Optical waveguides – With optical coupler – Particular coupling function
Reexamination Certificate
2000-05-31
2002-06-04
Schuberg, Darren (Department: 2872)
Optical waveguides
With optical coupler
Particular coupling function
C385S037000, C385S027000, C385S123000
Reexamination Certificate
active
06400865
ABSTRACT:
FIELD OF THE INVENTION
This application pertains to few-moded optical waveguides with a refractive index (Bragg) grating, and to optical communication systems that comprise such waveguides.
BACKGROUND
Bragg gratings (also referred to as refractive index gratings) in optical waveguides are known. Conventionally such gratings couple a forward-propagating core-guided mode in single mode fiber to the backreflected core mode.
Mode conversion gratings are also known. See, for instance, U.S. Pat. Nos. 5,717,798 and 5,740,292. The latter discloses reflective gratings that inter alia couple light in the fundamental mode (LP
01
) to the LP
11
mode.
Mode coupling gratings can find a variety of uses in optical waveguide systems. For instance, they can serve as wavelength routing filters in WDM networks.
However, it has been found that reflective grating mode converters that efficiently convert LP
01
radiation to LP
11
radiation frequently are difficult to manufacture, due to the spatial degeneracy of the LP
11
mode. Whereas one of the LP
11
spatial modes typically can be converted to LP
01
by a LP
01
to LP
11
mode converter, the other LP
11
spatial mode typically can not be so converted, due to the interference of two nearby degenerate spatial modes. This spatial degree of freedom is difficult to control. Thus, it would be desirable to have available a mode converter which does not involve coupling to or from a spatially degenerate mode.
In particular, there is a need for a mode converter which couples the LP
01
mode to the LP
02
mode, without coupling the LP
01
mode to any other guided mode, e.g., LP
11
and reflected LP
01
. Such an LP
01
-LP
02
mode converter could be used, for instance, in an add/drop multiplexer. This would avoid the need for an expensive and lossy circulator. Circulators are essential in implementing prior art Bragg grating-based add/drop filtering applications.
The LP
02
mode is not spatially degenerate and thus can be efficiently converted to LP
01
. However, a LP
01
to LP
02
mode converting reflective grating typically must be designed such that both the LP
01
to LP
11
mode conversion and the LP
01
to LP
01
mode conversion are substantially nulled. The LP
11
mode must exist in the optical fiber since, in order for the LP
01
to LP
02
mode conversion to be strong, the optical fiber must guide the LP
02
mode, and therefore must also guide the LP
11
mode. To the best of our knowledge, the prior art does not provide a technique for making such a reflective LP
01
to LP
02
mode converter. This application inter alia discloses such a mode converter.
C. X. Shi, IEEE Journal of Quantum Electronics, Vol. 32(8), August, 1996, page 1360, provides a theoretical treatment of a fiber-optic Fabry-Perot resonator with two mode conversion (LP
01
to LP
02
) “mirrors”. See also C. X. Shi et al., Optics Letters, Vol. 17(23), page 1655, December 1992; and F. Bilodeau et al., Electronics Letters, Vol. 27(8), page 682, April 1991.
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 0.6:1.0 in order to obtain the required relative photosensitivity. The Holmes et al. paper thus discloses fiber in which the core had 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 applications, and not to obtain a mode converter of the herein relevant type.
All cited references are incorporated herein by reference.
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.
The “coupling strength” between two guided core modes in a few-moded optical fiber is conventionally expressed in terms of an overlap integral, as shown in equation 2) below. The coupling strength typically depends on the refractive index profile n(r), the photosensitivity profile p(r), and the tilt angle &thgr;.
“Minimizing” the coupling between two guided core modes in a given waveguide means adjusting the tilt angle of a tilted grating such that the coupling strength between the two modes is less than −30 dB.
“Maximizing” the coupling between two guided core modes in a given waveguide means adjusting the tilt angle of a tilted grating such that the coupling strength between the two modes is at least about −10 dB.
By a “regular null” we mean herein a tilt angle region in a tilted (“blazed”) fiber Bragg grating that has a core mode coupling strength 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. Regular nulls occur for many tilt angles.
By a “super null” we mean two (or possibly more) regular nulls that occur at closely spaced tilt 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 0.5° 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. Por 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.
A “few-moded” optical waveguide supports the fundamental mode and one or more higher order modes, typically no more than about 10 guided modes total.
The description of the invention herein is generally in terms of conversion between the fundamental mode and a higher order mode such as LP
02
. This is for the sake of concreteness only, and the invention at least in principle can be embodied in an article for mode conversion between two appropriate higher order modes.
SUMMARY OF THE INVENTION
In an exemplary mode converter according to the invention, it is necessary that LP
01,f
light be strongly coupled into the LP
02,b
mode. However, in such a mode converter the coupling between LP
01,f
and all other guided reflected modes (exemplarily LP
11,b
and LP
01,b
) has to be small, exemplarily at least 20 dB less than LP
01,f
to LP
02,b
coupling. This simultaneous “nulling” of the coupling between LP
01,f
and the other guided modes (i.e., other than the desired coupling) can not be achieved with optical fiber that has uniform photosensitivity throughout the core, necessitating use of a more complex fiber design, as is described below.
The coupling strengths between the various guided modes in an optical fiber depend on the refractive index profile of the fiber and the electric fields of the various modes. Both of these parameters generally are fixed at the time of grating formation, and thus can not be varied to achieve a desired coupling. The only grating parameter which can be changed to significantly alter the relative coupling strengths is the tilt of the grating with respect to the core axis. However, with uniform photosensitivity in the fiber core, the control over the various coupling strengths that is achievable by introduction of a tilt in the grating is limited. In particular, with a uniform radial photosensitivity profile, it is impossible to “null” simultaneously an even-even reflection (e.g., LP
01,f
to LP
01,b
) and an even-odd reflection (eg., L
Strasser Thomas Andrew
Westbrook Paul Stephen
Assaf Fayez
Fitel USA Corp.
Pacher Eugen E.
Schuberg Darren
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