Patent Application
20100165352
The present invention relates to the field of integrated optics and particularly to the production of wavelength filtering devices whose essential optical characteristics do not depend on fluctuations in ambient temperature.
Integrated optical waveguide circuits combine miniaturized waveguides and optical devices into a functional optical system incorporated onto a planar substrate. These planar lightguide circuits (PLCs) can incorporate a multitude of devices many of which depend on filtering, or the ability to select and perform specific operations upon individual channels of a dense-wavelength-division-multiplexed (DWDM) optical system. Such devices, even when providing good performance at constant temperature, often deteriorate rapidly when subjected to thermal variations such as fluctuations in ambient temperature. The root cause is the sensitivity of an optical path length s=NL of guide length L to temperature T, where N is the effective refractive index of the waveguide mode in question. Optical path length variations of this kind give rise to thermally induced spectral shifts of the filter spectrum. The effect is monitored by the coefficient ds/dT which, per unit length of waveguide, takes the form
(1/L)ds/dT=dN/dT+Nα, (1)
in which α is the linear expansion coefficient along the waveguide length. In PLC geometry, α is therefore equal to the linear expansion coefficient of the substrate. The dependence of ds/dT upon α enters Eq. (1) implicitly via the thermo-optic coefficient dN/dT as well as explicitly via Nα. In conventional silica-on-silicon PLC guides the numerical value of Eq. (1) is about 1.0×10−5 per degree Celsius. For a transmission peak in a passband filter centered at about 1550 nm, this value translates to shift of about 0.8 nm (or equivalently about 100 GHz) when temperature changes between 0 and 80° C. This shift corresponds to one spacing between typical DWDM channels and is therefore completely unacceptable.
Different solutions have been proposed to remedy temperature sensitivity of a PLC-based filter. One group of solutions utilizes mechanical means of compensating for the wavelength shift in the filter response; for example see J. B. D. Soole, M. Schlax, C. Narayanan and R. Pafchek, Electronics Letters v. 39, no. 16, p. 1182 (2003). These solutions, however, are typically bulky and often not compatible with optical integration. In another approach, specially-designed compensating grooves normal to a waveguide length are filled with resin; see for example U.S. Pat. No. 6,304,687. However, this method typically suffers from excess radiation loss in the groove region. In the third group of solutions, a hybrid waveguide is manufactured for which the athermal condition
ds/dT=L[dN/dT+Nα]=0 (2)
is achieved by biasing dN/dT to negative values, and thereby compensating the thermal expansion of the substrate uniformly throughout the waveguide circuit. This is achieved by covering and encapsulating the PLC waveguide cores with overclads composed of polymer materials possessing highly negative values of dNpolymer/dT; see for example, Y. Kokubun, N. Funato and M. Takizawa, IEEE Photonics Technology Letters v. 5, p. 1297 (1993), E. Kang, W. Kim, D. Kim, and B. Bae, IEEE Photonics Technology Letters v. 16, p. 2625 (2004), or U.S. Pat. No. 6,421,472. This solution, while it offers distinct advantages over the two previous ones, is often not manufacturable or compatible with other PLC elements. This is largely due to the polymer upper cladding, which limits processing conditions in wafer manufacturing, limits design options in waveguide optimization, hinders device reliability and complicates chip attachment procedures during packaging.
The present invention discloses a method of PLC filter athermalization, which combines the advantages of the two latter approaches. In a tapered composite waveguide circuit only a small portion of a PLC circuit is made hybrid, i.e. composed of silica-based core material and another material with negative thermo-optic coefficient. The region between the regular PLC and hybrid PLC sections is adiabatically tapered, so that there is no optical loss between the two sections. The thermo-optic coefficient of the hybrid waveguide is designed in such a way as to be equal and opposite in sign to the ds/dT coefficient of the regular waveguide. Thus the thermally induced spectral shift of the filter built on this principle can be made negligibly small.
In one particular embodiment of the invention, a planar waveguide circuit is provided that includes a silica-based planar optical waveguide circuit having a lower cladding, a core and an upper cladding. At least one input waveguide and one output waveguide are each coupled to the optical waveguide circuit. At least one adiabatic tapered waveguide section is located in the waveguide circuit, which has an upper cladding segment that tapers down to at least the core to define a tapered recess. A filler material having a negative thermo-optic coefficient fills the tapered recess so that the optical waveguide circuit has an optical characteristic with a reduced temperature dependence.
a)-1(d) show cross sections of a composite planar waveguide used for filter athermalization in accordance with various embodiments of the invention.
