Patent Application
20130279006
1. Field of the Invention
This invention relates to the general field of telecommunications and, in particular, to multi-layered thin-film optical filters used in dense wavelength division multiplexing (DWDM) for telecommunications.
2. Description of the Prior Art
In optical communications, one fiber can carry many communication channels where each channel has its own carrier frequency. The light of different frequencies is merged into the fiber through a device called multiplexer (“mux”) in the art and is later separated into different ports through a device called de-multiplexer (“de-mux”). Mux and de-mux devices typically utilize technologies such as thin-film filters to isolate the wavelengths of interest; in telecommunications these are the frequencies set by the International Communication Union, the ITU grid.
A commonly used optical filter is based on the structure of the so-called Fabry-Pérot etalon, which is typically made of a transparent spacer with two reflective surfaces. The pacer defines the cavity of the etalon. For telecommunication applications, the spacer is a thin layer of dielectric material with a half-wave optical thickness tuned to the wavelength of the transmission peak of interest and the reflective surfaces are quarter-wave stacks with a broadband reflectance peaking at the design wavelength. The quarter-wave stacks and the dielectric spacer between are fabricated in successive continuous deposition steps and two or more such filters can be deposited on top of each other separated by so-called absentee layers to form multiple-cavity filters.
As illustrated in
In theory, so long as the optical thickness and phase of each half-wave stack spacer is the same, the transmission wavelength of each cavity will be the same. However, because the wavelength of the transmission peak of each etalon structure is very sensitive to minor differences in the structure of the spacer and the reflective surfaces, the passband peaks of stacked etalons are not always aligned and the resulting dichroic filter is often not suitable for telecommunication applications. Currently, the spacer of Fabry-Perot interferometers used in dense wavelength division multiplexing (DWDM) filters is only a few microns thick, but for some applications the spacer needs to be much greater. The traditional deposition method for making such thicker-spacer filters does not work because of the resulting structural non-uniformities and the attendant differences in transmittance. Thus, there continues to be a need for an economical and practical method for making such stackable filters and the present invention provides and alternative filter structure and method of fabrication that overcome these problems.
The invention lies in the idea of using a plane-parallel optical window, instead of dielectric material deposited with the reflective coatings, as the spacer of each filter in the stack used for DWDM applications. In the simplest embodiment, the optical window has a half-wave thickness and the highly reflective quarter-wave stacks are deposited on each of its polished sides. The optical window has a much larger area than that of a single filter, thereby obtaining a large coated optical window from which many single-cavity filters can be produced. Because the resulting half-wave spacer has the same thickness over the entire window area and the quarter-wave-stack deposition process is carried out throughout under the same conditions, each single-cavity manufactured from the optical window has the same transmission wavelength within the range of interest and is therefore readily stackable for DWDM applications.
In another embodiment of the invention, the optical window is selected with a thickness equal to one half of that required for a half-wave plate and is coated with a quarter-wave stack reflector on a single side. That further diminishes the opportunity for non-homogeneities in the structure of the coated plate because of the single deposition step instead of the two steps required to coat both sides of the optical window. The window is then divided in multiple identical components that can be combined in pairs by placing them in optical contact so as to form individual single-cavity filters with a resulting half-wave spacer and the same transmission wavelength. These filters are advantageously similarly stackable for DWDM applications.
In either embodiment of the invention, a material that is optically sensitive to thermal variations is preferably used to fabricate the stack, so that the transmission peak can be maintained at a desired set-point by a temperature control system coupled to the filter. A heater, an energy source for the heater, a thermistor to measure the filter temperature, and a processor/controller can be used in conventional feedback-control manner to tune and maintain the filter's transmission peak at the desired wavelength.
Various other advantages will become clear from the description of the invention in the specification that follows and from the novel features particularly pointed out in the appended claims. Therefore, to the accomplishment of the objectives described above, this invention consists of the features hereinafter illustrated in the drawings, fully described in the detailed description of the preferred embodiments, and particularly pointed out in the claims. However, such drawings and descriptions disclose only some of the various ways in which the invention may be practiced.
