Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2001-03-06
2003-08-05
Kim, Robert H. (Department: 2882)
Optical waveguides
With optical coupler
Input/output coupler
C385S043000, C385S050000, C385S039000, C065S385000, C065S378000, C065S376000, C065S377000, C065S398000, C065S392000
Reexamination Certificate
active
06603903
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to photonic couplers and methods of making the same, and more particularly to such components and methods used to fabricate reduced diameter, photosensitive optical fibers to and record grating patterns therein.
BACKGROUND OF THE INVENTION
Modern communication systems are increasingly based on optical transmission through optical fibers, because of the superior bandwidth capabilities of optical signals and the fact that a single optical fiber can transmit many different channels, as by wavelength division multiplexing. To realize the potential of such systems, wavelength selective devices, including couplers and filters, have been recently developed to meet the requisite design and performance specifications. These requirements include precise wavelength selectivity, low crosstalk, flat passbands, low dispersion and low insertion loss. These are all necessary to avoid diminution of signal strength and the introduction of signal distortion, as devices are cascaded to perform various multiplexing and demultiplexing functions.
Many wavelength selective components for these purposes are based upon the approach of embedding or writing a periodic pattern, such as a Bragg grating, in an optical fiber, so as to reflect or transmit only a very narrow wavelength band within a much broader spectral range, for example, the entire C or L WDM band. One example is a four terminal add/drop coupler formed from two optical fibers merged at an intermediate region and incorporating a Bragg grating. A substantial departure from prior concepts that use this basic configuration is described in U.S. Pat. No. 5,805,751 to Kewitsch et al., entitled “Wavelength Selective Optical Couplers”, and assigned to the assignee of the present invention. Devices as taught in this patent are grating assisted and typically asymmetric. They operate with high efficiency in typically a reflective mode or alternately in a transmission mode. They are further characterized by a non-evanescent, very small diameter coupling region in which two optical fibers are longitudinally fused. In this coupling or waist region, signals are guided in a glass-air waveguide mode, because the original cladding is now of small diameter and the doped cores of the fibers have been reduced to vestigial elements which have only a small effect on waveguiding. After the fibers are narrowed and merged, a periodic index of refraction pattern Bragg grating) is written in the small diameter coupling region, which is typically less than about 10 microns in cross-sectional dimensions but is photosensitive because of its dopant content, the use of in-diffusion of a photosensitizing gas, or both.
The process used to form a merged coupling region presents some unique problems involving multiple disciplines that extend well beyond the present day techniques used to produce fused splitters. For example, to illuminate the coupling region with uv light through a mask so as to record a grating pattern, the target material must remain photosensitive. However, the very small diameter coupling region must be formed by controlled elongation and fusion as the optical fiber is heated to the softening point, a process that can significantly affect the photosensitivity of the glass. To maintain low loss and control of elongation, the heating is generally best done with a reciprocating flame, recognizing that the temperature of the flame as well as the chemical composition of the heating gas can influence the subsequent photosensitivity. Furthermore, because the fibers in the coupling region are of micron range diametral size, the fibers cannot withstand the force of a flame of substantial velocity without deflecting and/or deforming. Moreover, the strength of the grating that is ultimately written is dependent on all stages of the process, from initial photosensitivity of the starting fiber cladding material, through heating and drawing, to the completion of an exposure step. The interrelationships of these factors have not heretofore been fully understood or utilized, but it is clear that improvements can be made in grating efficiency, passband characteristics and in product yields as well.
While achieving a photochemical state in which photosensitizing potential is brought to a high level is more than adequate in and of itself for many purposes, more is increasingly being required of photonic devices using index of refraction patterns. For example, workers in the art are now extending systems and devices toward 25 GHz and 50 GHz applications, thus requiring narrow bandwidth gratings in fibers and couplers. Higher performance is also being sought in add/drop devices for more general use. To meet the increasingly stringent requirements of the modern era, spatial variations in the effective index of variation change (chirp) must be very small, approximately a factor of 10 less than the desired DWDM periodicity. In numerical terms 100 GHz filters require a chirp of less than 0.08 nm, which equates to 0.0008 uniformity in the index of refraction change. For 25 GHz filters the chirp and uniformity of index of refraction change must be 4 times tighter.
Maintaining adequately low crosstalk (<−25 dB) further demands that the spatial variation of the index of refraction be extremely smooth along the grating length. Specifically, and superfluous periodicities (ripple) in the grating of between 0.5 microns to 1 mm must be removed to a level better than 5%. The problems of meeting such requirements are compounded when one considers that the exposure response of the photosensitive material varies non-linearly with exposure time, and in a variable manner dependent on the photochemistry of the material. In addition the photosensitivity of the target material varies non-linearly as a function of laser intensity, and the intensity of a beam projected through a varying (i.e. apodized) phase mask also is dependent on position relative to the phase mask.
SUMMARY OF THE INVENTION
Systems and methods in accordance with the invention include the use of photosensitizing dopants in a precursor element, such as an optical fiber, heating the fiber during drawing with a diffuse and distributed low hydrogen content flame of very low velocity and of controlled temperature. As the fiber is tensioned, it is locally heated in a repetitive manner by reciprocating movement of the flame until it is drawn down to a selected length of substantially uniform diameter. In illuminating this target region to write a periodic grating, the intensity of the actinic radiation is varied in controlled fashion as a photosensitizing gas is diffused into the fiber, preferably at elevated pressure. The index of refraction change in the target may be further enhanced by optimizing grating growth through balancing of light source intensity, scan velocity, and blue light luminescence from the target fiber.
In more specific examples of systems, devices and methods in accordance with the invention, the target region of a photonic device, i.e. an optical fiber or fibers in which a grating is to be written, includes a constituent (dopant) providing photosensitivity to uv illumination. This region is gently heated with a low velocity, inverted reciprocating flame that locally surrounds the target area of optical fiber. The flame is preferably a mixture of CO and O
2
, with an inert gas assuring that OH and water by-products will be minimized. Relative humidity and temperature of the surrounding air atmosphere are maintained within selected limits. Flame temperature can be reduced by mixing with an inert gas (such as N
2
), the amount of which can be adjusted to maintain a desired temperature. After the heated fiber is adequately elongated, the photochemical characteristics of dopants within the fiber, together with the exposure process, determine the grating growth characteristics. By subjecting the fiber during actinic illumination to indiffusion of high pressure deuterium or hydrogen (possibly heating the fiber at the same time) and by maintaining the uv illumination i
Kewitsch Anthony S.
Rakuljic George A.
Tong Xiaolin
Arroyo Optics Inc.
Jones Tullar & Cooper PC
Kim Robert H.
Wang George Y.
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