The present application is directed to techniques and systems for extension of unrepeatered submarine fiber links to provide an increase in reach of unrepeatered fiber transmission. Both single channel unrepeatered systems and multiple channel unrepeatered systems can be used. The multiple channel unrepeatered systems can further employ nonlinearity compensation. The present application is also directed to methods of signal transmission using the unrepeatered systems.

An optical transmission device includes: a frontend circuit, a converter, an equalizer, a recovery, spectrum detector a correction information generator, and a transmitter. The frontend circuit converts an optical signal received via an optical network into an electric signal. The converter converts an output signal of the frontend circuit into a digital signal. The equalizer equalizes the digital signal or a second digital signal that is generated based on the digital signal. The recovery recovers a symbol from an output signal of the equalizer. The spectrum detector detects a reception spectrum of the optical signal based on the digital signal or the second digital signal. The correction information generator generates, according to the reception spectrum, correction information for correcting a shape of a transmission spectrum of the optical signal. The transmitter transmits the correction information to the source device.

An optical transmission system is an optical transmission system for transmitting a signal from an optical transmitter to an optical receiver via at least a transmission line among one or more optical nodes and the transmission line, wherein the optical transmitter includes a digital filter that compensates for, between ripple components that are micro-fluctuating components in a frequency region representing a transmission characteristic of the signal in the optical transmitter and the optical receiver and non-ripple components of the transmission characteristic, at least the ripple components of the transmission characteristic, and at least one of the optical node and the optical transmitter includes an optical filter that compensates for the non-ripple components of the transmission characteristic.

A system and method for mitigating nonlinearity in an optical communication link with multiple carriers uses mutual frequency referencing to stabilize at least a portion of the multiple carriers. Using at least one frequency-referenced signal, carrier nonlinearity can be determined and compensated within the link by pre-distortion, back-propagation, or a combination of both. Mutual frequency referencing may be performed at the emitting end of the link, at the receiving end, or a combination of both.

Systems and methods are provided for enabling lower-bandwidth hardware components to support higher data rates. In particular, aspects of the disclosed systems and methods use Raised Cosine pulse shaping in short-reach links to band limit the signal spectra and thereby enable existing, such lower-bandwidth components to support higher data rates.

The present invention is directed to techniques and systems for extension of unrepeatered submarine fiber links to provide an increase in reach of unrepeatered fiber transmission. Both single channel unrepeatered systems and multiple channel unrepeatered systems can be used. The multiple channel unrepeatered systems can further employ nonlinearity compensation. The present invention is also directed to methods of signal transmission using the unrepeatered systems.

An optical communication system includes a transmission side system for multi-level pulse amplitude modulation (PAM) and a corresponding receiver side system, where the transmission side comprises a laser source providing an optical beam, a signal source of electrical signals to be modulated onto the optical beam, and a modulator coupled to the laser source and the signal source to modulate the electrical signals onto the optical beam using amplitude modulation and at least four signal levels, wherein the at least four signal levels are non-uniformly distributed. The receiver side includes a corresponding equalizer which is implemented as a filter of the form f.sub.1y+f.sub.2y.sup.2+f.sub.0, where y is the incoming signal and the parameters f.sub.0, f.sub.1 and f.sub.2 are obtained using an adaptive filter.

A system and method for mitigating nonlinearity in an optical communication link with multiple carriers uses mutual frequency referencing to stabilize at least a portion of the multiple carriers. Using at least one frequency-referenced signal, carrier nonlinearity can be determined and compensated within the link by pre-distortion, back-propagation, or a combination of both. Mutual frequency referencing may be performed at the emitting end of the link, at the receiving end, or a combination of both.

Systems and methods are provided for enabling lower-bandwidth hardware components to support higher data rates. In particular, aspects of the disclosed systems and methods use Raised Cosine pulse shaping in short-reach links to band limit the signal spectra and thereby enable existing, such lower-bandwidth components to support higher data rates.

A system for data transmission has a transmitter and a receiver connected by an optical channel. The transmitter has a transmitter laser and a transmitter-side electroabsorption modulator with an optical output. An electrical data input of the transmitter is connected to an electrical modulation terminal of the transmitter-side electroabsorption modulator. The receiver has a receiver laser and a receiver-side electroabsorption modulator with an optical output forming the input of the receiver. An electrical data output of the receiver is connected to the electrical modulation terminal of the receiver-side electroabsorption modulator. The transmitter and receiver lasers are detunable by specification of a physical variable, each provided by a respective control unit. The control units are synchronized and they specify the same signal at their outputs for establishing the physical variable for establishing the laser frequency.