Nonlinearity induced signal distortions are compensated by processing an input communications signal, in the electrical domain prior to Electrical-to-optical conversion and transmission through an optical link of a communications system. According to the invention, a compensation operator is determined that substantially mitigates the nonlinearity induced signal distortions imparted to an optical signal traversing the communications system. The input communications signal is then input to the compensation operator to generate a predistorted electrical signal. This predistorted electrical signal is then used to modulate an optical source to generate a corresponding predistorted optical signal for transmission through the optical communications system. With this arrangement, arbitrary nonlinearity induced signal distortions imparted by the optical link can be compensated in such a manner that a comparatively undistorted optical signal is obtained at the receiving end of the optical link.

Electrical domain compensation of non-linear effects in an optical communications system

Radio-over-fiber technology provides a simpler pathway for the distribution of wireless signals in broadband wireless networks via optical means. The technology has evolved over the last 30 years, with many challenges already overcome and with many more to address. This paper provides an overview of the technology evolutionary path of RoF with a review of its past, evaluation of its present, and a discussion of its challenges into the future.

Evolution of Radio-Over-Fiber Technology

Optical dispersion imposed on a communications signal conveyed through an optical communications system is compensated by modulating the communications signal in the electrical domain. A compensation function is determined that substantially mitigates the chromatic dispersion. The communications signal is then modulated in the electrical domain using the compensation function. In preferred embodiments, compensation is implemented in the transmitter, using a look-up-table and digital-to-analog converter to generate an electrical predistorted signal. The electrical predistorted signal is then used to modulate an optical source to generate a corresponding predistorted optical signal for transmission through the optical communications system.

Optical dispersion compensation in the electrical domain in an optical communications system

Polarization Dependent Effects (PDEs), including Polarization Mode Dispersion (PMD) and Polarization Dependent Loss (PDL) imposed on optical signals conveyed through an optical link are compensated by processing an input signal in the electrical domain prior to transmission. A compensation function is derived that at least partially compensates the PDEs. The communications signal is then processed in the electrical domain using the compensation function to generate an electrical predistorted signal. The electrical predistorted signal is then used to modulate an optical source to generate a corresponding predistorted optical signal for transmission through the optical link. The PDEs of the optical link operate of the predistorted optical signal such at that substantially undistorted optical signal is received at a receiving end of the link.

Electrical domain mitigation of polarization dependent effects in an optical communications system

A suppressed carrier optical communications signal is generated by driving (biasing) an optical modulator capable of complex modulation of an optical carrier signal to a bias point near a zero-crossing point of the modulator's E-field response. A complex input signal is then used to drive excursions of the E-field response to impress the input signal onto the optical carrier. The resulting lightwave emerging from the complex modulator exhibits an optical spectrum characterized by a pair of sidebands and a strongly suppressed carrier. Bias control of the complex modulator is implemented on the basis of the optical power detected at the output of the complex modulator. This enables the optical modulator to be treated as a “black box”, in that calculation of the bias signals does not relay on knowledge of the precise performance characteristics of the modulator.

Method for optical carrier suppression and quadrature control

A novel technique to improve an analog optical transmission performance by reducing both the nonlinearity and chromatic dispersion induced carrier suppression has been proposed. The intrinsic nonlinear distortion component of Mach-Zehnder modulator (MZM) was reduced by gain saturation characteristics of semiconductor optical amplifier (SOA). The composite triple beat (CTB) of MZM was reduced as much as 9 dB at the quadratic bias point and the following enhancement of dynamic range by 4.5 dB was obtained. The RF carrier suppression for the transmission length and operating frequency was reduced by a negative chirp characteristic of semiconductor optical amplifier (SOA) and the enhancement of CTB after transmission were also achieved as a result of the linearization using SOA.

Mitigation of dispersion-induced effects using SOA in analog optical transmission

A novel linearization process in an electroabsorption-modulated laser (EML) is proposed and experimentally demonstrated. A dual-parallel modulation scheme is used to compensate the nonlinear component of the EML by controlling the dc bias voltages of each EML separately. The validity of the proposed modulation scheme is confirmed through simulations and experiments. From a dual-parallel modulation experiment at 8 GHz, a reduction of 23 dB in IMD3 and a following increase of linear dynamic range of 19.6 dB are achieved compared to those of a single EML operation.

https://www.researchgate.net/profile/Sang_Kook_Han/publication/3291018_Linearity_enhancement_of_an_electroabsorption_modulated_laser_by_dual-parallel_modulation/links/54af6fec0cf2b48e8ed66588.pdf

Linearity enhancement of an electroabsorption modulated laser by dual-parallel modulation

We demonstrated experimentally the reduction of nonlinearity of an InGaAs/InGaAsP quantum-well electro-absorption (EA) modulator by a dual-parallel modulation scheme at 8 GHz frequency range. Through the experiments, the IMD3, which is the dominant nonlinear product in narrowband, was suppressed by 23 dB and the following 19.6 dB enhancement of SFDR were archived.

Duk-Ho Jeon

Papers

Mitigation of dispersion-induced effects using SOA in analog optical transmission

Linearity enhancement of an electroabsorption modulated laser by dual-parallel modulation

Dual-parallel electroabsorption modulated laser for analog optical transmitter