Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1999-05-17
2002-07-02
Chan, Jason (Department: 2733)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200
Reexamination Certificate
active
06414769
ABSTRACT:
DESCRIPTION
It is an object of the present invention to provide a bidirectional multichannel telecommunication system, a bidirectional optical amplifier, and a method for the bidirectional transmission of optical signals.
In the latest telecommunication technology, it is known to use optical fibers to send optical signals carrying information for long-distance communication.
It is also known that optical signals sent in an optical fiber undergo attenuation along the way, making it necessary to amplify the signal so that it will travel the entire required distance and reach the receiving station at a power level sufficient for correct signal reception.
Said amplification may be effected by means of appropriate amplifiers placed at predetermined intervals along the line, which periodically boost the power of the transmitted optical signal.
Optical amplifiers are suitably used for this purpose, by which the signal is amplified while remaining in optical form, i.e. without the optoelectronic detection and electrooptical regeneration of same.
Said optical amplifiers are based on the properties of a fluorescent dopant (e.g. erbium) which, if appropriately excited by the application of luminous energy, provides a strong emission in the wavelength band corresponding to the minimum attenuation of light in silica-based optical fibers.
Said amplifiers are unidirectional devices, i.e. having a predetermined direction of travel of the optical signal inside them.
This is due, as described for example in U.S. Pat. Nos. 5,204,923 and 5,210,808 of the Applicant, to the fact that the optical amplifiers, particularly if high gain values are required, incorporate unidirectional components that prevent signals reflected outside the amplifiers, e.g. due to Rayleigh scattering along the optical line fibers connected to the amplifiers, from returning into the amplifier, causing interferometric noise.
For the bidirectional transmission of optical signals, known technology generally calls for the use of two separate communication lines, equipped with their respective amplifiers, each of which is used to communicate in a single direction. This results in a high connection cost.
Nevertheless, some technologies are know whose objective is to permit bidirectional transmission on fiber-optic lines by means of bidirectional optical amplifiers.
Bidirectional amplification schemes have been presented with the use of a single unidirectional amplifier that exploit the possibility of fluorescent doped amplifiers to independently amplify signals at different wavelengths.
A bidirectional amplifier based on this principal is described in the article by S. Seikai et al.: “Novel Optical Circuit Suitable for Wavelength Division Bidirectional Optical amplification”, published in Electronics Letters, vol. 29, no. 14, Jul. 8, 1993, pages 1268-1270. It discusses a device placed along a fiber-optic transmission line in which two signals with different wavelengths propagate in opposite directions and which consists of wavelength selective couplers and a known type of unidirectional doped-fiber amplification unit connected by sections of passive optical fiber. The wavelengths of the signals are both internal to amplification band of the doped fiber. By means of selective couplers the two signals at different;wavelengths are input to different optical paths. The two optical paths coincide only in the section corresponding to the amplifying fiber, which is passed through by the two signals in the same direction. The device has a problem of instability caused by internal reflections at a wavelength between those of the propagating signals, a problem resolved through the addition of filters, some of them adjustable, which results in a highly complicated structure and the need to use devices to accurately and continuous adjust said filters.
Patent application EP96100586, filed on Jan. 17, 1996 in the name of the Applicant, describes, among other things, a bidirectional optical amplifier comprising:
an optical amplification unit including at least an optical isolator,
two optical input and output ports for at least two optical signals having opposite propagation directions, said signals having, respectively, a first and a second distinct wavelengths,
two first and two second wavelength selective optical couplers, having respectively a first wavelength passband, including said first wavelength, and a second wavelength passband, including said second wavelength, with no overlapping,
said amplification unit being connected between two opposite nodes of an optical bridge circuit, to whose other opposite nodes said input and output ports are connected, and said first and second selective optical couplers being present at the nodes of said bridge circuit, in which said first and second selective couplers are arranged symmetrically with respect to the amplification unit and with respect to the input and output ports of said optical signals.
Bidirectional amplification schemes have also been presented with separation of the counterpropagating signals and the use of a unidirectional amplifier for each direction.
As an example, the article by C. W. Barnard et al. “Bidirectional Fiber Amplifiers” , published in IEEE Photonics Technology Letters, vol. 4, no. 8, August 1992, pages 911-913, describes bidirectional erbium-doped fiber amplifier repeaters for bidirectional fiber networks and OTDR fault detection. At each repeater the counterpropagating signals are separated, amplified separately, then recombined. Signal separation is done by a bidirectional fiber coupler or an optical circulator. According to the authors, for example, one propagation direction could be assigned 1525-1535 nm, the other could be assigned 1550-1560 nm, and the OTDR wavelength could be 1548 nm.
Optical telecommunication systems are known with wavelength division multiplexing (WDM) transmission. In these systems, a number of channels are sent, i.e. a number of mutually independent transmission signals, on the same line, usually consisting of an optical fiber, by means of optical wavelength multiplexing. The transmitted channels can be either digital or analog and are mutually distinguished because each of them is associated with a specific wavelength.
U.S. Pat. No. 5,283,686, in the name of D. R. Huber, describes, among others, optical systems including an optical amplifier and a narrow-bandwidth optical filter for removing undesired spontaneous emission. An in-fiber Bragg grating reflector reflects substantially only the input amplified signal back to an optical circulator port. The undesired emission exits from the grating reflector and is removed from the system. Cascaded grating reflectors are used in a wavelength division multiplexing (WDM) system. The article of K. Y. Chen et al., “Demonstration of in service supervisory repeaterless bi-directional wavelength division multiplexing transmission system”, vol. 7, no. 9, Sep. 1, 1995, states that repeaterless long distance fiber transmission systems using erbium doped fiber amplifiers as a transmitter power amplifier and/or as a receiver preamplifier have many applications, in which cases it is infeasible or impossible to have an in line amplifier, such as island hopping and intracity links. In this letter, an in service supervisory repeaterless bi-directional six WDM channel transmission over a 200 Km single fiber link is demonstrated.
The patent application EP 0 535 590 discloses a two way repeater apparatus for directly amplifying optical signals, to be transmitted in mutually opposite directions. Said repeater receives an outward input optical signal S
1
of 1.552 microns in wavelength at an input/output terminal
1
, and receives an inward optical signal S
1
r of 1.536 microns in wavelength at an input/output terminal 20.
The patent application DE 36 32 047 A discloses a communication system used for digital narrow band and wideband signals transmitted in both directions via a single optical waveguide.
The article of M. J. Chawki et al., “Evaluation of an optical boosted add/drop multiplexer OBADM including circ
Meli Fausto
Sanches Marcos Antonio Brandão
Bello Agustin
Chan Jason
Corning O.T.I. Inc.
Foley Hoag & Eliot LLP
Oliver Kevin A.
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