Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
1999-10-19
2001-04-03
Schuberg, Darren (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S024000, C385S043000, C385S047000
Reexamination Certificate
active
06212318
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of fiber optic components. More particlarly, the present invention relates to wavelength division multiplexing using add/drop filters and multiplexers fabricated from cladding mode couplers and fiber Bragg gratings.
BACKGROUND OF THE INVENTION
Wavelength-division multiplexer are employed in optical fiber systems in which light waves of a particular spectrum or channel may be either multiplexed, i.e. added to, or demultiplexed, i.e. extracted from, a given transmission traveling in a core mode of a fiber optic cable. A channel extracted from a fiber optic cable is to be transferred to a second fiber optic cable while a channel being added to the first fiber optic cable are transferred from the second fiber optic cable. Various devices have been developed for transferring wavelength channels between two optical fiber cables.
One device is taught by Bilodeau et al., in “An all-fiber dense wavelength division multiplexer/demultiplexer using photoimprinted Bragg gratings” IEEE Photonics Technology Letters, Vol. 7, No. 4, pp. 388-390, which provides an all-fiber Mach-Zehnder interferometer with identical Bragg gratings in each of a pair of interferometer arms formed between two identical fused coupler regions. However, due to the interferometric nature of the device, the location of each component, i.e. the couplers, the fiber Bragg gratings, and the interfering arms needs to be controlled to a high degree of accuracy, thereby impeding its high-volume production.
A grating-frustrated coupler is disclosed by Arhambault et al. in Optics Letters Vol. 19 No. 3 Feb. 1, 1994, pp. 180-182, in which two single mode fibers form a 2×2 directional coupler. The two fiber cores are identical, except that only one of the cores contains a photorefractive Bragg grating coextensive with the coupling region. The grating frustrates the transfer of optical power from one fiber to the other within a narrow wavelength range. This approach also uses polished couplers which are difficult and expensive to fabricate. Additionally, the environmental stability of polished couplers tends to be lower than that of other approaches such as the fused taper method.
A grating-assisted coupler as disclosed by Dong et al. in IEEE Photonics Technology Letters, Vol. 8 No. 12, December 1996 includes a grating written in one fiber core over the coupling region of a polished 1000% coupler. The dropped channel sees a mismatched coupler due to the existence of the fiber Bragg grating and is therefore not coupled. Again, the environmental stability of polished couplers tends to be lower than that of other approaches such as the fused taper method.
U.S. Pat. No. 5,703,976 to Cullen discloses a wavelength selective 2×2 single mode fiber coupler having a weakly coupled coupling region in which the cladding modes of the two fibers are optically coupled while the core modes remain uncoupled. A matched pair of core-cladding mode converters are located on each fiber to opposite sides of the cladding region. This technique, however uses optical notch filters or bending to achieve conversion between the core and cladding modes. The forward propagating core mode is coupled to a forward propagating cladding mode and vice versa. Coupling from one forward propagating mode to another forward propagating mode requires a mode converter with a large period. Hence, in order to provide efficient coupling, these gratings must be relatively long. Furthermore, the notch filter and bending approaches are relatively difficult to control, particularly for narrow band devices.
It is therefore desirable to provide an add/drop filter for an optical fiber which may be more easily and reliably fabricated and without the need for both precisely positioning the components with respect to each other and for phase matching the components.
SUMMARY OF THE INVENTION
In view of the prior art, the present invention provides an optical fiber add/drop filter which includes a first and second elongate photosensitive optical fiber. Each optical fiber has opposed first and second ends and includes a core and a cladding. The cladding of the first optical fiber is optically coupled to the cladding of the second optical fiber at a coupler. A first fiber Bragg grating is etched into a second end of the first fiber for converting light propagating in a first direction through either of the core of cladding of the first fiber into light propagating in an opposite direction through the other of said core and cladding of said second fiber. A second fiber Bragg grating is etched into a first end of the second fiber for converting light propagating in a first direction either of the core or cladding of the second fiber into light propagating in an opposite direction through the other of the core and cladding of the second fiber. The first and second fiber Bragg gratings are located to opposite sides of the coupler.
The cladding mode coupler is fabricated so that there is substantially no coupling for the core modes of the coupler while the coupling ratio for the cladding is substantially 100 percent. The coupler may be fabricated by known methods including, but not limited to, the fuse taper method, the polished block method, or the fused polished method. However, the fibers may also be selected to have different propagation constants so the that the maximum coupling ratio of the coupler is about 50 percent and the coupling ratio is substantially constant over the entire wavelength range of interest. The advantage of the present invention is that the add/drop function is achieved without the need to phase match the constituent components.
The fiber Bragg gratings are written on the input and output ports of the coupler, on different fibers, at locations substantially outside the coupling region of the two fibers. The present invention avoids the need to control the position of the gratings relative to each other or with respect to the coupler and thereby simplifies and stabilizes the device. The fiber Bragg gratings function as wavelength selective mode converters. All incidental optical power at non-resonant wavelengths will pass substantially unaffected through the grating. Optical power at resonant wavelengths will couple between the core and cladding modes. In the
The add/drop filter of the present invention may be cascaded to form an optical add/drop multiplexer. As the present invention is not an interferometer, the position of the gratings relative to one another is not critical. The length of the interaction region between the two couplers is also not critical.
REFERENCES:
patent: 5703976 (1997-12-01), Cullen
patent: 5717798 (1998-02-01), Strasser et al.
Assai Tayez
Hoffman & Baron LLP
Schuberg Darren
Thomas & Betts International Inc.
LandOfFree
Add/drop filters and multiplexers fabricated from cladding... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Add/drop filters and multiplexers fabricated from cladding..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Add/drop filters and multiplexers fabricated from cladding... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2438368