Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2001-02-23
2003-07-22
Bovernick, Rodney (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S031000, C385S015000, C359S483010, C359S494010, C359S490020
Reexamination Certificate
active
06597839
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method and device for providing polarization compensation for a polarization sensitive device. This invention further provides a means of compensating for birefringence in a device or waveguide.
BACKGROUND OF THE INVENTION
Since most optical signals propagating through optical fiber have an arbitrary polarization state, it is preferred that the switching/routing devices or other devices through which these signals propagate, be substantially polarization insensitive. Notwithstanding, although measures are taken in the design of switches, routers, multiplexers and other components to lessen their polarization sensitivity, tests often indicate that required levels of polarization sensitivity are not met. Planar waveguides usually have different propagation constants for TE (transverse electric) and TM (transverse magnetic) waveguide modes and 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.
Two types of integrated wavelength multi/demultiplexers that have been investigated are phased waveguide arrays and grating-on-a-chip spectrometers.
Grating based devices require high quality, deeply etched grating facets. The optical loss of the device depends critically on the verticality and smoothness of the grating facets. However, the size of the grating device is usually much smaller than the phased array and the spectral finesse is much higher due to the fact that the number of grooves in the grating is much larger than the number of waveguides in the phased array. This allows the grating based device to have a larger number of channels available over its free spectral range (FSR) and consequently can be scaled-up easily to high density operation.
In waveguide array based devices, several approaches have been used to reduce the device polarization sensitivity; for example the insertion of a half wave plate in the middle of the waveguides array is described by H. Takahashi, Y. Hibino, and I. Nishi, in a paper entitled “Polarization-insensitive arrayed waveguide grating wavelength multiplexer on silicon”, Opt. Lett., vol. 17, no. 7, pp. 499-501, 1992.
One patent which provides a useful solution for polarization compensation in a slab waveguide device, is U.S. Pat. No. 5,937,113 issued Aug. 10, 1999 to He et al., entitled “Optical grating-based device having a slab waveguide polarization compensating region”.
Notwithstanding, the instant invention is believed to provide a more general solution to this problem of compensating for polarization sensitive optical devices, making them less sensitive to the polarization state of light passing therethrough.
The instant invention relies on the use of an in-line fibre grating to equalize the polarization dependence of an optical device. A similar type of blazed grating has been disclosed at OFC 1992, Feb. 4-7, 1992, San Jose Convention Center, San Jose, Calif., in a Tutorial Session (Tutorial TuK, page 132 of the Tutorial Digest) entitled “Fiber-Based Passive Components” presented by Kenneth O. Hill, the applicant on Feb. 4
th
, 1992.
Another later disclosure of a blazed grating used as a tap is found in U.S. Pat. No. 5,061,032 to G. Meltz et al. which discloses an optical fiber tap that comprises a blazed, chirped refractive index grating selected to redirect light guided in the fiber such that it comes to a focus at a point outside of the fiber. It is an object of Meltz et al. to provide a tap for extracting light from the grating section through the side of an optical fiber.
U.S. Pat. Nos. 5,850,302 and 5,832,156 in the name of Strasser et al. disclose improvements on the invention of Meltz et al. Strasser et al., further elucidate problems with the device of Meltz et al. by stating that “the tap of the '032 patent has some shortcomings. For instance, due to the relatively large (exemplary 22°) blaze angle that is required to achieve the desired redirection of the light guided in the fiber core to light in space outside of the fiber, the arrangement is subject to undesirable polarization effects, i.e., the fraction of light that is redirected by the grating depends on the polarization of the incident guided light. Whereas for low blaze angles (<10°) the polarization dependent difference in the amount of redirected light is at most about 0.54 dB, this difference increases rapidly with increasing blaze angle, being about 2.86 dB and about 6.02 dB for blaze angles of 22° and 30°, respectively. Furthermore, as those skilled in the art will appreciate, the fraction of redirected light decreases with increasing blaze angle, for a given index change. See, for instance, T. Erdogan et al., J. of the Optical Society of America-A, Vol. 13(2), p. 296 (1996).”
Strasser et al., further state that in view of the many important potential uses of an efficient, wavelength-selective fiber tap, it would be desirable to have available a fiber tap that is substantially free of the shortcomings such as the polarization dependence of the above discussed prior art tap. The Strasser specification illustrates the advantage of using blazed gratings having angles of about 15° or less.
It is clearly the goal of both Strasser et al., and Meltz et al., to provide a tap that is substantially polarization insensitive and which allows a suitable amount of light to be tapped. Both of these requirements rely on using blazed gratings having small angles, i.e., 15° or less.
Tilted or blazed fiber Bragg gratings (FBG's) have been used to fabricate devices that couple light from guided modes into radiative modes which are highly polarization sensitive at large blaze angles [4, 5]. For an FBG tilted 45° with respect to the fiber axis, there is a difference of two orders of magnitude between radiation mode coupling for s-polarized (in the plane of the tilted index modulation) and p-polarized light. [6] Recently an all fiber polarimeter was fabricated using blazed FBG's. [7]
In contrast, it is an object of the instant invention, to provide a blazed grating in combination with an optical element exhibiting polarization sensitivity, wherein the blazed grating taps a minimal quantity of light therefrom and is substantially of the opposite polarization of the element and therefore is suitable for compensating for the polarization sensitivity of the optical element.
Along with the need to provide polarization compensation in an optical device, there is often the requirement to provide a particular birefringence in order to lessen unwanted polarization mode dispersion. In some instances it may be desired to induce a certain amount of birefringence into the optical circuit. Along with providing a novel solution for lessening polarization sensitivity by providing a blazed grating, this invention further provides a method of providing an induced birefringence thereby providing compensation for polarization mode dispersion (PMD) well as polarization dependent loss (PDL).
Further, with the present invention, PDL compensating tilted FBG's are fabricated using excimer laser exposure through a zero-nulled order phase mask. For a given phase-matching condition and grating tilt, the overall PDL is controlled through the UV-induced refractive index variation and the grating length.
SUMMARY OF THE INVENTION
In accordance with an aspect of the invention, there is provided, an optical system comprising: an optical device for passing light in a predetermined wavelength range, having substantially unequal insertion losses f
Johnson Derwyn C.
Mihailov Stephen J.
Bovernick Rodney
Freedman & Associates
Her Majesty the Queen in right of Canada as represented by the
Pak Sung
LandOfFree
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