Article comprising a tilted grating in a single mode waveguide

Details Article comprising a tilted grating in a single mode waveguide Article comprising a tilted grating in a single mode waveguide

2000-05-31

2002-07-30

Sanghavi, Hemang (Department: 2874)

Optical waveguides

With optical coupler

Input/output coupler

Reexamination Certificate

active

06427041

ABSTRACT:

FIELD OF THE INVENTION
This invention pertains to tilted gratings in single mode waveguides, typically a single mode optical fiber, and to optical communication systems that comprise such gratings.
BACKGROUND
Bragg gratings in single mode waveguides typically couple a forward core-guided mode to backreflected modes in the core and the cladding. In at least some cases, it is desirable to control the relative strengths of these couplings to achieve a desired function. For instance, the coupling to backward-propagating cladding modes in single mode fibers may be used in loss filters. This typically requires that the undesirable core mode reflection be minimized in comparison to the cladding mode coupling.
The mode coupling strengths of gratings generally depend on the waveguide photosensitivity profile and the electric field of a given mode, both of which are largely fixed at the time of grating formation. A grating parameter which can be changed to alter the relative strength of the mode couplings is the tilt of the grating with respect to the waveguide axis. However, in prior art waveguides the degree of control that is achievable by means of the grating tilt is quite limited. For instance, in prior art single mode waveguides, the angular range of the tilt angle &thgr; over which a given mode coupling substantially is zero (defined herein as less than −30 dB) is quite limited, typically 0.1° or less. Such gratings are difficult to manufacture.
Tilted gratings in optical fiber are known. See, for instance, U.S. Pat. No. 5,740,292, which discloses tilted refractive index gratings for coupling light in a fundamental mode (e.g., LP
01
) into a higher order mode (e.g., LP
11
). Such a grating can find a variety of uses, e.g., as a wavelength-dependent loss element with abrupt wavelength dependence. See also U.S. Pat. No. 5,832,156, which discloses a tilted grating in a dispersive optical waveguide tap.
Thus, there exists a need for a tilted waveguide grating in a single mode waveguide that can provide a broader tilt angle range of substantially zero coupling into the backwards core mode. This application discloses such a tilted waveguide grating. Furthermore, prior art tilted Bragg gratings typically have relatively low cladding loss (typically substantially less than 20 dB) as well as relatively low bandwidth (typically substantially less than 20 nm). However, there is a need for tilted Bragg gratings in single mode optical waveguides that not only are readily manufacturable but that also have relatively large cladding loss (e.g., >20 dB) and relatively large bandwidth (e.g., >20 nm). Such gratings can, for instance, advantageously be used in Er-doped fiber amplifiers to reject undesired ASE (amplified spontaneous emission). See, for instance, R.P. Espindola et al., paper WD4, “Optical Amplifiers and Their Application” (OAA), 1999 Nara, Japan.
Prior art tilted Bragg grating filters in fibers with complex radial photosensitivity profile typically have a photosensitive cladding. For instance, M. J. Holmes et al., ECOC '98, September 1998, Madrid, Spain, pages 137-138, disclose sidetap filters that comprise a tilted Bragg grating in single mode fiber. The fiber had a non-photosensitive core dopant, and a photosensitive cladding doped with germania, to which boron was added in order to reduce the cladding refractive index to match the deposition tube. The fiber thus had a conventional refractive index profile, with the core refractive index greater than the cladding index, but had zero photosensitivity in the core and a non-zero photosensitivity in the cladding. See
FIGS. 1
a
and
1
b
below. See also E. Delevaque et al., Optical Fiber Communication Conference 1995, Paper PD5; C. W. Haggans et al., IEEE Photonics Technology Letters, Vol. 10(5), May 1998, page 690; I. Riant et al., Optical Fiber Communication Conference 1999, Paper ThJ6-1/147; L. Dong et al., Bragg Gratings, Photosensitivity and Poling in Glass Waveguides Conference, 1999, Paper PD3; and L. Brilland et al., Electronics Letters, Vol. 35(3), Feburary 1999, page 234.
The above-cited Delevaque and Dong papers describe fiber designs in which core and cladding photosensitivity are adjusted to reduce cladding mode loss in untilted gratings.
M. J. Holmes et al., ECOC '99, Sep. 26-30, 1999, Nice, France, pages I-216-217 disclose a fiber for sidetap filters. The fiber had a non-photosensitive core dopant for normalized radius less than 0.4, a combination of a non-photosensitive core dopant and germania for normalized radius 0.4-1, and a photosensitive cladding doped with germania out to a normalized radius of 3.5, to which boron was added to reduce the cladding index to match the deposition tube. The germania concentrations for the regions 0.4-1.0 and 1.0-3.5 were in the ratio 0.6-1 in order to obtain the required relative photosensitivity. See
FIGS. 2
a
and
2
b
below. The above cited 1999 Holmes et al. paper thus discloses fiber in which the core has two different photosensitivity levels, with the cladding also being photosensitive. The photosensitivity profile was chosen to optimize the wavelength dependence of the cladding mode loss spectrum for gain flattening filter applications. The Bragg grating in this fiber is limited to applications as a weak narrow bandwidth filter. Thus, there exists a need for a fiber grating that can readily be made to have very low core mode reflection, and that has large cladding mode loss (preferably greater than 20 dB) over a large bandwidth (preferably greater than 20 nm). Practice of the present invention is advantageous in prior art narrow bandwidth applications because it will provide an even lower level of core mode reflection.
GLOSSARY AND DEFINITIONS
For ease of exposition the discussion herein will generally refer to optical fibers. It will be appreciated, however, that similar results are obtainable in other optical waveguides, e.g., in planar waveguides.
By a “regular null” we mean herein a tilt angle region in a tilted (“blazed”) fiber Bragg grating that has a core mode coupling for light of a predetermined wavelength that is less than −30 dB over only a small (typically less than 0.1°) angular range of the tilt angle. See, for instance,
FIGs. 1
a
and
1
b
. Regular nulls occur for many tilt angles.
By a “super null” we mean two (or possibly more) regular nulls that occur at closely spaced blaze angles, thereby making the core mode coupling at the predetermined wavelength very low (typically less than −30 dB) over a relatively large (more than 0.1°, desirably more than 0.2°, or even 0.50 or more) range of tilt angles between the regular nulls.
Modes of the guided light are designated LP
mn
. in conventional fashion, with m and n being integers. For instance, LP
01
is the fundamental mode. LP
01,f
refers to the forward propagating fundamental mode, and LP
01,b
refers to the backward propagating fundamental mode.
“Photosensitivity” refers to the refractive index change in the waveguide that results if an appropriately doped waveguide is exposed to actinic radiation, typically UV radiation.
By “cladding mode loss” we mean herein waveguide loss which results from grating coupling of a core guided mode to cladding modes.
By the “band width” of a tilted Bragg grating we mean herein the wavelength interval over which the cladding loss is greater than 3 dB.
SUMMARY OF THE INVENTION
In a broad aspect the instant invention is embodied in an article that comprises a tilted Bragg grating of novel design in a single mode waveguide, the tilted grating selected to provide a relatively large (exemplarily >0.1°) range of tilt angle &thgr; wherein there is <−30 dB coupling of radiation of predetermined wavelenght &lgr; from a forward propagating core mode (e.g., LP
01,f
) into a backward propagating core mode (e.g., LP
01,b
), whereby manufacture of the tilted grating is facilitated. The relatively large range of tilt angle constitutes a “super null”, achieved through appropriate choice of the photosensitivity profile of the fiber core.
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