configured to send an optical signal in a first direction to a second device, wherein the optical signal in the first direction includes a direct current optical signal with a first wavelength and a modulated optical signal with a second wavelength; and configured to receive an optical signal in a second direction from the second device; and a first coherent receiver, connected with the first transceiver unit, and configured to take a part of the direct current optical signal with the first wavelength in the optical signal in the first direction as a Local Oscillator (LO) light for coherent reception, perform coherent frequency mixing between the LO light and the optical signal in the second direction, and demodulate the optical signal in the second direction.

In a system for converting digital data into a modulated optical signal, an electrically controllable device, including a modulator having one or more actuating electrodes, provides an analog-modulated optical signal that is modulated in response to output data bits of a digital-to-digital mapping. A digital-to-digital conversion provides the mapping of input data words to the output data bits. The mapping enables adjustments to correct for non-linearities and other undesirable characteristics, thereby improving signal quality.