Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
1999-06-25
2001-08-28
Bovernick, Rodney (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling structure
C385S015000, C385S028000
Reexamination Certificate
active
06282342
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to monolithic filters, and particularly to single-fiber coaxial Mach-Zehnder and lattice devices and their applications.
2. Technical Background
There is an emerging need for narrowband wavelength division multiplex (WDM) couplers and filters. Important applications include broadband gain flattening filters for rare earth and Raman amplifiers. For example, such devices can be utilized in the 1550 nm window for modifying the gain spectrum of erbium fiber amplifiers. They will also be widely used in trunk lines as well as in fiber-to-subscriber architectures. These components must environmentally stable and very reliable.
Mach-Zehnder filters are known for their narrow-band wavelength capabilities. It has been proposed that filters having pass bands as narrow as 1 nm be formed by connecting two evanescent couplers with unequal fiber lengths between them. However, it is difficult to reproduce an environmentally stable device with this approach. The connecting fibers are subject to external destabilizing conditions such as temperature changes and random bending forces.
An environmentally stable Mach-Zehnder device that is insensitive to temperature gradients and able to withstand forces that would tend to cause inadvertent bending has been proposed. The device includes an elongated body of matrix glass through which first and second dissimilar optical fibers extend. The body includes a phase shift region in which the fibers have different propagation constants, whereby optical signals propagate through the optical fibers at different velocities in the phase shift region. At opposite ends of the phase shift region the body further includes two spaced, tapered coupler regions where the diameter of the body and the diameters of the fibers are smaller than in the phase shift region. Even though the propagation constants of the fibers are different in the phase shift region, the difference in propagation constants of the fundamental modes propagating in those fibers inside the tapered coupling regions is insignificant due to the small size of the cores in the tapered regions wherein coupling occurs.
It has been proposed to use overclad Mach-Zehnder couplers for gain flattening filter applications. The typical sinusoidal dependence of the two-coupler device is useful for filtering the red band or the blue band gain of Erbium-doped fiber amplifiers. Broadband gain filters require a nonsinusoidal filter function. Such broadband functionality using a three coupler-two core overclad lattice structure has been demonstrated. However, it has been discovered that two-fiber overclad Mach-Zehnder narrowband filters tend to be polarization sensitive, because the cores deform in the phase shift region of the device as the tube collapses onto the fibers during manufacture of the device.
A coaxial geometry has been proposed to eliminate the polarization sensitivity. Such devices are formed from an optical fiber that defines two waveguides, a rod waveguide (the central core of the fiber) and a coaxial tubular or ring waveguide. The refractive indices of the central core and the ring waveguides are elevated relative to the refractive index of the cladding layer that is intermediate the core and ring waveguides and relative to the refractive index of the outer cladding that surrounds the ring waveguide. Implementation of the design is difficult for the following reasons. In order to couple light from the core waveguide to the ring mode in the ring waveguide, it is required that the propagation constants of these modes in the tapered regions be similar. But dissimilar propagation constants are required for good filtration. It is difficult to form a coaxial fiber coupler that meets these requirements. More importantly, in a coaxial device formed of a fiber having a central core waveguide and a ring waveguide, the ring mode may be so tightly bound to the ring waveguide that it is not readily stripped therefrom by the protective coating of the output fiber pigtail. This might necessitate the utilization of a further bath of index matching fluid to prevent light that propagates in the ring waveguide from reaching the output of the device. If the ring mode reaches the splice between the output pigtail and the system fiber, modal noise is generated. In addition, the characteristic of insertion loss with respect to wavelength of Mach-Zehnder devices made from a coaxial fiber having core and ring waveguides was highly non-reproducible.
SUMMARY OF THE INVENTION
The present invention provides environmentally stable interferometric and lattice devices that exhibit low excess loss and low polarization dependent loss. The interferometric and lattice devices of the present invention are inexpensive and simple to make. The modal noise at the splices between the device pigtails and the system fiber is minimized or eliminated.
One aspect of the present invention is an optical device for filtering a light signal. The optical device has a tunable spectral response. The optical device comprises: a first optical fiber having a first core and a first cladding with refractive index n
2
, the first core includes a first central region having a refractive index n
1
; and a first fiber coupling regulator integral with the first optical fiber, the first fiber coupling regulator couples the light signal between a first optical path and second optical path and substantially prevents the light signal from coupling into a third optical path.
In another aspect, the present invention includes a coaxial device for operation at an operating wavelength &lgr;
o
, the device comprising: a single optical fiber having a core having a maximum refractive index n
1
surrounded by a cladding having a maximum refractive index n
2
, and a refractive index pedestal having a maximum refractive index n
5
situated between the core and cladding, wherein n
1
>n
5
>n
2
; at least one tapered region in the fiber, that portion of the fiber that extends from one end of the tapered region having a protective coating thereon and constituting a fiber pigtail, the taper angle of the tapered region being sufficiently great to cause coupling between the LP01 and the LP02 modes, but not so great as to cause coupling to the LP03 mode, wherein the optical fiber has a cutoff wavelength &lgr;
co
more than 200 nm less than the operating wavelength &lgr;
o
.
In another aspect, the present invention includes a coaxial device comprising: a single optical fiber having a core having a maximum refractive index n
1
surrounded by a cladding having a maximum refractive index n
2
, and a refractive index pedestal having a maximum refractive index n
5
situated between the core and cladding, wherein n
1
>n
5
>n
2
; at least first and second axially spaced tapered regions along the fiber; a phase shift region of the fiber extending between the tapered regions; and a first fiber pigtail extending from that end of the first tapered region opposite the phase shift region, the taper angles of the tapered regions being sufficiently great to cause coupling between the LP01 and the LP02 modes, but not so great as to cause coupling to the LP03 mode.
In another aspect, the present invention includes a method for filtering a light signal with an optical device having a predetermined spectral response, the optical device including a first optical fiber having a first core region and a first cladding with refractive index n
2
, the first core region includes a first core having a refractive index n
1
, the method comprising: providing a first fiber coupling regulator integral with the first optical fiber; directing the light signal into the first optical fiber; and coupling the light signal from an LP
01
mode into an LP
02
mode, wherein the first fiber coupling regulator couples the light signal between a LP
01
mode and LP
02
mode and substantially prevents the light signal from coupling into a LP
03
mode.
Additional features and advantages of the invention will be set forth in the
Berkey George E.
Nolan Daniel A.
Bovernick Rodney
Corning Incorporated
Kim Ellen E.
Malley Daniel P.
Smith Eric M.
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