Optical waveguides – Planar optical waveguide – Thin film optical waveguide
Reexamination Certificate
1999-11-10
2001-10-30
Ullah, Akm E. (Department: 2874)
Optical waveguides
Planar optical waveguide
Thin film optical waveguide
Reexamination Certificate
active
06311004
ABSTRACT:
TECHNICAL FIELD
The invention provides integrated photonic devices for use in fiberoptic communications networks as well as other applications that require optical data signals either to be transported from one location to the next or modified prior to their use by other photonic devices.
BACKGROUND AND SUMMARY OF THE INVENTION
Photonic devices for optical network management and wavelength multiplexing and demultiplexing applications have been extensively researched for a number of years. A significant class of such devices is commonly called “planar lightwave circuits” or “planar lightwave chips” or just PLC's. PLC's comprise technologies wherein complex optical components and networks are disposed monolithically within a stack or stacks of optical thin films supported by a common mechanical substrate such as a semiconductor or glass wafer. PLC's are designed to provide specific transport or routing functions for use within fiberoptic communications networks. These networks are distributed over a multitude of geographically dispersed terminations and commonly include transport between terminations via single-mode optical fiber. For a device in such a network to provide transparent management of the optical signals it must maintain the single-mode nature of the optical signal. As such, the PLC's are commonly, though not strictly, based on configurations of single-mode waveguides. Since optical signals do not require return paths, these waveguide configurations do not typically conform to the classic definition of “circuits”, but due to their physical and functional resemblance to electronic circuits, the waveguide systems are also often referred to as circuits.
A key performance issue in the practical application of PLC's is the efficiency of the circuit in transporting the optical energy of the signal. This performance is characterized in terms of the fraction of energy lost from the signal passing through the device, expressed as “loss” in units of decibels (dB) or “loss rate” in units of dB/cm. The standard family of materials for PLC waveguides, widely demonstrated to have superior loss characteristics, is based on silicon dioxide (SiO
2
), commonly called silica. The silica stack includes layers that may be pure silica as well as layers that may be doped with other elements such as Boron, Phosphorous, Germanium, or other elements or materials. The doping is done to control index-of-refraction and other necessary physical properties of the layers. Silica, including doped silica, as well as a few less commonly used oxides of other elements, are commonly also referred to collectively as just “oxides”. Furthermore, although technically the term “glass” refers to a state of matter that can be achieved by a broad spectrum of materials, it is common for “glass” to be taken to mean a clear, non crystalline material, typically SiO
2
based. It is therefore also common to hear of oxide PLC's being referred to as “glass” waveguides. Subsequently, the moniker “silica” is used to refer to those silicon oxide materials suitable for making PLC waveguides or other integrated photonic devices.
One of the promising features of PLC's is the ability to integrate transport and interconnect functions with dynamically selectable functions such as routing, switching, attenuation, and programmable filtering. These dynamic functions typically are used to provide integrated, solid-state replacements for functions that would otherwise need to be performed by discrete components using mechanical-displacement-based devices. These functions are achieved based on the phenomena that certain stimuli applied to the active area of a waveguide device will change the magnitude of the refractive indices in and around the waveguides in that area. Typically-used stimuli for changing the refractive index are electric field (electro-optic), heat (thermo-optic), or dynamic stress (acousto-optic). Less commonly, other effects such as piezo-optic, static-stress, photo-refractive, etc., are employed. in waveguide applications In the current state-of-the-art, thermo-optics is being accepted for the broadest range of applications and can provide a more predictable response to the randomly varying polarization of the optical signal presented by the telecommunications network. As such, discussions in this application will focus on thermo-optics, where a heat source in the vicinity of the active region of the waveguide device is used to change the temperature and thus select an index change and effect the operation of the device. The optical behavior of these devices is simply determined by the refractive-index distributions generated. It should be recognized that it would be readily apparent to those skilled and experienced in these technologies that the devices and structures described in this teaching can be applied in substance to electro-optic and other methods of stimulating the appropriate refractive-index profiles.
As previously stated, silica waveguides have superior loss characteristics for transporting the optical signals used in these communications networks. Their performance is also very stable with respect to reasonable changes of their local environment. This behavior is desirable for transport and interconnect functions, but it leads to difficulties in achieving any of the stimulated functions. There are a few thermo-optic devices for these applications currently being made from silica waveguides. Since the index changes that can be commanded are on the order of only 10
−4
per micron and can not be localized within regions a few microns wide, thermo-optic silica devices exhibit marginally acceptable performance while requiring uncommonly sophisticated control of the driving conditions.
Other classes of materials, notably optical polymers, have superior response for stimulated functions. They however can not match the low-loss transport qualities of good silica at relevant optical wavelengths. There also are various thermo-optic devices and circuits being made with optical-polymer waveguides and they achieve reasonable and robust levels of performance for their active functions, but the overall device loss is fairly high, typically a few dB for simple functions like a 2×2 switch. Currently, the main impediment to wide-spread deployment of such solid-state switches in fiber-optic telecommunications networks is a lack of availability of robust switches having less than 1 dB of device loss.
When one wishes to make a thermo-optic waveguide switch, it would be expected that the optical signals should be appropriately coupled with high efficiency between the input and output ports in response to the driven stimulus, and that this performance should not be significantly altered by any undriven stimulus. That is, for a thermo-optic switch one would want to simultaneously achieve: (1) the heating signal applied or removed by direction of the user should robustly establish the switching; and (2) any heat applied or removed by conditions arising from the environment should not effect the switching. To exhibit this behavior the device must be shielded from environmental changes and/or the switch must be designed to be sensitive to an applied heating that cannot occur naturally and insensitive to thermal patterns that can occur naturally. The former approach is occasionally used when there is no other way to achieve superior performance, but the latter approach is invariably preferable when available, since stabilization techniques are more expensive to realize and support and typically reduce the reliability of the overall device.
There is a class of devices, commonly referred to as “digital optical switches” that exhibit some very desirable characteristics in regard to the conditions cited above. Firstly, the response of these devices saturates as the driving temperature is increased, so a device may be “over driven” to isolate the response from any changes not arising from the drive signal. Secondly, these devices are typically configured to respond only to a strong th
Burke James
Kenney John T.
Love John
Midgley John
Morozov Valentine N.
Lightwave Microsystems
Morrison & Foerster / LLP
Ullah Akm E.
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