Optics: measuring and testing – For optical fiber or waveguide inspection
Reexamination Certificate
2002-05-16
2003-11-11
Nguyen, Tu T. (Department: 2877)
Optics: measuring and testing
For optical fiber or waveguide inspection
Reexamination Certificate
active
06646727
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable
REFERENCE TO COMPACT DISK APPENDIX
Not applicable
BACKGROUND OF THE INVENTION
The present invention generally relates to the field of optical measurements. In particular, the present invention relates to a quantum optical system for the characterization of polarization mode dispersion in an optical system and the method of performing the characterization. The invention also relates to performing said characterization on an active (viz., in-use) optical communications channel.
As the demand for increased bandwidth continues, telecommunications providers are looking for new ways to provide the additional bandwidth. The ultimate bandwidth available in an optical communications channel (e.g., an optical fiber) is limited by its optical properties. In particular, if the transmission time of an optical pulse through an optical channel is dependent on its polarization, the channel is said to exhibit polarization mode dispersion (PMD). PMD results from the birefringence of optical materials in the optical path which leads to a differential propagation delay between the orthogonal polarization components of light transmitted through the optical channel. PMD limits the bandwidth of the optical channel because it broadens the optical pulses and increases the bit error rate (BER). As modulation speeds increase, pulse durations decrease, and accurate compensation of PMD are required to maintain a low BER. To control such compensators, precise characterization of the PMD of the optical channel is required. Additionally, the PMD of an optical channel depends on the wavelength of the propagating light. Presently, optical communications fiber are wavelength multiplexed. That is, one physical channel is used to carry many communications channels, where each communications channel is identified uniquely by the wavelength of the light it uses. Thus, in addition to accurate and precise PMD characterization, the co-temporal characterization of the PMD of each of the multiplexed wavelengths in the channel is required.
Prior art methods of characterizing PMD have depended on classical optical (as opposed to quantum optical) phenomena. For example, the NetTest NEXUS Polarization Mode Dispersion Measurement System employs a Michelson interferometric technique to analyze PMD. Essentially these prior art systems attempt to measure the amplitude and relative phase of the two vector components of the polarized light. Other prior art systems use an optical signal analyzer (OSA) to measure the effects of PMD (that is, the system measures power variations at a fixed set of output polarization states as function of wavelength). In the former case, the light that has passed through the device under test must be divided into two arms of an interferometer, potentially introducing non-common path errors, while with the second approach the dispersive phase delay is not measured directly, it being inferred from the measured intensity variations.
One object of the present invention is to provide an apparatus that uses quantum-optical phenomena to measure the effective time delay between polarization states of light the have propagated through an optical element. A second objective of the invention is to provide a method of performing PMD characterization on an optical element. A further objective of the invention is to provide a PMD characterization apparatus that may be used on an active communications channel, that is, in the presence of signal photons. Yet another objective of this invention is to provide a method of characterizing PMD in an active communications channel.
BRIEF SUMMARY OF THE INVENTION
The present invention relates to an apparatus and method for determining the PMD of an optical element and specifically of an optical communications fiber. The method includes the generation of a beam of “twinons”. Twinons are a pair of quantum mechanically entangled photons, typically emitted from a parametric down conversion optical process. Each photon in a twinon has a corresponding twin photon that is correlated with it in frequency (or energy), direction (or momentum) and polarization. Each of these photons loses its individuality when it becomes one half of an entangled pair. In the invention, the apparatus is arranged such that each of the twin photons travels in substantially the same direction but differ in wavelength and polarization state. Specifically, the twin photons in this invention have orthogonal polarizations. Although every twinon in this invention has substantially the same total energy, each of the two twin photons generally has a different, random energy, within a range of energies determined by the configuration of the parametric down conversion.
The key to the invention is understanding that each twinon is a single entity that happens to be made up of two photons. The behavior of one photon is correlated with the behavior of the other, even when they appear to be in separate locations. When a twinon traverses an optical system in which there are multiple indistinguishable paths, quantum optical interference determines in which of the paths the photons will be detected. For example, destructive interference can prevent two different detectors from observing a photon simultaneously while quantum optical constructive interference can “force” one photon to appear at each detector. Thus, in the absence of any differential delay (viz., PMD) between the two orthogonally polarized twin photons, quantum-interference effects can either eliminate or reinforce coincident detections (“CD's”) on two separated detectors.
In this invention, the twinon beam propagates through the optical element or device under test (DUT) and then impinges on a beamsplitter at the input of a quantum-interferometric device (QID). Unlike classical interferometers, a QID does not bring two interfering photons together on a single detector. Instead, the two arms of the device each terminate at a separate detector.
In the presence of a polarization-specific delay (viz., PMD) the twinon acts like two un-entangled photons. In this case, as in classical optics, each photon may be reflected or transmitted at the beamsplitter. About half of the time one photon will propagate down one arm of the QID and one photon will propagate down the other arm of the QID. Thus, when the photons are acting independently (that is, when they are distinguishable) the CD rate is substantially one half the maximum observable rate.
In the invention, one arm of the QID includes a variable, polarization-specific delay element. When this inserted delay from this element exactly compensates for the PMD induced delay, the twin photons are within a coherence length of each other and quantum interference takes hold. Depending on the phase of the photons, the CD count rate either dips to near zero or rises significantly. The inserted delay for which rate of coincident detections exhibits its maximum change is a measure of the PMD.
One arm of the QID used in this invention also includes a wavelength demultiplexer and an array of detectors. The demultiplexer directs photons in different wavelength bands into individual detectors in the array. Comparing the output of each detector in the array with the single detector in the other arm of the QID generates a wavelength histogram of detection coincidences as a function of polarization-specific delay. As in the single wavelength case, the variable delay at which each wavelength channel sees the CD rate dip or peak is the PMD for that wavelength.
In one embodiment the system includes an entangled photon source which projects a beam into the optical element to be measured, a beam dividing element to divide the light exiting the optical element to be measured into two beams, a polarization-specific fixed delay element and a polarization-specific, variable delay element in one of the two beams, an optical demultiplexer in one of the two beams, a plurality of first detectors to detect the light emerging from the optical demultiplexer, and
Bielagus Steven J.
Merhar Milan J.
Saleh Bahaa E. A.
Sergienko Alexander
Teich Malvin C.
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