Systems employing optical wavelength division multiplexing (WDM) and demultiplexing (WDDM) are widely employed. A bandwidth, or bundle, of different wavelength sub-ranges, defined as bands or channels, can be carried over a common optical fiber waveguide and separated into multiple channels, each carrying a predetermined wavelength range. Conversely, it is possible to reverse the process and to combine, or multiplex, two or more separate channels into a common output signal.
In a WDM system, it is often advantageous to add an extra band or remove a band to an optical signal; this approach is known in the industry as an "add or drop" respectively.
In a demultiplexing mode of operation, it is desirable to isolate a predetermined band from the remaining wavelength bundle rather than leave a portion of the respective wavelength sub-range with the bundle which may cause transmission noise later on. A well-isolated wavelength band, or sub-range can be represented graphically as an (ideally) rectangular shape on the intensity/wavelength graph. The band should ideally be separated from an adjacent, usually closely spaced, band or wavelength range and free of "fringe" wavelengths, i.e. signals extending beyond the predetermined wavelength sub-range. One of the problems occurring in WDM systems is unsatisfactory isolation of separate channels filtered out (demultiplexed) from the original bandwidth signal. In the known wavelength-selective optical filters known to date, the filtering of a predetermined band (sub-range) through an optical interference filter is inherently much less than complete. While the efficiency is quite substantial, an unfiltered part of the sub-range remains with the other bands and constitutes an undesirable noise. It is certainly desirable to improve the efficiency of removal of a specific band before the remaining signal is directed to the filter corresponding to a next wavelength sub-range or channel.
Known in the art are systems wherein an input signal is passed from a waveguide through a collimating means (e.g. GRIN lens) onto a first wavelength-selective filter, a predetermined band having a first wavelength sub-range is filtered transmitted through the filter, and the remaining signal, reflected from the first filter, is returned through the GRIN lens and another waveguide to a second filter of the same wavelength range as the first filter. While this system offers multiple filtering and is effective in removing (isolating) a substantial part of the first sub-range from the input signal, the associated transmission losses incurred by the multiple passage through lenses and waveguides are significant.
U.S. Pat. No. 4,244,045 to Nosu et al. discloses a multiplexing/demultiplexing system. The system has a plurality of optical filters each of which transmits a predetermined wavelength and reflects other wavelengths. The filters are arranged such that an optical beam is transmitted or reflected via each optical filter in sequence in a zigzag fashion. A light source or light detector is provided behind each optical filter to project or receive a collimated optical beam. Another optical means is provided to connect the multiplexer/demultiplexer with an outside optical filter, wherein the transmission wavelength of each optical filter is different from the others.
U.S. Pat. No. 4,777,064 to Dobrowolski et al. proposes an optical mixing/demixing device having a series of solid, light transmitting blocks, each having opposed, front and rear parallel planar faces coated with optical interference multilayer coatings constituting bandpass filters, and light transmitting faces arranged one on each side of the front planar face. The blocks are arranged side by side with a collimating lens on the first light transmitting face and further lenses on each of the second light transmitting faces. The multilayer interference coatings are such as to reflect light of a particular spectral sub-range and transmit the remaining wavelengths. When the device is used in the demixing (demultiplexing) mode, a light beam passes through at least one of the blocks wherein a particular spectral band of the beam will be reflected internally several times by the interference coating before exiting the block. The device can function in a multiplexing and demultiplexing mode.
U.S. Pat. No. 5,583,683 to Scobey describes a multiplexing device having an optical block with an input optical port to admit an input multiple wavelength collimated light signal, and a variable thickness interference filter forming arrayed ports along the surface of the optical block, the filter being transparent at each of the ports to a different sub-range of the input signal and reflective to other wavelengths. The input light signal is cascaded along a multipoint travel path from one to another of the arrayed multiple ports.
In analyzing the system of the U.S. Pat. No. `683` Scobey patent, it has been realized that the system employs a sequence of filters each of which corresponds to a different distinctive wavelength band. While the system keeps the signal within the optical block and is effective in reducing transmission losses due to the passage of a signal through multiple lenses and waveguides, it has an inherent drawback in that each wavelength band exiting the optical block passes through only one wavelength-specific filter. The single pass is insufficient to isolate a selected wavelength from the remaining multi-wavelength input signal with sufficiently high efficiency, and a significant amount of the band remains in the signal as a noise.
It is desirable, as explained above, to improve the efficiency of isolation of predetermined wavelength bands from the remaining signal, preferably without incurring excessive signal losses. As explained, transmission through a number of lenses and waveguides does incur such losses. On the other hand, forming a multiple filter by stacking filters of the same wavelength characteristics is likely to result in a marked loss of reliability.