Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2001-04-06
2002-04-16
Palmer, Phan T. H. (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S011000, C385S129000
Reexamination Certificate
active
06374016
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to planar optical circuits and more particularly to a method for lessening unwanted polarization dependence within planar waveguides of such circuits.
BACKGROUND OF THE INVENTION
The invention is directed to a method for modifying the refractive index of planar optical waveguides by ultraviolet light irradiation, including but not restricted to forming an optical structure such as a Bragg or long period grating. More particularly, the novel method includes steps, which can minimize birefringence effects normally associated with writing such structures in multi-layer devices supported by a relatively thick substrate.
The sensitivity of optical waveguide fibers to light of certain wavelength and intensity has been known since the late 1970's. It was found that the loss characteristic and refractive index of a waveguide fiber could be permanently changed by exposing the waveguide to light of a given wavelength and intensity. A publication which describes the effect and how it may be used is, “Light-sensitive optical fibers and planar waveguides”, Kashyap et al., BT Techno., 1, Vol. 11, No. 2, April 1993. The publication discusses the making of light-induced reflection gratings, page 150, section 2.1, and notes that the amount of refractive index change increases as light wavelength is reduced from 600 nm to 240 nm, where the photosensitivity of the waveguide appears to peak, with the notable exception of irradiation using strong 193 nm light where the photosensitivity can be very large (as demonstrated by Malo et al. Electron. Lett. Vol. 31, p.879 (1995)).
In “Bragg grating formation and germanosilicate fiber photosensitivity”, SPIE V. 1516, Intn'l Workshop on Photoinduced Self-Organization Effects In Optical Fiber, Meltz et al., 1991, the mechanism and magnitude of photosensitivity is discussed (page 185, first paragraph, section 1.). This publication also discusses an interferometric technique of writing gratings (pp. 185-6, section 2.) At page 189, first paragraph, a measurement of induced birefringence is presented. See also FIG. 6 of that publication.
Another publication, “Characterization of UV-induced birefringence in photosensitive Ge-doped silica optical fibers”, Erdogan et al., J. Opt. Soc. Am. B/V.11, No. 10, October 1994, notes the dependence of induced birefringence on the orientation of the polarization direction of the light incident upon the waveguide fiber. In particular, data presented in the publication shows that the induced birefringence is greatest when the polarization direction is oriented perpendicular to the long axis of the fiber and least when the polarization direction is parallel to the long axis of the fiber. See FIG. 3
a
. and FIG. 4. of the publication.
The Erdogan et al. publication points out that the induced birefringence polarization anisotropy can be used to make such devices, “as polarization mode converters and rocking filters”, page 2100, first paragraph. However, in devices using resonant propagation, “the birefringence can result in substantial polarization dependence of resonant grating properties, such as reflectivity”, page 2100, first paragraph.
The Erdogan, et al., data shows that even in the configuration where the polarization direction is along the long axis of the waveguide, some birefringence is still induced in the waveguide. Comparing the curves of FIG. 3
a
. and FIG. 4., the non-polarization dependent induced birefringence is a factor in the range of about 4 to 12 smaller than the polarization dependent induced birefringence. However, even this smaller amount of birefringence is undesirable. A more versatile and effective grating would result from a writing method which produces a grating having minimal birefringence.
Notwithstanding, polarization effects or sensitivity from irradiating waveguides in multilayer structures exhibited as a result of disposing a relatively thin waveguide comprised of an assembly of material layers supporting low loss light propagation deposited on a thick substrate comprised of a material having different characteristics from those of the deposited layers, is significantly more evident and problematic than the effects and causes described by Erdogan et al. It is this polarization sensitivity caused by mismatching and thickness differences in layered material that is addressed by this invention.
Planar optical circuits, often termed planar lightwave circuits (PLCs) are well known and for particular applications some of which include optical gratings formed therein, such as Bragg gratings, or long period gratings. Since most signals propagating through optical fiber have an indeterminate polarization state, it is preferred that the gratings through which these signals propagate, be substantially polarization insensitive. J. Albert et al., the applicants have disclosed in a paper entitled “Polarization-independent strong Bragg gratings in Planar Lightwave Circuits” Electron Lett. 34, 485-486 (1998), a method of lessening the polarization dependence or “polarization sensitivity” of planar waveguides having Bragg gratings formed therein. By using an intense ArF excimer laser a refractive index change is produced and is birefringent. This birefringence is large enough and of the proper sign to compensate the inherent birefringence exhibited in most PLCs.
Notwithstanding, in the instant invention the birefringence can be controlled independently of the size of the index change. An instance where this control is particularly useful is in the path length trimming of a Mach-Zehnder interferometer that is initially polarization independent. Of course it is preferred that the trimming be nonbirefringent to maintain the polarization independence of the device. However, this invention can be used in other phased array devices, or arrayed wave guides (AWGs), requiring similar polarization insensitively in the arms of the AWG.
Planar waveguides usually have different propagation constants for TE (transverse electric) and TM (transverse magnetic) waveguide modes and therefore are known to be polarization sensitive. Stated more simply, the response of these waveguides differs for orthogonally polarized light beams. For wavelength multi/demultiplexers, this difference in propagation constants results in a wavelength shift in the spectral response peak or the passband of each wavelength channel. This wavelength shift is sensitive to the design of the planar waveguide, and can be as large as 3 nm. As WDM systems are being designed towards smaller and smaller channel spacing (from 1.6 nm to 0.8 nm or even less in the future), even a small polarization dependent wavelength shift (e.g. 0.3~0.4 nm) is of concern.
Quite surprisingly, the inventors of the instant application have discovered that the size of the beam, relative to the size of the waveguide in which a grating is to be photo-induced, greatly affects the polarization dependence of the grating or structure being written into the waveguide. For example, photo-induced birefringence occurs when irradiating a planar waveguide as described with a beam of suitable intensity having a spot size that is substantially greater than the width of the waveguide region. In some instances this birefringence offsets or compensates for the birefringence present in the planar waveguide prior to irradiation. However, most often, when writing an optical structure by photoinducing a refractive index change in the waveguide using current techniques, the amount of photo-induced birefringence cannot be accurately controlled; achieving as a desired refractive index change &Dgr;n does not always occur at the point where irradiation of the waveguide induces a birefringence that yields a substantially polarization insensitive device. However, by utilizing conventional techniques of irradiating with a beam sized larger than the waveguide width in combination with irradiating the waveguide with a smaller beam spot size less than or equal to the width of the waveguide channel, improved control over the polarization sensitivity of the de
Albert Jacques
Bilodeau Francois
Hill Kenneth O.
Johnson Derwyn C.
Mihailov Stephen J.
Freedman & Associates
Her Majesty the Queen in right of Canada as represented by the
Palmer Phan T. H.
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