a)-1(d) show schematic drawings of various embodiments of a composite waveguide structure constructed in accordance with the present invention. The waveguide structure includes a lower cladding 101, waveguide core 102, upper cladding 103 and filler material 104. The filler material fills the tapered region on top of the upper cladding and the waveguide core, which could either be tapered or untapered i.e. its height could be smaller or unchanged. Various configurations of the tapered region (as shown by the corresponding configuration of the filler material 104) are shown in
An optical filter is often characterized by a particular free spectrum range (FSR), which in turn is given by a characteristic waveguide length L and an effective (refractive) index N determined by the PLC design (e.g. L is the ring waveguide length in an etalon filter or the length difference between waveguide arms in interferometric filters, as for example shown in
in which c is the velocity of light. The dependence of N (and in some cases L) on temperature usually determines the temperature sensitivity of the filter. In order to compensate it, a composite waveguide structure may be introduced which, in addition to regular waveguides with modified length L+ΔL′, includes a hybrid section with a characteristic length ΔL″ and an effective index N″. An example is shown in
ΔL′N′+ΔL″N″=0 (4)
where N′ is the effective index in the modified portion of the regular waveguide structure. In general N and N′ could be different depending on the exact location of the waveguide inside the filter circuit.
In addition, the athermal condition follows from Eq. (2) as
L(dN/dT)+ΔL′(dN′/dT)+ΔL″(dN″/dT)=0. (5)
Here we have assumed a negligible contribution from the thermal expansion of the substrate (an approximation that is usually valid to better than 10% for PLCs on a silicon platform) but the equations are readily embellished to include substrate expansivity if required. There is an infinite number of solutions to Equations 4 and 5, the most obvious one being ΔL′=−L, ΔL″=L, N′≈N″≈N, dN″/dT=0. The implementation of this solution requires accurate index matching of the filler material to that of the upper cladding and waveguide mode design in the tapered region, in which dN″/dT=0. The latter can be easily accomplished by properly choosing the modal confinement factor in the upper cladding (or filler) region.
In some embodiments the athermal condition may be satisfied by using a tapered region 105 that is adiabatic. The use of an adiabatic tapered region 105 ensures that the light traveling therein is always guided throughout the waveguide structure and that any scattering losses are eliminated. The adiabatic tapered region 105 is formed by providing adiabatic transitions between the various interfaces of the device with which the region 105 is in contact. An adiabatic transition is achieved by changing the structure and/or the refractive index of the tapered region 105 in a sufficiently slow manner so the light is not scattered from its mode when it is incident on the tapered regions and continues propagating in this same mode when it exits the tapered region and enters the next portion of the waveguide. That is, the light experiences a gradual transition between the tapered region 105 and the adjacent regions of the waveguide structure such that the mode of the light does not change and no significant loss of light takes place. The provision of such modal control of the light beam is very beneficial in the design of an athermal waveguide structure because it provides more flexibility, lower losses, better control, and an ability to athermalize the device over a larger temperature range in the presence of refractive index nonlinearities that may arise in the filler material 104.
In contrast to the adiabatic tapered waveguide used in the aforementioned embodiment, conventional arrangements that compensate for the temperature dependence of the refractive index of waveguide structures typically use a filler material having an opposite temperature dependence to that of the remainder of the waveguide. In these cases the filler material causes un-guided propagation of the beam as it enters the taper. In addition, light is lost at the abrupt interface that is formed between the filler material and the adjacent portions of the waveguide, which are formed from a different material.
It should be noted that equation 4 is not a necessary condition for athermalization. A breakdown of this condition will simply shift the center wavelength of a filter, without affecting its thermal sensitivity. This shift can be taken into account in the original filter design approach. Alternatively, an appropriate trimming method may be applied to bring back the center wavelength, e.g. using UV light or thermal annealing. Also, it may be preferable to have filler material with its refractive index lower than that of a core material across an entire operating temperature range, in order to preserve guiding properties of the waveguides.
Several material groups could be used as filler materials, e.g. deuterated polysiloxane, UV cured epoxy resin, fluorinated polymers, etc. It may be desired to match the filler's refractive index closely to the refractive index of the upper cladding. Commercial polymers in this category are now available from such companies as Zen Photonics, Optical Polymer Research, and RPO optical polymer waveguides. For instance fluoracrylate polymers from Zen Photonics have already been demonstrated to achieve acceptable optical performance in D. Kim, Y. Han, J. Shin, S. Park, H. Sung, S. Lee, Y. Lee, and D. Kim OFC 2003 Proceedings, v. 61, p. 61 (2003).
Although this invention has been described in terms of circuits made from silica-based planar waveguides, the invention also encompasses planar waveguide formed from other materials such as nonsilicate glasses, amorphous materials, organic and inorganic semiconductors.
ΔL″i=ΔL*i (6)
This is a continuation-in-part of U.S. patent application Ser. No. 11/522,853, filed Sep. 18, 2006, entitled “Tapered Composite Waveguide For Athermalization”, which is incorporated by reference in its entirety herein.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11522853 | Sep 2006 | US |
| Child | 12607892 | US |
