Method and arrangement in connection with optical...

Optical waveguides – Directional optical modulation within an optical waveguide – Electro-optic

Reexamination Certificate

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C385S007000, C385S037000, C359S573000

Reexamination Certificate

active

06510256

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention relates to optical Bragg-reflectors, and more particularly to the alteration of the reflective properties of optical Bragg-reflectors.
BACKGROUND OF THE INVENTION
The need for capacity in communications networks is increasing exponentially, and presently capacity demands are doubling every eighteen months or even less. Efforts are being made in the telecom and computer business to meet this incredible expansion of bandwidth requirements.
One obvious action is to simply draw new optical cables between nodes of the network. However, this approach is costly and hardly a successful way of meeting the upcoming demands. Instead, measures are focused, when possible, on augmenting the capacity of existing fiber networks.
One widespread method of achieving increased transfer capacity in existing fiber networks is wavelength division multiplexing (WDM). In WDM, a single optical fiber is used for the transmission of several channels of information, each channel being associated with a specific wavelength. For a background of this technology, the reader is directed to, “A review of WDM technology and applications”,
Opt. Fiber Technol.,
5, pp. 3-39 (1999).
While the use of WDM in optical networks yields a substantial increase in transfer capacity, the complexity of the communications systems increases correspondingly. Furthermore, the number of physical components in each link of the network increases, the requirements on each component increase, and detecting and localizing errors or imperfections in the network is rendered more difficult.
The optical networks of today mainly use point-to-point communication, wherein electronically encoded information in one node of the network is transformed into optically encoded information, and then transferred, through a sequence of optical fiber amplifiers and optical transport fibers, to another node, where the optically encoded information is transformed back into electrically encoded information.
However, this prior art is associated with major drawbacks and limitations.
If, for instance, an error occurs in one of the fiber amplifiers, it is very hard to determine which amplifier, in the sequence of amplifiers, is the faulty one.
Furthermore, technology is being pushed towards communications systems in which the optical signal is passed via all-optical switching nodes, where the signal is routed without any need for optical-to-electrical conversion or vice versa. Thus, if an error is detected in a receiving node of the communications network, it is extremely hard to localize the origin of error.
If the light signal propagating in an optical transport fiber could be analyzed in an accurate and simple fashion, without interfering with the signal, any flaws in the optical communications network could be detected at an early stage. In the prior art, such analysis has been too complicated, too expensive and too inaccurate to gain widespread acceptance.
Obviously, it is of utmost importance to be able to monitor the power spectrum of the light propagating in the optical transport fiber.
It is known in the prior art to use a phase grating in an optical fiber in order to filter out one desired wavelength from a broadband light signal. A phase grating will reflect one predefined wavelength and leave other wavelengths essentially undisturbed.
An optical phase grating is a structure of essentially periodically varying refractive index in an optically transparent medium. An overview of the technology is given in M. C. Hutley, “Diffraction gratings”, Academic Press, London (1982). When light is incident on an optical phase grating, a small part of the incident light is reflected off each grating element (period). When a multiplicity of grating elements are arranged after each other (i.e., arranged as a phase grating), the total reflected light would be the sum of all these single reflections. The fraction of the incident light reflected off each grating element is determined by the depth (amplitude) of the refractive index modulation in the phase grating. The deeper the modulation, the larger fraction of the incident light is reflected off each grating element. If the incident light is essentially normal to the grating (i.e., to the grating elements), the grating is said to act in the Bragg domain and is named “Bragg grating”. Light reflected off each grating element will thus overlap the light reflected off the other elements, causing interference. For a certain wavelength, all these reflections are in phase, whereby constructive interference is effected. Although each reflection is small, a substantial reflection is obtained due to constructive interference. The wavelength for which constructive interference is effected is called the “Bragg wavelength,” &lgr;
Bragg
, and is given by (at normal angle of incidence)
&lgr;
bragg
=2
n&Lgr;
where n is the average of the refractive index, and &Lgr; is the grating period.
If the grating period, &Lgr;, varies along the grating, the grating is said to be a “chirped” Bragg grating. In a chirped Bragg grating, different wavelengths are reflected in different portions of the grating, in fact, making the Bragg grating a broadband reflector, a chirped Bragg-reflector.
U.S. Pat. No. 6,052,179 discloses a system for determining the average wavelength of light transmitted through an optical fiber. In this system, a chirped Bragg grating is provided in an optical fiber, the grating having a modulation amplitude that varies from a first end to a second end of the fiber grating. Thus, the reflectivity at one wavelength is different from the reflectivity at another wavelength, since different wavelengths are reflected at different portions of the grating. Based on the output from two photo detectors, one detecting the light transmitted through the grating and the other acting as a reference, the average wavelength of light transmitted through the grating is determined. However, this approach has several disadvantages. Firstly, the light to be analyzed has to be divided into two separate fibers, which is a serious complication in itself. Secondly, only the average wavelength can be determined. There is no possibility for proper spectrum analysis by the system disclosed in the above reference. Furthermore, the system is intended for sensing applications, where external influence changes the average wavelength coming into the system.
Previous attempts to alter the reflective properties of an optical Bragg grating include creation of an acousto-optic superlattice imposed on a chirped Bragg grating. Such an arrangement is described by Chen et al. in “Superchirped moirégrating based on an acousto-optic superlattice with a chirped fiber Bragg grating”,
Optics Letters,
Vol. 24, No. 22, pp. 1558-1560. According to this reference, multiple transmission peaks are obtained by superimposing an acoustic wave on a chirped Bragg grating. The spacing of the transmission peaks is varied by the acoustic frequency. However, this arrangement does not permit transmission of one single wavelength only, and the variation of the spacing of the transmission peaks by acoustic frequency is very limited.
SUMMARY OF THE INVENTION
The present invention provides new methods and arrangements for establishing transmission of light through a reflecting Bragg grating, and for utilizing such transmission for analysis of the characteristics of a light signal. The drawbacks and limitations associated with the prior art are effectively eliminated by a method and an arrangement of the general kind set forth in the accompanying claims.
The present invention has further advantages, which will be apparent from the detailed description set forth below.
It is a general object of the present invention to provide a method of establishing transmission of light through a broadband, chirped Bragg-reflector. The method can be used for analysis of the power spectrum of a light signal, as well as for other applications where it is desirable to transmit a certain wavelength component of light through a structure at a

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