This invention relates generally to switches for optical networks and in particular to a feed-back based control system for optical switching fabrics with tilting mirrors.
As optical fiber progressively supplements and replaces metal wire as the backbone of telecommunications networks, the switches that route optical signals have emerged as a significant bottleneck. Transmission systems move information as optical photons but the switching systems and so-called crossconnect fabrics that switch, route, multiplex, and demultiplex optical signals have generally been electronic. Electronic switching requires light to be converted to an electronic signal to pass through the switch and then be reconverted to light in a process termed optical-electronic-optical (OEO) conversion that introduces both time delay and cost.
There is great interest in the telecommunications industry, therefore, in developing all optical switching to avoid the necessity of multiple OEO conversions. On long haul networks, ten""s or hundred""s of individual wavelengths, each carrying a signal, are multiplexed onto each fiber. Switches are desired that provide all optical switching at the fiber level, the wavelength level, or at both levels. As described, for example, by Bishop et al. in Scientific American (January, 2001, pp 88-94), all optical switches based on a number of underlying technologies including Micro Electro Mechanical Systems (MEMS) tilting mirrors, thermo-optical devices, bubbles formed by inkjet printing heads, and liquid crystals, have been proposed. Optical fiber switches based on MEMS mirrors are particularly attractive because they can incorporate very large scale integrated circuits and can be robust, long-lived, and scalable.
An optical fiber switch described in U.S. Pat. No. 5,960,132 to Lin, for example, includes an array of hinged MEMS mirrors, each of which can be rotated about its hinge between a reflective state and a non-reflective state. An array of N2 such mirrors is required to switch signals carried by N input optical fibers from one to another of N output optical fibers. Unfortunately, N2 scaling results in unmanageably complex devices for large N.
Another optical fiber switch described in Bishop et al., cited above, as well as in Bishop et al., Photonics Spectra (March 2000, pp. 167-169) includes an array of MEMS mirrors disposed on a single surface. Each mirror tilts independently to direct light received from an array of input/output optical fibers to any other mirror and thus to any input/output fiber. No internal optical diagnostics for this switch have been described in publications to date.
Still other optical fiber switches are based on two arrays of MEMS mirrors that can be tilted in any direction. Incoming light is directed onto a mirror in the first array which deflects it onto a predetermined mirror in the second array. The mirror in the second array, in turn, directs the lights to the predetermined output port. In these so-called 2N configurations, the position of the mirrors has to be controlled very precisely, to small fractions of degrees, to provide the desired connections.
Therefore, optical fiber switches having a low insertion loss and that can be finely tuned to cross-connect large numbers of input and output fibers would further the development of fiber optic telecommunications networks. Furthermore, control systems for controlling the tuning of optical fiber switches are needed.
In accordance with the present invention, a control system for controlling individual mirrors in a micro-electro-mechanical system (MEMS) based optical switching fabric is presented. An optical switching fabric refers to an optical switch with multiple input ports and multiple output ports that allows an optical signal entering the device on any input port to be directed to any output port. As such, the optical switching fabric receives instructions from a node controller and directs light from input ports to assigned output ports based on those instructions.
A MEMS-based optical switching fabric, then, can include an input mirror array receiving signal beams from the input ports and reflecting the signal beams to an output mirror array. The output mirror array receives signal beams from the input mirror array and directs the signal beams to the output ports. A particular signal beam, for example, enters the optical switching fabric through a first input port and is then routed by reflections from an individual mirror of the input mirror array to an individual mirror of the output mirror array. The individual mirror of the output mirror array directs the particular signal beam to its assigned output port in accordance with instructions from the node controller. In some embodiments of the invention, signal beams can travel in both directions through the optical switching fabric so that beams can be received by the output mirror array and directed out of the switching fabric by the input mirror array.
