Abstract:
A method includes evaluating an optical signal spectrum for estimated filtering parameters of an optical spectral filtering device for shaping optical signal spectrum, determining a feedback for fine tuning the optical spectral filtering device for nonlinearity tolerance enhancement in the optical transmission system, responsive to received optical signal quality in the optical signal spectrum; and using the feedback to adjust said optical spectral filtering device for predetermined shaping and predetermined fiber nonlinearity tolerance in the optical transmission system.

Description:
RELATED APPLICATION INFORMATION 
       [0001]    This application claims priority to both provisional application No. 61/548,315 filed Oct. 18, 2011, the contents thereof are incorporated herein by reference 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates generally to optical communications, and more particularly, to optical spectral shaping for nonlinearity enhancement in high speed digital coherent optical transmission. 
         [0003]    Coherent optical communication has triggered the intensive research in improving system capacity, transmission distance and spectral efficiency by using high-order modulation formats and advanced digital signal processing (DSP) techniques. Among the various modulation formats, dual-polarization (DP) quadrature phase shift-keying (QPSK) is one of the popular candidates for upgrading system capacity at 100 Gb/s per channel with trans-oceanic transmission reach [1]. Instead of laying new dispersion uncompensated fiber links (which have the optimal dispersion map for digital coherent transmission systems), it is highly desirable to upgrade the widely deployed dispersion managed fiber (DMF) links to 100 Gb/s and beyond to reduce the system cost and increase the network data capacity. However, one of the challenges in digital coherent transmission over DMF links is caused by the its chromatic dispersion (CD) map, which was designed to have full CD compensation by using dispersion-compensated fiber (DCF) for analog receivers. This type of dispersion map would bring two critical issues when upgrading such system to 100G using coherent receivers: the small effective area of DCF (˜25-40 μm 2 ) brings strong self-phase modulation (SPM) effects; the dispersion map does not provide large walking-off between neighboring WDM channels, thus enhancing the cross-phase modulation (XPM) impact. As a result, the 100G upgrade over legacy DMF fiber is a very challenging yet beneficial project for both system providers and network carriers. 
         [0004]    With the aid of coherent receivers, enhanced DSP algorithms have been proposed to compensate for linear distortions and even fiber nonlinearity, such as SPM and XPM. Recent demonstration has shown 1.7 dB Q-factor improvement using SPM compensation and maximum likelihood sequence estimation. Compared with new advanced DSP algorithms, which generally require high investment for developing new ASIC chips, innovative optics solutions can be more attractive. In this paper, we propose to use low cost multi-channel optical spectral shaping device for enhancement of the fiber nonlinearity tolerance of 100G DP-QPSK transmission over legacy DMF fibers. 
         [0005]    Optical fiber nonlinearity is one of the major limiting factors for high-speed optical communications. In the past, various measures have been proposed to improve the optical transmission system tolerance to fiber nonlinearity. As for digital coherent optical communication systems, some of the prior research efforts include the following outputs: 
         [0006]    Since digital coherent receivers can have high tolerance to fiber chromatic dispersion, one approach is to adopt dispersion uncompensated link for high-speed optical transmission systems, and all the residual dispersion is compensated by the electronic digital signal processing (DSP) units at the receiver end. For example, an optical link consists of uncompensated standard single mode fiber (SMF-28) with dispersion at ˜18 ps/nm/km. However, in transoceanic optical cable communications, very large dispersion can be accumulated and go beyond the dispersion compensation capabilities of the digital coherent receiver. Therefore, dispersion managed fiber links, which can manage the total residual dispersion, are more practical and popular for undersea optical cable communication systems. As pointed in session A1, dispersion managed fiber links generally incur large fiber nonlinearity to the optical signals. 
         [0007]    In a digital coherent optical communication system, fiber nonlinearity has been shown to be compensated through high-speed DSP processing. One fiber nonlinearity compensation algorithm is known as “back propagation” algorithm, in which the received signal is back-propagated thorough the transmission line using DSP processing. Due to the lack of the analog solution to the Nonlinear Schrodinger Equation describing the optical signal propagation in optical fiber, the back propagation algorithms virtually solve the Nonlinear Schrodinger Equation wither reverse signs numerically. The existing nonlinearity compensation algorithms are generally demanding on computation resource and requires high investment for building new ASIC chips. 
         [0008]    Accordingly there is a need for improved optical spectral shaping for nonlinearity enhancement in high speed digital coherent optical transmission 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    The present invention is directed to a method for using an optical spectral shaping to improve optical power distribution in spectra for enhanced fiber nonlinearity tolerance in an optical transmission system. The method includes evaluating an optical signal spectrum for estimated filtering parameters of an optical spectral filtering device for shaping optical signal spectrum, determining a feedback for fine tuning the optical spectral filtering device for nonlinearity tolerance enhancement in the optical transmission system, responsive to received optical signal quality in the optical signal spectrum; and using the feedback to adjust said optical spectral filtering device for predetermined shaping and predetermined fiber nonlinearity tolerance in the optical transmission system. 
