Optics: measuring and testing – By dispersed light spectroscopy – Utilizing a spectrometer
Reexamination Certificate
2000-12-13
2003-12-02
Evans, F. L. (Department: 2877)
Optics: measuring and testing
By dispersed light spectroscopy
Utilizing a spectrometer
C385S014000
Reexamination Certificate
active
06657723
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to multimode planar spectrographs and methods for their fabrication, and more particularly to planar spectrographs for demultiplexing coarse wavelength division multiplexed (CWDM) optical signals, and methods for fabrication of such spectrographs employing silicon-based processing.
2. Description of the Related Art
Emerging applications of wavelength-division-multiplexing (WDM) in local-area networks require wavelength demultiplexers that are compact, low-cost, manufacturable in high volumes, and most of all compatible with multimode fiber input. Multimode operation precludes the many single-mode waveguide demultiplexers used in dense WDM systems for telecommunications purposes.
A planar spectrograph demultiplexer is a two-dimensional grating spectrometer comprised of a slab waveguide which confines the light in the vertical direction and a concave reflection grating which simultaneously diffracts and images the input light, separating different wavelengths in an output image plane.
Referring to
FIG. 1
, formation of a planar spectrograph
10
within a layered dielectric slab waveguide for single-mode fiber applications is illustratively shown. Spectrograph
10
is formed by depositing a plurality of glass layers
17
to form slab waveguide
16
having a numerical aperture and a thickness of a center core layer
17
b
closely matched to single-mode fiber. Upper and lower cladding layers
17
a
and
17
c
are also provided. In
FIG. 1
, a concave trench
12
is lithographically patterned and then etched into the slab waveguide
16
with a grating structure
14
formed on a near face. The grating facets are then metallized by access through the open trench
12
.
To enable the highest performance within the smallest possible footprint of spectrograph
10
, aberrations in the diffracted images are reduced by means of an acircular grating curve with variable grating facet pitch (i.e., chirp). Manipulating the two degrees of grating freedom, curvature and pitch, allows for two stigmatic wavelengths. Grating resolution is optimized by varying the two stigmatic wavelengths to minimize aberrations over the full wavelength range of operation. The grating theory and design procedure have been developed along the lines described in the art.
In addition, for maximum diffraction efficiency, grating facets
28
are blazed (i.e., angled) to spectrally reflect the incident light to the diffracted image of a wavelength (the blaze wavelength) central to the wavelength range of operation. A saw-toothed echelette grating facet
28
may be employed. This optimum blaze angle will vary with facet position along the curved grating.
By allowing for arbitrary lateral profiles, the lithographic patterning and etching of grating
14
in spectrograph
10
facilitate the realization of such acircular, chirped gratings having continuously variable blazing.
For local area network (LAN) WDM applications, a critical demultiplexer feature is compatibility with input from multimode fiber (MMF). In particular, this would require a thick core layer (e.g., ≧62.5 microns) for the spectrograph slab waveguide
16
. Depositing and etching glass layers
17
to form a MMF-compatible planar spectrograph in a manner similar to that for the single-mode device are no longer practically or technically feasible for glass slab waveguide layers of these thicknesses. Past efforts at realizing MMF-compatible spectrographs fabricated thick-core glass slab waveguides by stacking thin glass sheets or by performing ion-exchange. Gratings were formed on an end face of the slab waveguide by replication of a ruled master grating, by holography, or by epoxying a separately ruled or etched grating. These approaches suffer from several drawbacks and difficulties with respect to their ultimate levels of performance and their ability to be packaged.
Referring to
FIG. 2
, a multimode planar spectrograph
29
representative of the prior art is illustrated where a reflective grating
24
is affixed to a slab waveguide glass stack
22
on an end that has been ground and polished into a concave surface. Stack
22
includes cladding layers
21
and a core layer
19
. Grating
24
is pre-processed to form an echelette grating
26
before being attached to slab waveguide
22
. Fabrication of an optimized chirped, acircular grating complete with continuously variable blazing similar to that for the single-mode spectrograph is non-trivial by traditional means. Control over one of the grating design parameters, chirp or curvature, must often be sacrificed, affording the possibility for only one stigmatic wavelength versus two for the grating patterned lithographically in spectrograph
10
(FIG.
1
). Grating
24
will typically be formed through a grating ruling process or a holographic printing process. Grating ruling employs mechanical scribing with a diamond tip to create facets
28
. To chirp the grating and/or vary the blazing, it is necessary to adjust the diamond tip between scribes. Since continual adjustment between successive scribes would be laborious, time-consuming, and expensive, the grating pitch and blaze are normally changed step-wise for only a few segments of the length of a ruled grating. Holographic grating definition can provide continuous chirping for aberration correction but with only one resultant stigmatic wavelength. Furthermore, blazing of holographic grating facets is limited.
Since grating
24
, input fiber
27
, and output array
31
are attached to slab waveguide
22
in separate steps, costly low throughput active alignment of either the grating or the input/output elements is required when using a corrected grating. Relative misalignments may occur and are an additional source of concern for aberrations in the output images.
By means of the LIGA process, multimode planar spectrographs have been made in polymer material systems using deep-etch X-ray lithography with parallel synchrotron radiation. Polymers, however, are generally frowned upon for use in LAN datacom transceivers due to their uptake of moisture in non-hermetic packaging, high sensitivity to operating temperature changes, and/or inability to survive elevated temperatures during transceiver solder reflow processing.
Meanwhile, grating spectrometers constitute a platform that will scale to higher WDM channel counts and tighter channel spacings with less added cost and more consistent performance than will presently competing serial-processing multimode demultiplexers that use dielectric interference filters.
Therefore, a need exists for multimode spectrographs, and a method for fabrication thereof, which are inexpensive, easy to manufacture, environmentally rugged, and unrestricted in terms of their grating design.
SUMMARY OF THE INVENTION
The present invention provides for multimode-fiber compatible spectrographs which are fabricated employing planar, batch processing of silicon wafers. By means of silicon deep reactive-ion etching, high-quality, aberration-corrected gratings defined lithographically and capable of realizing minimum device dimensions are etched into a silicon substrate. The diffraction grating is integrally formed in the substrate so as to be in operative relationship with input light to diffract and image the wavelength components of the input light to an output detector, fiber, or integrated waveguide array. Input/output coupling and passive alignment features can be integrated directly into the silicon substrate to facilitate low-cost, high-volume packaging. The thick-core slab waveguide responsible for coupling input light to and diffracted light from the etched grating may be formed in a plurality of ways, for example, as the top silicon layer in a thick-film silicon-on-insulator (SOI) wafer or as a hybrid thin glass element dimensioned and configured to fit within a recess formed within the silicon substrate.
A planar spectrograph for demultiplexing optical wavelength signals of the present invention includes a monolithic substrate. The substrate
Cohen Mitchell S.
Sefler George A.
Speidell James L.
Evans F. L.
F. Chau&Associates, LLC
Geisel Kara
International Business Machines - Corporation
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