Modem wavelength division multiplexing (WDM) systems many times have channel spacings of less than 100 Gigahertz. In fact, systems are being proposed that have spacings as narrow as 10 to 20 Gigahertz.
Optical spectrum analyzers are typically used to scan the spectrum of the WDM signal in order to confirm the proper operation of the WDM system. Because of the fine spectral detail implicit in the WDM signals, narrow bandwidth tunable filters are many times used in the optical spectrum analyzers.
High finesse, high-performance filters require high reflectivity (HR) layers made with thin film, dielectric coatings to define the filters"" optical cavities. Common configurations include microelectromechanical system (MEMS) tunable Fabry-Perot filters. Actuation mechanisms, such as thermal or electrostatic, are used to modulate the distance between these HR layers and thus, the spectral location of the filter""s passband.
The size of the optical cavity of the tunable filter must be stable. The change in size of the optical cavity of, for example, less than a nanometer, changes the location of the passband of the optical spectrum analyzer the spectral distance between channels, in some systems. When these changes occur in an uncontrolled fashion, confusion can occur in the interpretation of the data from the system. The stability is even required when a separate monitoring signal, with known spectral characteristics is used, to calibrate the MEMS tunable filter. Typically, the drift in the size of the cavity must be less than the capture range of the reference signal system.
Getters have been used in the hermetic packages of active optoelectronic systems. Water or organics in the package can increase the temperature at the semiconductor laser""s facets, increasing the risk of catastrophic optical damage (COD).
Moisture getters have also been proposed for use with MEMS devices. A common problem associated with MEMS devices is stiction, or the undesirable interfacial adhesion between a movable MEMS structure and an adjoining surface. Stiction forces are sometimes generated by surface moisture. Placing moisture getters in the hermetic package can reduce the moisture level and thus, the risk of stiction attachment in these devices. Lowering moisture can also prevent propagation of microcracks.
We have isolated another modality by which moisture in a MEMS hermetic package can lead to an unintended operation. Specifically, it surrounds the interaction between package moisture and stress levels in dielectric coatings on MEMS devices. Specifically, as the moisture content in these dielectric coatings changes, there are concomitant changes in the material stress. These changes in material stress can affect the operation of the overall MEMS device. Specifically, in the context of tunable filters, moisture can lead to a temporal drift in the size of the optical resonant cavity as changes in material stress affect the MEMS structures. Also electrostatic charging effects in devices with high bias voltages (e.g., greater than 50 Volts) sometimes occur with module moisture.
In general, according to one aspect, the invention features an optical filter system. In the illustrated embodiments, this optical filter system is an optical spectrum analyzer. The system comprises a package and an optical filter, which is installed within the package. This filter has at least two thin film, dielectric mirrors, which define an optical resonant cavity. According to the invention, a getter is added to the package to absorb moisture, and thereby stabilize the operation of the optical filter, and specifically prevent uncontrolled drift in the size of the optical cavity.
In the preferred embodiment, the package is a hermetic package. Thus, the inclusion of the getter in the package leads to a long-term stable, low moisture environment.
In the present implementation, the optical filter is a tunable filter and at least one of the mirrors is disposed on a release structure. Presently, a membrane-type release structure is being used.
In operation, the getter absorbs moisture in the package to stabilize a material stress in the thin film mirrors and possibly other thin film coatings, such as antireflective AR coatings, and thereby stabilize the dimensions of the optical resonant cavity.
Presently, the tunable filter is a high-performance tunable filter. Its mirrors comprise alternating layers of tantalum pentoxide and silicon dioxide or titanium oxide and silicon dioxide. Its AR coatings comprise layers of silicon dioxide and titanium oxide. Even when these layers are hardened in an ion-assisted deposition system, they are still susceptible to material stress changes from environmental moisture. The effect is reduced by the use of the getter according to the present invention.
In general, according to another aspect, the invention features a process for packaging an optical filter. This process comprises installing an optical filter having at least two thin-film mirrors in a package. A getter is placed in the package to absorb moisture during operation. The package is then sealed with the optical filter and getter inside.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.