In-line all-fiber polarimeter

Details In-line all-fiber polarimeter In-line all-fiber polarimeter

2000-03-03

2001-04-03

Font, Frank G. (Department: 2877)

Optics: measuring and testing

By polarized light examination

C356S367000, C356S366000, C385S011000

Reexamination Certificate

active

06211957

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a polarimeter and, more particularly, to an in-line., all-fiber polarimeter for measuring the state of polarization (SOP) of a propagating lightwave signal.
BACKGROUND OF THE INVENTION
As a result of the inherent degeneracy associated with the polarization of light traveling in an optical fiber, it is often important to be able to measure the state of polarization (SOP) of an optical signal at a given point in time and space. Polarization measurement is important, for example, in accurately orienting polarization maintaining fiber during a splicing operation, in measuring the polarization dependent loss (PDL) of components or systems, as well as in determining the polarization-mode dispersion (PMD) in optical transmission systems. Polarization mode dispersion is an increasingly critical phenomenon that will need to be controlled with high precision in the emerging high-speed (multi-Gbps), long-haul systems.
Most prior art arrangements for measuring polarization mode dispersion rely on a statistical sampling of polarization states. See, for example, U.S. Pat. No. 5,440,390 issued to Tirri et al. on Aug. 8, 1995. As networks grow increasingly complex, especially with the advent of local access, optical signal monitoring will become more important to ensure reliable operation. Real-time measurement of polarization mode dispersion and correlation of this data with bit-error rate (BER) or system quality would be a useful tool in measuring the optical system performance. Moreover, some processes could be improved, or at least made more deterministic, if a simple in-line technique were available for providing knowledge of the exact state of polarization within the transmission fiber.
An exemplary prior art “in-line” fiber optic polarimeter is described in an article entitled “Fiber-optic four-detector polarimeter” by A. Bouzid et al. appearing in
Optics Communications
, Vol. 18, 1995, at pp. 329-324. In the Bouzid et al. arrangement a set of four externally-induced, in-core fiber gratings are used in association with four photodetectors to perform the polarization measurements. The article purports to provide for the measurement of all four Stokes parameters simultaneously by measuring the intensity of light absorbed (or reflected or transmitted) by four photodetectors at four different planes of incidence. However, the arrangement lacks any phase retardation element, which is necessary in order to accurately define the required Stokes parameters. Additionally, this polarimeter lacks sufficient bandwidth to be useful in telecommunications applications.
Thus, a need remains in the art for an in-line polarization measurement device that is readily available for use in systems, particularly where it could be combined with a polarization controller to enable polarization-sensitive devices or functions to be utilized.
SUMMARY OF THE INVENTION
The need remaining in the prior art is addressed by the present invention, which relates to a polarimeter and, more particularly, to an in-line, all-fiber polarimeter for measuring the state of polarization (SOP) of a propagating lightwave signal.
In accordance with the present invention, an in-line, all-fiber polarimeter comprises a plurality of gratings formed in the fiber itself which, when coupled with fiber birefringence (phase retardation), is capable of coupling light out of the fiber in a highly polarization-selective fashion. A plurality of detectors are disposed near (or on) the outside of the fiber and are used to receive the outcoupled radiation. The polarization state of the fiber can then be ascertained, in real time, by reviewing the readings on the plurality of detectors. In general, a set of four gratings, in combination with a single elliptical retardation element, can be used to precisely define the polarization state of the optical signal.
In one embodiment of the present invention, a first blazed grating formed within the fiber is used to out-couple a first, predetermined polarization. Disposed beyond the first grating is a second blazed grating oriented to out-couple a second, orthogonal polarization. A third blazed grating is linearly polarized and disposed to out-couple light at an angle of 45° with respect to the first two output beams, thus measuring both polarization components. The remaining optical signal then passes through a birefringent section of fiber whose principle axis is at 0°, after which another measurement of linear polarization at 135° is made. The set of four measurements can be used to calculate the set of well-known Stokes parameters S
1
-S
4
and thus determine the state of polarization (SOP) of the transmission fiber.
In a preferred embodiment of the present invention, the gratings are disposed at a blaze angle of 45° to optimize the out-coupled signal at the polarization sensitive angle of 90°.
Preferably, the blazed gratings of the in-line all-fiber polarimeter of the present invention are formed using an interferometric technique with an excimer-pumped, frequency-doubled, pulsed dye laser. Other alternatives are possible.
Various other aspects of the present invention will become apparent during the course of the following discussion and by reference to the accompanying drawings.


REFERENCES:
patent: 5440390 (1995-08-01), Tirri
patent: 5815270 (1998-09-01), Lee
Ahmed Bouzid, Mustafa A.G. Abushagur, A&D El-Sabae, R.M.A. Azzam “Fiber Optic Four-Detector Polarimeter” Optics Comm. Jul. 15, 1995.

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