The present invention relates to a polarization-independent optical pulse characterization instrument which analyzes time and frequency (wavelength) properties of an optical pulse in an arbitrary polarized state.
As a technology for analyzing time and frequency properties of an optical pulse by measuring a spectrogram which is a function of delay time and a frequency (or a wavelength), a method called frequency-resolved optical gating (FROG) has been developed. Change with time or change with frequency (wavelength) in intensity and phase of an optical pulse to be measured can be obtained from a spectrogram. This technique is reported in the following documents: Review of Scientific Instruments, Vol. 68, No. 9, pp. 3277-3295, 1997; Physica Status Solidi (b) Vol. 206, pp. 119-124, 1998; and IEEE Journal of Quantum Electronics, Vol. 35, No. 4, pp. 421-431, 1999.
As a technology for characterizing a feeble ultrashort optical pulse with high sensitivity and high time resolution at the time of optical fiber transmission, FROG to which two-photon absorption in a semiconductor is applied as an optical gate has been developed. This technique is reported in Optics Express, Vol. 7, pp. 135-140, 2000. With this technique, the following method for measuring a spectrogram is disclosed: a probe optical pulse and a gate optical pulse in a linear polarization state, in which both of the optical pulses are orthogonal to each other, are colinearly entered in a two-photon absorption medium; and thereby a spectrogram is measured as a function of delay time between the gate optical pulse and the probe optical pulse, and as a function of a frequency or a wavelength.
In a long-distance optical fiber transmission system, it is expected that polarization mixing and polarization mode dispersion caused by double refraction in an optical fiber will exert a serious influence upon signal degradation. Therefore, characterization of the influence of polarization mixing and polarization mode dispersion in the optical fiber exerted upon ultrashort optical pulse transmission is indispensable for enhancing performance of the long-distance optical fiber transmission system which uses an ultrashort optical pulse. However, in the conventional method for characterizing an optical pulse using FROG which applies two-photon absorption in a semiconductor as an optical gate, an optical pulse to be measured and a gate optical pulse must always be in a linear polarization state. Therefore, the conventional method cannot be applied to an arbitrary polarized optical pulse. It is impossible to correctly characterize a randomly polarized optical pulse, and an optical pulse, a waveform of which is distorted, which are caused by polarization mixing and polarization mode dispersion.
An object of the present invention is to provide an optical transmission system, a signal error rate of which is low. Another object of the present invention is to provide an instrument of optical pulse characterization which is useful to provide such an optical system.
According to a first typical aspect of the present invention, there is provided an instrument of optical pulse characterization, wherein: analysis of optical pulse properties, resulting from polarization mode dispersion, becomes possible by the steps of: discriminating between an optical pulse to be measured and an optical pulse by four-wave mixing to eliminate noise generated by interference of the optical pulse to be measured with the optical pulse by four-wave mixing; and while measuring, with high sensitivity, a spectrogram of the optical pulse to be measured in an arbitrary polarized state, separating the spectrogram into two polarized components which are independent of, or orthogonal to, each other.
According to a second typical aspect of the present invention, there is provided an instrument of optical pulse characterization, wherein: said instrument of optical pulse characterization can select a method, measurement sensitivity of which is high, by selecting an optical pulse to be measured, intensity of which has been changed by two-photon absorption, and a third optical pulse generated by four-wave mixing, as objects to be measured.
An optical transmission system according to the present invention can be realized by using the above-mentioned instrument of optical pulse characterization.
To be more specific, according to a main mode of the present invention, a two-photon transition medium where efficiency of two-photon transition does not depend on polarization is prepared; and an optical pulse to be measured is split into the optical pulse to be measured itself (probe optical pulse) and a gate optical pulse by a polarization independent beam splitter. After adding variable delay time to this gate optical pulse, the probe optical pulse and the gate optical pulse are entered into a highly efficient two-photon absorption medium so that both of the pulses cross each other. Then, in a state in which a new optical pulse generated by four-wave mixing of the probe optical pulse and the gate optical pulse is spatially separated from the transmitted probe optical pulse itself so that both of the pulses are discriminated, an optical gate function is generated, and thereby a spectrum of the transmitted probe optical pulse, or of the optical pulse by four-wave mixing, is resolved before detecting the spectrum by a photodetector. By means of the optical detection, intensity of electric-field absorption of the optical pulse to be measured is measured as a function of delay time and a frequency. Thus, characterization by measuring time and frequency properties of the optical pulse to be measured in an arbitrary polarized state becomes possible.
Moreover, optical pulse characterization can also be achieved by the following steps: using an optical pulse to be measured as a probe optical pulse, and using an optical pulse independent of the optical pulse to be measured as a gate optical pulse; and entering both of the optical pulses into the two-photon transition medium described above. In this case, an optical pulse which is free from intensity distortion and phase distortion can be used as the gate optical pulse, which enables improvement in accuracy of measurement. Additionally, at the same time, a spectrogram of an optical pulse constituted of different wavelength components in wavelength-multiplexed communication can be measured collectively for a common gate optical pulse.
Furthermore, high-sensitivity optical pulse characterization becomes possible by the following steps: colinearly entering the probe optical pulse and the gate optical pulse into the two-photon transition medium; and utilizing a difference in beat frequency caused by interference with a reference optical pulse, a carrier frequency of which is different from that of the probe optical pulse, to discriminate between the optical pulse to be measured and the optical pulse by four-wave mixing.
According to a further aspect of the present invention, there is provided an optical communication system comprising: an optical transmission line through which an optical pulse propagates; an element for compensating chromatic dispersion or polarization mode dispersion; an element for extracting an optical pulse for characterization by diverting a part of optical power from the optical transmission line; an instrument of optical pulse characterization, which is connected to the element for extracting an optical pulse for characterization, according to any one of claim 2 through 8; and a control unit by which at least one of chromatic dispersion, and polarization mode dispersion, of the optical transmission line is measured by reading properties of a waveform of an optical pulse output from the instrument of optical pulse characterization, and thereby at least one of chromatic dispersion and polarization mode dispersion, which occur in the element for compensating at least one of chromatic dispersion and polarization mode dispersion, is controlled so that, for example, at least one of chromatic dispersion, and polarization mode dispersion, of the optical transmission line is minimized.
A still further aspect of an optical communication system according to the present invention relates to wavelength-multiplexed transmission. To be more specific, it is an optical communication system comprising: an optical transmission line for wavelength-multiplexed transmission, through which a wavelength-multiplexed optical pulse is transmitted; an element for compensating at least one of chromatic dispersion, polarization mode dispersion, and propagation time between wavelength-multiplexed channels; an element for extracting an optical pulse for characterization by diverting a part of optical power from the optical transmission line; an instrument of optical pulse characterization, which is connected to the element for extracting an optical pulse for characterization, according to any one of claim 2 through 8; and a control unit by which from among chromatic dispersion, polarization mode dispersion, and crosstalk between wavelength-multiplexed channels, which occur in the optical transmission line, at least one of them is measured by reading properties of a waveform of an optical pulse output from the instrument of optical pulse characterization, and thereby at least one of chromatic dispersion, polarization mode dispersion, and crosstalk between wavelength-multiplexed channels, which occur in the element for compensating at least one of chromatic dispersion, polarization mode dispersion, and crosstalk between wavelength-multiplexed channels, is controlled so that, for example, from among chromatic dispersion, polarization mode dispersion, and crosstalk between wavelength-multiplexed channels, which occur in the optical transmission line, at least one of them is minimized.