An adaptive equalizer (70) according to this invention includes an adaptive equalization filter (71) configured to adaptively compensate for a waveform distortion caused by a polarization fluctuation of a received signal (61) by updating a tap coefficient, a first tap coefficient updater (72) configured to calculate the tap coefficient according to the polarization fluctuation of the received signal (61) using a variable step size and update the tap coefficient of the adaptive equalization filter (71), a second tap coefficient updater (73) configured to calculate the tap coefficient according to the polarization fluctuation of the received signal (61) using a fixed step size ?.sub.0, a polarization state estimator (74) configured to estimate a polarization state of the received signal (61) using the tap coefficient calculated by the second tap coefficient updater (73), and a step size updater (75) configured to obtain the step size corresponding to the polarization state estimated by the polarization state estimator (74) and update the variable step size. According to this invention, it is possible to provide an adaptive equalizer that always implements stable followability to various SOP fluctuations.

An object of the present invention is to provide an optical transmission system capable of easily controlling a transmission capacity and an optical signal quality even if a MIMO equalizer is provided. The optical transmission system according to the present invention is provided with an N?M MIMO equalizer, includes receivers (N units) equal in number to the spatial multiplexing order L of an optical fiber, and changes the number M of the signal beams of light transmitted through the optical fiber in the range of the spatial multiplex order L or less. The adjustment of the number M of signal beams of light makes it possible to adjust a transmission capacity and a signal quality of the optical transmission system even after construction of the transmission line.

A system and method including polarization modulation of supervisory signals for reducing interference with data signals in a wavelength division multiplexed optical communication system. At least one supervisory signal for monitoring a transmission path and/or elements coupled to the transmission path is fast polarization modulated and launched with data signals onto the path. Polarization modulating of the supervisory signal reduces impact of the supervisory signal on the data signals and improves system performance.

A system and method including polarization modulation of supervisory signals for reducing interference with data signals in a wavelength division multiplexed optical communication system. At least one supervisory signal for monitoring a transmission path and/or elements coupled to the transmission path is fast polarization modulated and launched with data signals onto the path. Polarization modulating of the supervisory signal reduces impact of the supervisory signal on the data signals and improves system performance.

A coherent optical reception device includes a local oscillation laser that supplies laser light, a coherent optical reception front-end unit that receives a multi-level modulated optical signal, demodulates the optical signal on the basis of the laser light, and converts a demodulated optical signal into an electrical analog signal, an analog-to-digital converter that converts the analog signal into a digital signal, a compensation unit that compensates for an influence of dispersion due to a wavelength or a polarized wave of the optical signal and recovers a carrier phase of the digital signal, a constellation distortion compensation unit that compensates for constellation distortion of the multi-level modulation included in the digital signal in which an influence of dispersion is compensated for by the compensation unit, and an error correction decoding unit that performs error correction of the digital signal in which the constellation distortion is compensated for.

An amplitude adjustment circuit includes a memory that stores correspondence information between frequency distributions of an amplitude and adjustment coefficients, a processor configured to generate a frequency distribution of amplitude of data for which adaptive equalization processing has been executed, acquire the correspondence information between frequency distributions of the amplitude and adjustment coefficients from the memory, select the adjustment coefficient based on a result of comparison between the frequency distributions included in the correspondence information acquired by the acquiring unit and the frequency distribution generated by the generating unit, and adjust a gain of the data based on the adjustment coefficient selected by the selecting unit.

An amplitude adjustment circuit includes a memory that stores correspondence information between frequency distributions of an amplitude and adjustment coefficients, a processor configured to generate a frequency distribution of amplitude of data for which adaptive equalization processing has been executed, acquire the correspondence information between frequency distributions of the amplitude and adjustment coefficients from the memory, select the adjustment coefficient based on a result of comparison between the frequency distributions included in the correspondence information acquired by the acquiring unit and the frequency distribution generated by the generating unit, and adjust a gain of the data based on the adjustment coefficient selected by the selecting unit.

Methods and systems are disclosed for characterizing a polarization-dependent loss or gain (PDL/G) of an optical device under test (DUT), such as an optical fiber link, from an optical signal having passed through the optical DUT. The optical signal is substantially unpolarized upon entering the optical DUT. The method can include varying a state of polarization (SOP) of the optical signal over a plurality of sampled SOP conditions to produce a respective plurality of SOP-varied optical signals; performing a polarization-analysis and detection operation on the plurality of SOP-varied optical signals to acquire a respective plurality of detected signal sets, each detected signal set including at least one polarization-analyzed detected signal; and determining, as the PDL/G of the optical DUT, a polarization extinction ratio parameter representative of a ratio of maximum to minimum power levels measured among the polarization-analyzed detected signals of the plurality of detected signal sets.

The present disclosure provides a method and system of identifying macro-bends in at least one test fiber. The method includes generation of modulated optical pulses and scrambling the state of polarization of the modulated optical pulses to random states of polarization. The method includes injection of the modulated optical pulses in at least one test fiber and reception of backscattered optical pulses and splitting of the backscattered optical pulses to a first optical component and a second component. The method includes measurement of a first power of the first optical component and a second power of the second optical component of the backscattered optical pulses. The method includes calculation of discrete values of polarization dependent loss as a function of distance and identification of the macro-bends by analysis of peaks in one or more plots of one or more traces of the discrete values of the polarization dependent loss.

The present disclosure provides a method and system of identifying macro-bends in at least one test fiber. The method includes generation of modulated optical pulses and scrambling the state of polarization of the modulated optical pulses to random states of polarization. The method includes injection of the modulated optical pulses in at least one test fiber and reception of backscattered optical pulses and splitting of the backscattered optical pulses to a first optical component and a second component. The method includes measurement of a first power of the first optical component and a second power of the second optical component of the backscattered optical pulses. The method includes calculation of discrete values of polarization dependent loss as a function of distance and identification of the macro-bends by analysis of peaks in one or more plots of one or more traces of the discrete values of the polarization dependent loss.