Wavelength selective optical couplers

Details Wavelength selective optical couplers Wavelength selective optical couplers

1998-03-06

2001-09-18

Derrington, James (Department: 1731)

Glass manufacturing

Processes of manufacturing fibers, filaments, or preforms

Process of manufacturing optical fibers, waveguides, or...

C065S378000, C065S399000, C065S400000, C065S408000, C065S411000, C065S425000, C065S435000, C264S001250, C264S001260, C385S009000, C385S037000, C385S050000, C385S141000, C385S144000

Reexamination Certificate

active

06289699

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the communication of signals via optical fibers, and particularly to an optical fiber coupler and methods for making the same. More particularly, the invention relates to a wavelength selective optical coupler and other devices using a refractive index grating in a coupling region.
DESCRIPTION OF RELATED ART
Low loss, wavelength selective couplers are important components for optical fiber communication networks based on wavelength division multiplexing (WDM). WDM enables an individual optical fiber to transmit several channels simultaneously, the channels being distinguished by their center wavelengths. An objective is to provide a precise wavelength selective coupler that is readily manufactured and possesses high efficiency and low loss. One technology to fabricate wavelength selective elements is based on recording an index of refraction grating in the core of an optical fiber. See, for instance, Hill et al., U.S. Pat. No. 4,474,427 (1984) and Glenn et al., U.S. Pat. No. 4,725,110 (1988). The currently preferred method of recording an in-line grating in optical fiber is to subject a photosensitive core to the interference pattern between two beams of actinic (typically UV) radiation passing through the photoinsensitive cladding.
Various techniques such as flame brushing and hydrogen loading have been introduced to increase fiber photosensitivity and produce index of refraction changes in excess of 10
−2
, as described by Ouellette et al., Applied Physics Letters, Vol. 54, p. 1087 (1989), and Applied Physics Letters, Vol. 58, p. 1813 (1991) and Atkins et al., U.S. Pat. No. 5,287,427. Alternately, J.-L. Archambault et al., Electronics Letters, Vol. 29, p. 453 (1993) reported that an index change as large as 0.006 was obtained in untreated optical fiber (15 mol % Ge core) using a single high energy (40 mJ) pulse at 248 nm.
A method of recording Bragg gratings in single mode optical fibers by UV exposure through a phase mask was reported by Anderson et al., U.S. Pat. No. 5,327,515 (1994), Snitzer et al., U.S. Pat. No. 5,351,321 (1994), Hill et al., U.S. Pat. No. 5,367,588 (1994). This phase mask is typically a transparent substrate with periodic variations in thickness or index of refraction that is illuminated by an optical beam to produce a spatially modulated light pattern of the desired periodicity behind the mask.
Optical fiber gratings reported in the prior art almost universally operate in the reflection mode. Because of the small numerical aperture of single mode optical fibers (N.A.~0.11), grating components transverse to the longitudinal axis of the optical fiber couple light into the lossy cladding modes. The maximum allowable angular offset of the grating fringes with the longitudinal axis is generally less than 1 degree. Furthermore, to gain access to this reflected mode in a power efficient manner is difficult, because the wave is reflected backwards within the same fiber. A first method to access this reflected light is to insert a 3 dB coupler before the grating, which introduces a net 6 dB loss on the backwards reflected and outcoupled light A second method is to insert an optical circulator before the grating to redirect the backwards propagating mode into another fiber. This circulator introduces an insertion loss of 1 dB or more and involves complicated bulk optic components. A method to combine the filtering function of a fiber grating with the splitting function of a coupler in a low loss and elegantly packaged manner would be highly attractive for WDM communication networks.
Another method well known in the prior art uses directional coupling to transfer energy from one waveguide to another by evanescent coupling (D. Marcuse, “Theory of Dielectric Waveguides,” Academic Press 1991 and A. Yariv, “Optical Electronics,” Saunders College Publishing, 1991). This evanescent coupling arises from the overlap of the exponential tails of the modes of two closely adjacent waveguides, and is the typical mode of operation for directional coupler based devices. In contrast, non-evanescent coupling occurs when the entire optical modes substantially overlap, as is the case when the two waveguides are merged into a single waveguide. Devices that rely on evanescent coupling (e.g., directional couplers) in contrast to non-evanescent coupling have inherently weaker interaction strengths.
One realization of a directional coupling based device uses gratings recorded in a coupler composed of two identical polished fibers placed longitudinally adjacent to one another (J.-L. Archambault et al., Optics Letters, Vol. 19, p.180 (1994)). Since the two waveguides are identical in the coupling region, both waveguides possess the same propagation constant and energy is transferred between them. This results in poor isolation of the optical signals traveling through the two waveguides, because optical power leaks from one fiber to the other. Another device also based on evanescent coupling was patented by E. Snitzer, U.S. Pat. No. 5,459,801 (Oct. 17, 1995). This device consists of two identical single mode fibers whose cores are brought close together by fusing and elongating the fibers. The length of the coupling region should be precisely equal to an even or odd multiple of the mode interaction length for the output light to emerge entirely in one of the two output ports. A precisely positioned Bragg grating is then UV recorded in the cores of the waist region.
An alternative grating assisted directional coupler design reported by R. Alferness et al., U.S. Pat. No. 4,737,007 and M. S. Whalen et al., Electronics Letters, Vol. 22, p. 681 (1986) uses locally dissimilar optical fibers. The resulting asymmetry of the two fibers improves the isolation of the optical signals within the two fibers. However, this device used a reflection grating etched in a thin surface layer on one of the polished fibers, dramatically reducing the coupling strength of the grating. It also is based on evanescent coupling. A serious drawback of this device is that the wavelength for which light is backwards coupled into the adjacent fiber is very close to the wavelength for which light is backreflected within the original fiber (about 1 nm). This leads to undesirable pass-band characteristics that are ill suited for add/drop devices that are designed to add or drop only one wavelength. For optical communications applications in the Er doped fiber amplifier (EDFA) gain window (1520 to 1560 nm), this backreflection should occur at a wavelength outside this window to prevent undesirable crosstalk. The separation between the backreflected and backwards coupled wavelengths is impractically small for the all-fiber, grating assisted directional coupler approaches of the prior art.
Alternatively, F. Bilodeau et al., IEEE Photonics Technology Letters, Vol. 7, p. 388 (1995) fabricated a Mach-Zender interferometer which served as a wavelength selective coupler. This device relies on the precisely controlled phase difference between two interferometer arms and is highly sensitive to environmental fluctuations and manufacturing variations. In addition, a significant fraction of the input signal is backreflected. Therefore, it is uncertain whether this device will be able to meet the demanding reliability requirements for telecommunications components.
The conventional grating assisted directional coupler suffers from both a relatively low coupling strength and small wavelength separation of back-reflected and backwards coupled light These problems arise because the two coupled optical waveguides remain physically separate and the light remains guided primarily in the original cores. The light propagating in each of the two coupled waveguides overlaps only at the evanescent tails of the optical modes, corresponds to evanescent coupling. The two original optical fibers can instead be fused and elongated locally to form a single merged waveguide core of much smaller diameter. The resulting optical mode propagation characteristics are effectively those of a multimode

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