Referring to the figures, wherein like parts are designated throughout with like numerals and symbols,
A comparable result can be obtained by coating a single side of an optical window 30 to produce an intermediate structure 32 for a half-wave filter etalon. Two such structures 32 can be bonded together along their uncoated surface 34 and then diced as needed to manufacture a filter of a given smaller size, as shown in
When the spacer of a Fabry-Perot interferometer is relatively thick (e.g., 50 um), it becomes impractical and uneconomical to manufacture it by deposition because of the length of deposition time and the degraded quality of very thick deposited films. The insertion loss becomes much greater than that of bulk material of the same thickness. For example, as illustrated in the tables below for a free spectral range (FSR)=400 GHz, the spacer thickness for silicon is 107 μm and or BK7 glass is 250 μm. Therefore, using an optical window as the spacer has tremendous advantages. No lengthy deposition process is needed and the spacer will automatically have the same uniform properties throughout as the bulk material of the window.
BK7 Glass and Silicon Spacer Thickness as a Function of FSR
The shift in wavelength as a function of changes dL in the thickness of the spacer of a Fabry-Perot etalon is given by the relation
Shift=wavelength×dL/L,
where L is the thickness of the spacer. The free spectral range FRS=c/(2nL). Therefore, the tables above show that the thickness variation (dL) of the spacer needs to be kept at a minimum, in the order of few nanometers, in order to have a small enough wavelength shift to allow stacking of filters for DWDM applications. This can be easily achieved using a plane-parallel optical window as the etalon spacer, but not so by deposition of the spacer. Therefore, multiple filters obtained from the same window can be stacked successfully for DWDM applications so long as the parallelism of the window is well controlled. On the other hand, it is extremely difficult to match two windows produced separately within such a tight thickness tolerance.
Moreover, to achieve a two-cavity filter, two single-cavity filters can simply be bonded using a fusion bonding or other conventional process, for example. In practice, two large portions of a cavity 20 composed of a coated window can be bonded together, as illustrated in
The deposition of the absentee layer is preferably carried out over the reflector layer or layers deposited over the window. In one embodiment, illustrated in
In another embodiment, illustrated in
The use of plane-parallel optical windows affords another valuable advantage over the traditional method of filter manufacture. Inasmuch as the filter consists of solid etalon cavities made of a spacer material with a refractive index that is temperature sensitive, such as silicon, this property can be exploited advantageously to provide fine filter tuning. It is known that the wavelength of the transmission peak is related to temperature changes by the following relationship,
Δλ=λ[(dn/dT+n×α)/n]ΔT,
where λ is wavelength, n is refractive index, T is temperature, dn/dT is thermal coefficient of refractive index, and α is the thermal expansion coefficient. For example, at λ=1,550 nm, for silicon with n=3.5, α=2.6×10−6/° C., and dn/dT=190×10−6/° C., the wavelength shift Δλ/ΔT is ˜88 pm/° C. or ˜11 GHz/° C. This means that for a silicon cavity with a FSR of 3.6 nm, a temperature change of about 41° C. would cause a transmission peak shift of a full FSR. Such thermal sensitivity makes it possible to thermally tune the transmission peak of the stackable filter.
As illustrated in
Thus, a simple approach has been disclosed to enable the fabrication of single-cavity filters having substantially identical optical properties suitable for stacking to produce multi-cavity filters ideal for DWDM applications. While the invention has been shown and described in what are believed to be the most practical and preferred embodiments, it is recognized that departures can be made therefrom within the scope of the invention. For example, the thermal control has been described above using temperature as the control parameter because of the direct and predictable relationship between material temperature and transmissivity. However, the optical thermal property of the material can be controlled in equivalent manner by monitoring the transmission peak wavelength and using that parameter for feedback-control purposes. Therefore, the invention is not to be limited to the details disclosed herein, but is to be accorded the full scope of the claims so as to embrace any and all equivalent apparatus and methods.
This application is a continuation-in-part application of U.S. Ser. No. 13/660,909, filed on Oct. 25, 2012, which is based on and claims the priority of U.S. Provisional Application Ser. No. 61/551,428, filed Oct. 25, 2011.
| Number | Date | Country | |
|---|---|---|---|
| 61551428 | Oct 2011 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13660909 | Oct 2012 | US |
| Child | 13924598 | US |
