Optical add/drop multiplexers (OADMs) is typically considered to be a generic term for devices that have the functionality of selecting a subset of optical channels from an incoming optical fiber, or other optical transport medium, and dropping the light carrying these channels into a second optical fiber, and adding a subset of channels from a third optical fiber to an output fiber carrying the undropped optical channels from the incoming fiber. In other words, OADMs are used for adding, dropping and exchanging optical channels in wavelength division multiplexing (WDM) systems. In WDM networks, each optical channel is designated a different wavelength, referred to as a wavelength channel. The introduction of OADMs in optical networks enables new and more efficient network architectures and provides for protection switching and dynamic provisioning.
In modern OADMs for use in optical communication networks, wavelength selective optical coupling between optical waveguides, particularly between optical fibers, is a key functionality.
In addition, wavelength selective optical couplers have proven to be useful outside the field of telecommunications, for example in applications such as spectroscopy, metrology and sensor interrogation systems.
A subclass of OADMs of particular interest is the reconfigurable OADM (ROADM). In ROADMs, there is a functionality that enables tuning or selection of the wavelength channel(s) to be dropped or added. The use of ROADMs provide operators of WDM networks greater network dynamics and the possibility of optimizing and adapting the network to the current traffic situation.
Despite the apparent advantages of ROADMs in network design, these devices have not yet become widely used. One main reason may be the cost currently associated with the construction and implementation of such devices, but there are also other issues. A good ROADM should exhibit low loss, particularly for those channels that are not dropped, the so-called express channels. It should also be able to operate over the entire relevant wavelength band, such as the C-band; have low channel cross-talk; be of a sufficiently small size; have low energy consumption, etc.
One additional criterion for ROADMs which may be important from a network operability point of view is that the ROADM should be “hitless”. The feature of being hitless means that when changing the channel(s) to be dropped or added, all traffic carrying wavelength channels not involved in this change are left undistorted. Most often, this is considered from a power distortion point of view, but phase distortion, which may cause anomalous dispersion, can be equally detrimental.
The prior art has proposed a plethora of implementations that accomplish ROADM functionality.
The possibly most straightforward implementation proposed is to use a combination of two or more tunable three-port filters. A three-port filter (in the drop configuration) selects one or several wavelengths from a broadband, multi-wavelength signal entering at port 1 of the filter and passes the or each selected wavelength to filter port 3. Any remaining wavelengths from the broadband signal entering at port 1 leave the filter at port 2. In the add configuration, the same type of filter may be used, but the channels to be added are then fed into port 3, the express channels are fed into port 2, and the combined, multiplexed wavelength channels (expressed and added) leave the filter at port 1.
A three-port filter may be implemented, for example, using thin film interference filters (TTFs) together with fiber connected mirco-optics. A TTF is typically designed to reflect all wavelengths except a narrow wavelength range, which may be matched to the channel width of the wavelength channels at interest. By using a geometry wherein the angle of incidence of the beams hitting the TTF in not normal to the major plane of the interference filter, it is possible to separate the incoming light from the reflected light and thereby attain a three-port design. To make these three-port filters tunable, some mechanical means may be used for rotating the interference filter and thereby adjust the pass wavelength. However, TTF-based ROADMs are associated with several drawbacks. For example, the loss suffered by the express channels is relatively high and the tuning mechanisms are sensitive to vibrations. It has also proven to be very complicated and costly to provide for hitless tuning in TTF-based ROADMs.
Another implementation of a three-port filter is based on the use of fiber Bragg gratings (FBGs) in combination with optical circulators. Whereas a TTF-based filter is a band pass filter, the FBG-based filter is a band stop filter. To accomplish a wavelength drop, the incoming broadband signal is connected to port 1 of an optical circulator, and port 2 thereof is connected to one or several FBGs. The FBGs reflect wavelengths for the channels to be dropped, which return to port 2 of the circulator and leave the circulator at port 3. The express channels pass through the FBG(s). The add operation is accomplished by appending a second optical circulator after the FBGs. By adding wavelength channels at port 3 of this second circulator, they will be reflected by the FBGs and combine with the express channels passing through the same FBGs, and eventually leave the second circulator at port 2 thereof. In order to make the FBG-based filters tunable, use is typically made of a stretching or compressing mechanical force leading to a change of the stop band wavelengths. One major drawback of FBG-based three-port filters is that the optical circulators introduce significant loss. The circulators are also associated with a high cost. It has also proven to be problematic to make the FBGs tunable over the entire wavelength range of interest, and to make the filters hitless during tuning.
