Nanotechnology – Kensuke Ogawa, Fujikura Ltd
Abstract for “Instruments for optical pulse characterization”
An optical transmission system can be created by creating a two-photon medium that is efficient in two-photon transformation. After splitting the optical pulse into a probe and gate pulses by a polarization-independent beam splitter, enter the probe and gate optical pulses into a highly efficient two photon absorption medium. The spectrum of each pulse is then resolved and detected by a photodetector. Intensity of electric-field absorption for the probe opticalpulse is measured as a delay time and frequency
Background for “Instruments for optical pulse characterization”
“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.”
“A technology to analyze time and frequency properties in an optical pulse by measuring its spectrogram, which is a function delay time and a frequency (or wavelength), has been developed. This method is called frequency-resolved optic gating (FROG). A spectrogram can show the phase and change in intensity of an optical pulse. This technique is described in the following publications: 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.”
“A technology to characterize a feeble ultrashort optic pulse with high sensitivity, high time resolution at optical fiber transmission has been developed. FROG is a two-photon absorption method in a semiconductor that can be used as an optical gate. Optics Express Vol. 7 reports this technique. 7, pp. 135-140, 2000. This technique allows for the measurement of a spectrum. A probe optical pulse is colinearly entered into a two-photon absorb medium. The delay time between the gate and probe optical pulses is then measured. It can also be used to determine a frequency or wavelength.
“In a long distance optical fiber transmission system, it’s expected that polarization mixing (or polarization mode dispersion) caused by double reflection in an optical fibre will have a significant influence on signal degradation. The characterization of the effects of polarization mixing in an optical fiber and polarization mod dispersion in an optical fiber upon ultrashort optical impulse transmission is essential for improving the performance of long-distance optical fibre transmission systems that use an ultrashort pulse. The conventional method of characterizing an optical pulse using two-photon absorption within a semiconductor as an optic gate requires that an optical pulse be measured. A gate optical pulse must also be measured. The conventional method cannot be used to characterize an arbitrary polarized pulse. It is difficult to accurately characterize an optical pulse that is randomly polarized and one that is distorted due to polarization mixing or dispersion.
“The present invention aims to provide an optical transmission device with a low signal error rate. An instrument for optical pulse characterization is another object of the present invention that is useful in providing such an optical system.
“Accordingly to a first characteristic aspect of the invention, there is an instrument for optical pulse characterization. This instrument allows you to: distinguish between an optical impulse to be measured and an optic pulse by four wave mixing to remove noise from interference of the opticalpulse by four-wave mixture; while measuring with high sensitivity an optical pulse spectrogram in an arbitrary, polarized state, and then separating the spectrogram into polarized components that are independent of each other, or orthogonal.
“Accordingly to a second characteristic aspect of the invention, there is an instrument for optical pulse characterization. This instrument can choose a method with high measurement sensitivity by selecting an optical pulse to measure, whose intensity has been altered by two-photon absorption and a third optical impulse generated by four wave mixing as the objects to be measured.
“An optical transmission device according to the invention can be realized using the above-mentioned instrument for optical pulse characterization.”
To be more precise, according to the main mode of the invention, a two photon transition medium in which efficiency of two-photon transformation does not depend upon polarization is prepared. An optical pulse to measure is divided into the optical pulse (probe optical pulse), and a gate optic pulse by a beam splitter that is polarization independent. The gate optical pulse and probe optical pulse are then entered into a two-photon absorption medium. Both pulses will cross each other after adding a variable delay time. In this state, the new optical pulse that is created by four-wave mixture of the probe and gate optical pulses is spatially separated from each other so that they can be discriminated. An optical gate function is then generated. The spectrum of either the transmitted probe or the optical pulse by 4-wave mixing is then resolved and the spectrum is detected by a photodetector. The optical detection measures the intensity of electric field absorption of the optical impulse to be measured. This is done as a function time and frequency. It is possible to characterize the optical pulse in an arbitrarily polarized state by measuring its frequency and time properties.
“Optical pulse characterization can also take place using an optical probe to measure the pulse and an independent optical pulse to measure the gate optical pulse. Finally, both optical pulses are entered into the two-photon medium. An optical pulse that is free of phase distortion and intensity distortion can be used to measure the gate optical pulse. This improves measurement accuracy. 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.”
“In addition, high-sensitivity optical-pulse characterization is possible by colinearly entering both the probe and gate optical pulses into the two-photon transformation medium. Then, using a difference between beat frequencies caused by interference with a reference pulse (a carrier frequency that is different than the probe optical pulse) to discriminate between the optical impulse to be measured and the optical Pulse by Four-wave Mixing.
