Fiberoptic networks are increasingly being deployed to provide high bandwidth transmission of voice, data, video, and internet services. Optical signals are sent through the fiber and switched either optically or electrically at switching nodes.
Wavelength division multiplexing (“WDM”) is an optical technique that enables multiple channels of different wavelengths to be carried in the same fiber, thus increasing the bandwidth per fiber to the sum of the bit rates of each wavelength. The WDM technique uses different wavelengths of light transported down the same fiber to carry different channels of information. DWDM systems multiplex a large number of wavelengths—such as more than 40 wavelengths—in each fiber, thus further increasing single fiber bandwidth. The direction of technology is to increase the number of wavelengths carried by DWDM systems, which decreases the channel spacing between wavelengths or channels. For example, certain prior art DWDM systems have a channel spacing of 0.2 nanometers (“nm”).
A Fabry-Perot optical filter comprises two high reflectance mirrors, such as dielectric multilayers, separated by a space layer. There is multiple interference in the space layer of the filter, which causes the output spectral characteristic of the filter to peak sharply over a narrow band of wavelengths.
FIG. 1 shows a typical transmittance spectrum of a tunable Fabry-Perot optical filter at a single voltage. The transmittance spectrum shown in FIG. 1 is relatively narrow, which shows why Fabry-Perot optical filters are useful as band-pass filters. For the example of FIG. 1, the Fabry-Perot filter has a 3-dB bandwidth of 0.1 nm.
The Fabry-Perot optical filter is tunable given the presence of a piezoelectric transducer in the space layer of the filter. The piezoelectric transducer of the tunable filter expands when an increasing voltage is applied to the transducer. The expanding piezoelectric transducer in turn expands the thickness of the space layer. Changing the thickness of the space layer in turn changes the transmittance spectrum of the filter. In particular, the peak of the transmittance spectrum moves towards higher wavelengths as the voltage applied to the piezoelectric transducer increases. FIG. 2 shows two transmittance spectra for the same tunable Fabry-Perot optical filter, but with two different direct current (“DC”) voltages applied to the piezoelectric transducer of the Fabry-Perot filter.
An example of one prior use for a tunable Fabry-Perot optical filter is in a prior art optical channel analyzer for a DWDM system. The optical channel analyzer measures wavelength, optical power, and optical signal-to-noise ratio of an optical channel in order to monitor the performance of the optical channels in the DWDM system.
One disadvantage of a typical prior art tunable Fabry-Perot optical filter is that it does not have an ideal transmittance spectrum. The typical tunable Fabry-Perot optical filter has a transmittance spectrum like the one shown in FIG. 1, with a wide skirt towards the bottom of the transmittance spectrum, which limits the resolving power of the typical Fabry-Perot optical filter.
To help to overcome this problem, a very narrow tunable Fabry-Perot optical filter could be used to extract wavelength channels from the DWDM signals. Such an approach is disadvantageous, however, because expensive high-technology is needed to create such narrow bandwidth Fabry-Perot filters. Moreover, with such narrow bandwidth Fabry-Perot filters, measurement sensitivity is reduced because the light intensity typically weakens upon passing through a narrow bandwidth Fabry-Perot filter.