Another, quite different, approach uses a combination of array waveguide gratings (AWGs) together with 2×2 switches. A first AWG demultiplexes the incoming broadband optical signal into separate, wavelength specific waveguides. Each wavelength channel is then fed into an input port of a 2×2 switch that determines whether the wavelength channel is to be dropped or not if a wavelength channel is dropped, new information at the same wavelength may be added at the other input port of the 2×2 switch. Finally, a second AWG operates to recombine all wavelength channels, express and added, into an output fiber. One major drawback associated with this approach is that the demultiplexing/multiplexing operations introduce significant loss. Moreover, the required components are typically costly.
Yet another approach proposed in the prior art is the so-called broadcast-and-select method. In one version, a coupler is used for splitting the incoming broadband signal into two waveguides carrying the same information. One of the two signals is fed through a band pass filter, transmitting one or more channels to be dropped. The light in the second waveguide is fed through one or more wavelength blockers that block the same wavelengths that are dropped in the other waveguide. The add operation is then performed in a similar manner using a combination of filters and couplers. In order for this approach to provide a ROADM, the filters need to be tunable at least half a channel spacing. The loss suffered in a device according to this approach is significant, and typically amplifiers need to be introduced. The large number of components involved also make this an expensive and bulky approach.
In EP 1 535 096, there is disclosed a wavelength selective optical coupler based on resonant coupling of light from a waveguide, typically an optical fiber. In that device, portions of a first and a second optical fiber are provided with deflecting means, typically a tilted FBG. The deflecting means directs some of the light propagating in the fiber into a narrow lobe protruding out of the fiber in a direction being close to orthogonal to the propagation direction of light within the fiber. The two fibers are placed adjacent and parallel to each other in the plane of the protruding lobes of deflected light, and with the deflecting portions arranged such that an overlap between the protruding lobes is obtained. Further, the device comprises an external resonator formed by two highly reflecting mirrors outside the two fibers, the external resonator being arranged such that the resonator modes are essentially in the same geometrical plane as the lobes from the deflecting portions. The working principle of this coupler is as follows. The tilted FBG deflects some (typically a few percent of the light) of the broadband light propagating in the first fiber into the external resonator. Light of a wavelength that is in resonance with the external resonator will, after one round trip in the resonator, interfere constructively with subsequently deflected light of the same wavelength. Thus, a resonantly enhanced coupling of such wavelength is achieved, and substantially all light of that wavelength will couple to the resonator mode. Since the resonator mode overlaps with the deflecting portion of the second fiber, light of the resonant mode will be coupled into this second fiber. Under certain conditions, most of the light in the resonant mode will in fact be coupled to the second fiber. By adjusting the separation between the mirrors of the external resonator, it is possible to tune the wavelength to be coupled to the second fiber.
The separation between two wavelengths that fulfill the resonance condition is called the free spectral range (FSR) and is determined by the separation between the mirrors of the resonator. In order to be able to select only one wavelength at the time using the resonant coupler of EP 1 535 096, the FSR must be larger than the bandwidth of the broadband signal involved in the coupling. In a typical WDM system, the width of a wavelength band is in the order of 40 nm. To accomplish an FSR of about 40 nm, the separation between the mirrors of the resonator needs to be about 20 μm. In order for two fibers to fit into such resonator, the diameter of each fiber must be less than about 10 μm. Hence, it is clear that the small separation required between the mirrors of the external resonator leads to quite some challenges from a manufacturing standpoint.