Optical waveguides – Planar optical waveguide
Reexamination Certificate
2001-01-02
2003-09-30
Ullah, Akm Enayet (Department: 2874)
Optical waveguides
Planar optical waveguide
C219S121600
Reexamination Certificate
active
06628877
ABSTRACT:
BACKGROUND OF THE INVENTION
The insatiable drive for bandwidth in telecommunication systems is forcing component manufacturers to increase channel counts and decrease channel spacing while maintaining uniformity in both loss and polarization sensitivity across many wavelength channels. Furthermore, wavelength selective devices, used for Dense Wavelength Division Multiplexing (DWDM)/Demultiplexing and channel Add/Drop Multiplexing (ADM) to name a few, must maintain strict adherence to an International Telecommunications Union channel wavelength standard, the so-called ITU grid. Increasing channel capacity under rigid industry standards (e.g. Telecordia certification) has greatly compromised manufacturing yields of some of the industries most promising DWDM technology, for example arrayed waveguide gratings (AWG) and Interleavers. These devices operate on the basis of optical interference and consequently even minor changes in their optical properties can degrade performance—or destroy functionality.
In terms of scaleable channel capacity, functional integration, and uniformity, these (so-called) planar waveguide structures offer greater potential for DWDM than devices constructed using thin film filters or fiber (waveguide) Bragg gratings structures. Though their potential has been demonstrated in DWDM test beds, manufacturing yields are marginal at best, which make them very expensive. These devices are lithographically fabricated chips of silicon on isolator (SOI) or silica on silicon (SOS) planar waveguide structures. They separate or combine different channels based on interference of signals after propagating down two or more waveguides with a well-defined difference is length. The wavelength or channel position and the adjacent channel separation, as reflected in the devices transfer function, is a sensitive measure of the differential phase acquired by the signal propagating in the different waveguides. Such length or phase difference can also be induced by a difference in the index of refraction between waveguides—making the transfer function temperature dependent. Since these devices must function reliably in various field conditions, temperature control is needed to stabilize the environment and not relax manufacturing tolerances. Therefore the need exists for the ability to change the index of refraction and/or the index profile on these chips in a spatially controlled manner during a post processing, quality control stage of the manufacturing process.
Besides phase errors corrupting the transmission response in these interference-based devices, polarization dependent loss (PDL), and/or polarization mode dispersion (PMD), and/or unbalanced channel loss can be the reason behind failure of planar waveguide structures to pass quality control tests. For example, at this stage in device integration, a common component in optical waveguide circuits is a variable optical attenuator (VOA). VOA's either follow a device with a multi-port output (e.g. AWG DeMux) in the optical path or precede a multi-port input device (e.g. Mux). Uniformity over channels in a DWDM or ADM device may be caused by non-uniform gain over the number of transmission channels or channel dependent loss. The latter case most often results from the inability to control losses during fabrication. VOA's are included in these circuits to actively ensure channel uniformity by adjusting signal strength in each channel.
The capability of ultra-short laser pulses to direct-write arbitrary three-dimensional refractive index patterns in transparent materials is very desirable for index trimming of SOI, SOS, or other glass (or polycrystalline) (planar) waveguide structures. Patterned index trimming offers a procedure for correcting fabrication defects that result in phase errors, PDL, PMD, unbalanced loss, and degradations in performance related to how light propagates through these devices. Beneficially, this technique does not require special environmental conditions like clean room facilities or special sample preparation. It is therefore ideal for optimizing performance in a post-fabrication or quality control step designed to optimize device performance and/or correct defect(s) that arise during the manufacturing process.
BRIEF SUMMARY OF THE INVENTION
This invention makes use of ultrafast pulse beams of light to direct-write three-dimensional index profiles in materials using the unique material changing capabilities of ultra-short (i.e. <10 picosecond) laser pulses. The invention for writing waveguides was reported in a previous invention disclosure (ID# 468049, submitted to USPTO Feb. 3, 2000). In this invention, an existing waveguide or waveguide circuit fabricated by some technique (for example but not limited to photolithography, flame hydrolysis deposition, modified chemical vapor deposition, or ultra-fast laser pulse direct writing) is modified by altering the index of refraction in a localized region or different local regions of the waveguide structure. Hereafter the local change of the refractive index (either by shaping the index profile through which light passes or by altering how the index-of-refraction varies as a function of position within the structure) will be referred to as index trimming. A localized region of the waveguide structure may constitute any region from a portion of the cross section of the waveguide to the entire waveguide structure itself. The waveguide structure consists of but is not limited to the core and surrounding cladding regions anywhere within the boundary of the structure itself.
Index trimming is accomplished through the action of a focused laser beam (or multiple focused beams) consisting of one or more ultra-short laser pulses and is generally performed at a wavelength in which the material is transparent or weakly absorbing, to the fundamental wavelength of the beam of light. The trimmed index pattern is generated by, but not limited to, moving the focal position of the beam or by moving the sample (i.e. waveguide device) relative to a fixed beam focused. Trimming occurs only at or near the focus of the beam. The focus may be a beam waist or a reduced replica of the input beam as might be created by a simple lens or collection of lenses. Or the focus could be where a pattern encoded onto the phase front of the beam is imaged onto or into the sample as, for example, by use of a mask or diffractive optical element (DOE). Trimming is intended to include any and all of these options, configurations, and derivative modes of altering the optical properties of a planar waveguide structure.
An example of index trimming of an optical waveguide contained within a planar waveguide structure will be illustrated. The illustration is by no means intended to exhaust the application of index trimming of general planar waveguide structures but rather to illustrate the idea. Those skilled in the art will recognize variations in both method and performance of a device modified with this invention—all of which are intended to be included in the claims. The data obtained from the direct writing of linear waveguides in bulk silica glass using a transverse writing configuration gives rise to an elliptically shape waveguide. This is because the intensity profile in the confocal region is an ellipse with minor and major axis scaled by the beam waist and Rayleigh range. When such a beam is focused inside a waveguide of dimensions larger than the confocal region and scanned along the axis of the waveguide a non-isotropic index change is induced which gives rise to a birefringence. The waveguide is then polarization sensitive. To reduce the birefringence and make the index change more uniform over the waveguides cross-section, the writing beam may be displaced side-to-side and scanned along the axis of the waveguide. Or, alternatively, we might shape the beam profile at the focus in order to shape the waveguide. By choosing sub-waveguide focusing parameters a prescribed polarization sensitivity can be written into or effectively erased from a waveguide structure while in
Dugan Mark A.
Maynard Robert L.
Said Ali A.
Clark-MXR, Inc.
Harter Secrest & Emery LLP
Salai Stephen B.
Suchy Donna P.
Ullah Akm Enayet
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