Optical: systems and elements – Single channel simultaneously to or from plural channels
Reexamination Certificate
1999-03-19
2001-04-10
Ben, Loha (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
C359S588000, C359S589000, C359S260000, C359S199200, C359S885000, C385S027000, C385S047000, C372S010000
Reexamination Certificate
active
06215592
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to optical filters, and in particular to Fabry-Perot optical filters for channel selection in Wavelength Division Multiplexed systems.
BACKGROUND OF THE INVENTION
Wavelength Division Multiplexing (WDM) techniques in optical fiber systems have been utilized to significantly enhance the data carrying capacity of optical fibers. Essentially, in a WDM system multiple information streams are simultaneously transmitted on a single optical fiber at different wavelengths or channels. Early WDM systems transmitted up to four distinct channels over a single fiber. Recent technological advances are, however, allowing ever-increasing numbers of channels to be transmitted over a single fiber. Generally, systems that transmit in excess of four channels are referred to as Dense Wavelength Division Multiplexed (DWDM) systems in recognition of the closer spacing between the respective channels.
In a typical optical WDM or DWDM communication system, the distinct optical wavelengths or channels are multiplexed and propagated over an optical medium to a plurality of receivers. To ensure interoperability with other system equipment, the channels or wavelengths chosen for transmission, as well as the channel spacings, are selected to correspond to an International Telecommunication Union (ITU) channel grid. According to one such ITU channel grid, the channel spacing is 100 GHz with, for example, channel 15 at 191,500.00 GHz and channel 72 at 197,200.00 GHz.
One or more of the propagated ITU channels are selected for detection within the receiver by interposing appropriate filters between the medium and each receiver. For example, optical signals from each of N different optical signal generators with ITU channel wavelengths of &lgr;
1
, &lgr;
2
, . . . , &lgr;
N
, respectively, are multiplexed and propagated over a system fiber connecting the various receivers. A given filter may pass only one of the ITU channel wavelengths, e.g., &lgr;
i
, from the multiplexed wavelengths present on the fiber through to the associated receiver, while the other wavelengths are reflected.
Obviously, the ability of the filter to effectively pass the desired channel(s) or wavelength(s) is critical to the operation of the overall system. Another important aspect of the filter is its effect on the system loss budget, i.e., the total amount of optical loss that a given optical link can tolerate while maintaining signal integrity. One type of filter which has been successfully employed in Wide Area Networks (WANs) is a diffraction grating. Diffraction gratings generally offer appropriate spectral resolution for reliably passing a plurality of selected channels. Unfortunately, however, diffraction gratings are bulky, lossy, and expensive. The expense of diffraction gratings and their effect on system loss budget, makes diffraction gratings impractical where cost considerations are important, e.g., for in-line, short transmission length applications such as in Local Area Networks (LANs).
A more cost-effective approach to filtering is to use a Fabry-Perot filter. Generally, a Fabry-Perot filter includes at least one pair of reflective elements, e.g. mirrors, separated by a fixed distance. By adjusting the distance between the reflective elements, the filter can be tuned to filter a selected channel. Advantageously, Fabry-Perot filters are less expensive and generate less optical loss than diffraction gratings.
One disadvantage associated with conventional Fabry-Perot filters is that they provide a very narrow resonant frequency passband, i.e. on the order of about 1-2% of the filter free spectral range (FSR). The narrow passband requires precise tuning of the filter to the signal transmitting and receiving elements, resulting in increased equipment costs. Moreover, where the filter is to be applied for filtering a plurality of spaced channels, i.e., as a comb filter, it is necessary to manufacture the filter with highly precise dimensions to ensure that the filter resonance frequencies match the desired transmission characteristics within the narrow passband. Accordingly, where Fabry-Perot filters have been used to filter a plurality of spaced channels, a separate filter has been used to separate each desired channel from the WDM signal. Using multiple Fabry-Perot filters having narrow frequency passbands is inefficient and expensive.
Accordingly, there is a need in the art for a Fabry-Perot filter which has an increased frequency passband compared to prior art designs, and which is capable of transmitting a first set of wavelengths from an input signal composed of a plurality of multiplexed optical signals and reflecting a corresponding second set of wavelengths. There is also a need in the art for a Fabry-Perot filter which may be efficiently and cost-effectively produced, and which may be used for transmitting a first set of wavelengths and reflecting a second set of wavelengths from an input composed of a plurality of multiplexed optical signals.
SUMMARY OF INVENTION
The present invention is organized about the concept of providing a Fabry-Perot optical filter for separating an optical signal including a plurality of multiplexed ITU channels into a first set of transmitted odd or even ITU channels a second set of corresponding reflected even or odd ITU channels. An exemplary three-cavity filter according to the invention includes a pair of inner mirrors separated substantially by an inner spacer, a first outer mirror separated from a first one of the inner mirrors substantially by a first outer spacer, and a second outer mirror separated from a second one of the first inner mirrors substantially by a second outer spacer. The mirrors are formed by depositing alternating layers of high and low index of refraction materials to achieve desired reflectivities. Generally, the reflectivity of the inner mirrors should be higher than the reflectivity of the outer mirrors. For example, advantageous filter characteristics may be obtained by providing inner mirrors with reflectivity of from about 70-80%, and providing the outer mirrors with a reflectivity of from about 20-30%.
The spacers may be, for example, solid or hollow, or formed by protrusions on the mirrors. The spacer optical thicknesses are adapted to cause transmission of the first set of ITU channels and reflection of the second set of ITU channels. Advantageously, a three-cavity filter according to the invention displays transmission characteristics having a broader passband at the resonant frequencies than a single-cavity Fabry-Perot filter. The broad passband transmission characteristics obviates the need for highly precise signal transmitting and receiving components in an optical communication system incorporating the filter. The overall system cost is, therefore, reduced.
In a method of making a filter according to the invention having at least two mirrors and at least one spacer, the spacer thickness is calculated to obtain an appropriate FSR for transmitting the first set of wavelenghts. A wafer of spacer material is formed to the calculated spacer thickness, and an optical beam is transmitted through the wafer while monitoring the resonant frequencies of the wafer. The wafer thickness is adjusted until the observed wafer resonant frequencies align with the first set of ITU channels. The filter is then built using sections of the wafer as the filter spacer(s).
In an exemplary method of making a three-cavity filter according to the invention, a wafer of spacer material is formed to a spacer thickness which will provide a filter free spectral range in alignment with the first set of ITU channels. At least one dielectric layer is deposited on the top surface of the wafer, and at least one dielectric layer is deposited on the bottom surface of the wafer. The dielectric layers form portions of the filter mirrors.
At least one protrusion is deposited on the top dielectric layer on the top surface of the wafer in area defining a first outer segment of the filter, a portion top dielectric layer defining an
Ben Loha
Ciena Corporation
Daisak Daniel N.
Soltz David L.
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