In some embodiments, each of the individual mirrors of the input mirror array receives light from one of the input ports and each of the individual mirrors of the output mirror array directs light to one of the output ports. An individual mirror of the input mirror array can be oriented to direct light from its corresponding input port to one of the individual mirrors of the output mirror array. The individual mirror of the output mirror array receives the light from the individual mirror of the input mirror array and directs it towards an associated output port. Input ports are optically coupled to selected output ports, then, by appropriately orienting the individual mirrors of the input mirror array to direct light to the appropriate mirrors of the output mirror array and appropriately orienting individual mirrors of the output mirror array to receive light from associated individual mirrors of the input mirror array and direct that light to the output port associated with that individual mirror.
In accordance with the present invention, a position sensitive detector is positioned relative to a first mirror array to monitor the orientations of each of the mirrors in the first mirror array. A first control beam, which can be generated by a laser or other optical source, can be directed by dichroic optical elements to be reflected from at least the first mirror array onto the first position sensitive detector. The first mirror array can be either one of the input mirror array or output mirror array. In some embodiments, a first calibration beam can further be directed onto the first position sensitive detector. The orientation of individual mirrors of the first mirror array, then, is directly related to the position of first control beams from each of the individual mirrors of the first mirror array on the cells of the first position sensitive detector.
In some embodiments, a second control beam can be directed colinearily along the signal beam path through a second mirror array and onto cells of a second position sensitive detector. In some embodiments, a second calibration beam can also be directed onto the second position sensitive detector. The orientation of individual mirrors of the second mirror array is directly related to the position of the second control beam from each of the individual mirrors of the second mirror array on the cells of the second position sensitive detector. The second mirror array can be either of the input mirror array or the output mirror array.
In some further embodiments, a third control beam can travel along the signal beam path through the first mirror array and the second mirror array and be directed onto cells of a third position sensitive detector. The third position sensitive detector, which receives the third control beams directed through the first mirror array and the second mirror array, provides data for a fine alignment of the orientation of the mirrors. In some embodiments, a third calibration beam can also be directed onto the third position sensitive detectors in order to calibrate the third position sensitive detector.
In some embodiments, multiple control and calibration beams can be generated by multiple lasers and directed onto position sensitive detectors. For example, the first control beam and the second control beam can be generated by a first laser; the third control beam and the second calibration beam can be generated by a second laser; and the first calibration beam and the third calibration beam can be generated by a third laser. Control beams are directed onto position sensitive detectors by being reflected from one or more of the first mirror array or the second mirror array onto the position sensitive detectors. Calibration beams are not reflected from the first mirror array or the second mirror array. In some embodiments, then, a timing sequence that includes turning a first laser on to generate a first control beam which is reflected from the first mirror array onto the first position sensitive detector and second control beam which is reflected from the second mirror array onto the second position sensitive detector; turning the first laser off and turning a second laser on which generates a third control beam which is reflected from the first mirror array and the second mirror array onto a third position sensitive detector and the second calibration beam which is incident on the second position detector; and turning the second laser off and turning a third laser on which generates the first calibration beam which is incident on the first position detector and a third calibration beam which is incident on the third position sensitive detector. PSD signals from each of the first position detector, the second position detector, and the third position detector, in some embodiments, is integrated, digitized, and stored for later readout by an integration and digitization circuit. A digital signal processor reads out the accumulated data and adjusts the orientation of individual mirrors in the first mirror array and the second mirror array accordingly.
In some embodiments, one or more digital signal processors reads the integrated and digitized data from the position sensitive detectors and, based on that data, adjusts the orientation of individual mirrors in the first mirror array and the second mirror array. The orientation of the individual mirrors in the first mirror array and the second mirror array are held such that control beams reflected from them arrive at cells of the associated position sensitive detectors at particular coordinates. In other words, the orientation of individual mirrors in the first mirror array and the second mirror array, which determines the routing of light from the input ports to the output ports, is monitored through positions on cells in associated position sensitive detectors.
Some embodiments of the invention may include more than two mirror arrays. In those embodiments, PSD detectors can be placed to monitor individual mirrors of each of the mirror arrays in the switching fabric.
These and other embodiments are further discussed below with respect to the following figures.