“Accordingly to another aspect of the invention, an optical communication system is provided. It comprises: an optical transmission link through which an optical impulse propagates; an electronic element for compensating either chromatic or polarization-mode dispersion; an eyepiece for extracting an optic pulse for characterisation by diverting some optical power from it; and an instrument for optical pulse analysis, which reads properties of an optical waveform from the instrument for optical pulse characterization and determines whether at least one chromatic and one polarization
“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.”
Summary for “Instruments for optical pulse characterization”
“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.”
“A technology to analyze time and frequency properties in an optical pulse by measuring its spectrogram, which is a function delay time and a frequency (or wavelength), has been developed. This method is called frequency-resolved optic gating (FROG). A spectrogram can show the phase and change in intensity of an optical pulse. This technique is described in the following publications: 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.”
“A technology to characterize a feeble ultrashort optic pulse with high sensitivity, high time resolution at optical fiber transmission has been developed. FROG is a two-photon absorption method in a semiconductor that can be used as an optical gate. Optics Express Vol. 7 reports this technique. 7, pp. 135-140, 2000. This technique allows for the measurement of a spectrum. A probe optical pulse is colinearly entered into a two-photon absorb medium. The delay time between the gate and probe optical pulses is then measured. It can also be used to determine a frequency or wavelength.
“In a long distance optical fiber transmission system, it’s expected that polarization mixing (or polarization mode dispersion) caused by double reflection in an optical fibre will have a significant influence on signal degradation. The characterization of the effects of polarization mixing in an optical fiber and polarization mod dispersion in an optical fiber upon ultrashort optical impulse transmission is essential for improving the performance of long-distance optical fibre transmission systems that use an ultrashort pulse. The conventional method of characterizing an optical pulse using two-photon absorption within a semiconductor as an optic gate requires that an optical pulse be measured. A gate optical pulse must also be measured. The conventional method cannot be used to characterize an arbitrary polarized pulse. It is difficult to accurately characterize an optical pulse that is randomly polarized and one that is distorted due to polarization mixing or dispersion.
“The present invention aims to provide an optical transmission device with a low signal error rate. An instrument for optical pulse characterization is another object of the present invention that is useful in providing such an optical system.
“Accordingly to a first characteristic aspect of the invention, there is an instrument for optical pulse characterization. This instrument allows you to: distinguish between an optical impulse to be measured and an optic pulse by four wave mixing to remove noise from interference of the opticalpulse by four-wave mixture; while measuring with high sensitivity an optical pulse spectrogram in an arbitrary, polarized state, and then separating the spectrogram into polarized components that are independent of each other, or orthogonal.
“Accordingly to a second characteristic aspect of the invention, there is an instrument for optical pulse characterization. This instrument can choose a method with high measurement sensitivity by selecting an optical pulse to measure, whose intensity has been altered by two-photon absorption and a third optical impulse generated by four wave mixing as the objects to be measured.
“An optical transmission device according to the invention can be realized using the above-mentioned instrument for optical pulse characterization.”
To be more precise, according to the main mode of the invention, a two photon transition medium in which efficiency of two-photon transformation does not depend upon polarization is prepared. An optical pulse to measure is divided into the optical pulse (probe optical pulse), and a gate optic pulse by a beam splitter that is polarization independent. The gate optical pulse and probe optical pulse are then entered into a two-photon absorption medium. Both pulses will cross each other after adding a variable delay time. In this state, the new optical pulse that is created by four-wave mixture of the probe and gate optical pulses is spatially separated from each other so that they can be discriminated. An optical gate function is then generated. The spectrum of either the transmitted probe or the optical pulse by 4-wave mixing is then resolved and the spectrum is detected by a photodetector. The optical detection measures the intensity of electric field absorption of the optical impulse to be measured. This is done as a function time and frequency. It is possible to characterize the optical pulse in an arbitrarily polarized state by measuring its frequency and time properties.
“Optical pulse characterization can also take place using an optical probe to measure the pulse and an independent optical pulse to measure the gate optical pulse. Finally, both optical pulses are entered into the two-photon medium. An optical pulse that is free of phase distortion and intensity distortion can be used to measure the gate optical pulse. This improves measurement accuracy. 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.”
“In addition, high-sensitivity optical-pulse characterization is possible by colinearly entering both the probe and gate optical pulses into the two-photon transformation medium. Then, using a difference between beat frequencies caused by interference with a reference pulse (a carrier frequency that is different than the probe optical pulse) to discriminate between the optical impulse to be measured and the optical Pulse by Four-wave Mixing.
“Accordingly to another aspect of the invention, an optical communication system is provided. It comprises: an optical transmission link through which an optical impulse propagates; an electronic element for compensating either chromatic or polarization-mode dispersion; an eyepiece for extracting an optic pulse for characterisation by diverting some optical power from it; and an instrument for optical pulse analysis, which reads properties of an optical waveform from the instrument for optical pulse characterization and determines whether at least one chromatic and one polarization
“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.”
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