         [0010]    These and other advantages of the invention will be apparent to those of ordinary skill in the art by reference to the following detailed description and the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  shows an exemplary system application of optical spectral shaping for enhancing optical transmission system tolerance to fiber non-linearity, in accordance with the invention; 
           [0012]      FIG. 2  shows an example of optical spectral filtering for improved fiber nonlinearity tolerance; 
           [0013]      FIG. 3  shows spectra of 8WDM channels with optical spectral shaping at (a) 5.4 dB and (b) 9.5 dB center notch depth (optical spectral analyzer resolution @0.2 mm), in accordance with the invention; and 
           [0014]      FIG. 4  shows key aspects of the inventive method for spectral shaping for improved fiber nonlinearity tolerance in an optical transmission system. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    The present invention is directed to a method for optical spectral shaping to enhance the optical system tolerance to fiber nonlinearity. Through optical spectral shaping, the total optical power is better distributed evenly among the signal spectrum. In many cases, the low frequency part of the optical spectra is generally suppressed and the relative intensity of the high-frequency components is increased. With evenly distributed optical power among the different spectral components, the fiber nonlinear effects can be suppressed. 
         [0016]      FIG. 1  shows the system application of optical spectral shaping for enhancing the optical transmission system tolerance to fiber nonlinearity. The optical signal can be generated using different modulation and multiplexing schemes, including polarization multiplexed quadrature phase shift keying (DP-QPSK) modulation. The generated optical signals at different wavelengths are combined and transmit through a single piece of optical fiber. Afterwards, an optical spectral shaping device is used to reshape the optical signal spectra to improve the power distribution for enhancement of fiber nonlinearity. 
         [0017]    Data for transmission (Data Tx) under the inventive method starts with multiple high-speed generations in two orthogonal polarizations at different wavelengths  101 . The multiple input channels are combined and fed into a single transmission media  102 . An optical spectral shaping device is used to improve the power distribution of the optical signals media  103 . Optical spectral shaping is a general term used to describe the manipulation of optical signal spectrum for a certain purposes. Previously, optical spectral shaping was used for optimal spectral shape for higher optical spectral efficiency, by squeezing high-speed optical signal into a narrow spectral band. With e present invention, it is shown that optical spectral shaping can improve the optical power distribution among the optical spectral components to enhance the fiber nonlinearity tolerance. 
         [0018]    Returning to  FIG. 1 , the resulting optical transmission media exhibits non-linear effects  104 . A local oscillator  111  enables coherent receiving of the incoming optical signals  105 . Digitization of the received signal and digital signal processing is applied for signal recovery  106 . Received signal quality is evaluated for a feedback signal for adjustment of the optical spectral shaping device  107 . The feedback signal for optical spectral shaping  121  is looped back to the optical spectral shaping device  103 . When the quality of the received signal is satisfactory, the data reception RX continues. 
         [0019]      FIG. 2  shows one example of optical spectral shape filtering according to the invention, which can be used to enhance the optical transmission system nonlinearity tolerance. Its periodic filtering enables the device to support multiple channels simultaneously. Using a shaping device according to the wave pattern of  FIG. 2  and adjusting the filtering notch depth, one can change the optical power distribution among the different optical signal spectral components, as shown in  FIG. 3 . 
         [0020]    The wave patterns of  FIG. 3  show the optical spectra of 8 WDM (wavelength division multiplexing) channels. Using a large notch depth, the per-channel optical spectral width is broadened when comparing  FIG. 3  ( a ) and ( b ). For the same optical signal, a broadened spectrum can result in a more even distribution of optical signal power among the different spectral components, which can improve the optical signal tolerance to fiber nonlinearity. 
         [0021]    Referring now to  FIG. 4 , key aspects of the inventive method for optical spectral shaping in an optical transmission system are shown. In response to nonlinear distortion as a major factor in limiting high speed optical transmission systems, optical spectral shaping is used to improve the optical power distribution in spectra for enhanced fiber nonlinearity tolerance. The inventive method has the advantages of a low cost solution and is transparent to optical modulation formats. 
         [0022]    The optical signal spectrum will be evaluated to decide estimated filtering parameters of the optical spectral filtering device  201 . The filtering parameters may include periodicity, filtering notch center and depth, and dispersion, etc. The optical spectral shaping device can handle one channel or multiple channels simultaneously by having periodic filtering  202 . Based on the receiver optical signal quality, feedback comments and adjustments can be made to the fine tuning of optical spectral shaping device for nonlinearity tolerance enhancement  203 . A tunable optical spectral shaping device can be used to find optimal filtering characteristics for a fixed optical transmission system  204 . Once the optimal filtering characteristics is identified, a fixed optical filtering device can be used for lower system cost. 
         [0023]    From the foregoing, it can be appreciated that with the inventive method, optical spectral shaping can be used to improve the optical transmission system tolerance to the fiber nonlinearity. In comparison with other approaches, the inventive method has much less complexity (by using a single optical filtering device), lower cost (by using commercially available device. The device can be specifically designed for the system. One device with multiple periodic filtering characteristics can be applied for WDM channels simultaneously, transparent to the modulation format of the optical signal (e.g. DPSK, DQPSK, Dual polarization(DP)-QPSK, DP-8QAM, DP-32QAM, etc). 
         [0024]    The foregoing is to be understood as being in every respect illustrative and exemplary, but not restrictive, and the scope of the invention disclosed herein is not to be determined from the Detailed Description, but rather from the claims as interpreted according to the full breadth permitted by the patent laws. Additional information is provided in an appendix to the application entitle, “Additional Information”. It is to be understood that the embodiments shown and described herein are only illustrative of the principles of the present invention and that those skilled in the art may implement various modifications without departing from the scope and spirit of the invention. Those skilled in the art could implement various other feature combinations without departing from the scope and spirit of the invention.