CA2328756A1 - Control mechanism for large optical cross-connect switches - Google Patents

Description

' CA 02328756 2000-12-19 Doc. No. 10-425 CA Patent Control Mechanism For Optical Cross-Connect Switches Field of the Invention The present invention relates to a control mechanism for optical cross-connect switches, particularly for optical cross- connects using tiltable mirrors for detecting and correcting error in mirror positioning.
Background of the Invention IO
One of the major challenges of designing an optical cross-connect (OXC ) switch using tiltable MEMS mirrors consists in controlling accurately each of the individual mirrors so that low fiber-to-fiber losses can be maintained over the operation lifetime of the switch.
Optical matrix switches are commonly used in communications systems for transmitting voice, video and data signals. Generally, optical matrix switches include multiple input and/or output ports and have the ability to connect, for purposes of signal transfer, any input port/output port combination, and preferably, for N x M switching applications, to allow for multiple connections at one time. At each port, optical signals are transmitted and/or received via an end of an optical waveguide. The waveguide ends of the input and output ports are optically connected across a switch interface. In this regard, for example, the input and output waveguide ends can be physically located on opposite sides of a switch interface for direct or folded optical pathway communication therebetween, in side-by-side matrices on the same physical side of a switch interface facing a mirror, or they can be interspersed in a single matrix arrangement facing a mirror.
In the OXC in accordance with the present invention the optical path between an input port and an output port involves the use of one or more moveable mirrors interposed between the input and output ports. The input and output waveguide ends remain stationary and the mirrors are used for switching. The mirrors can allow for two-dimensional targeting to optically connect any of the input port fibers to any of the output port fibers.
The major obstacle to creating an optical switch is the necessary control for precisely addressing each of the mirrors to achieve accurate switching with low loss.
Small errors in angle over the optical path length of the switch result in large coupling errors.
An optical cross-connect switch is proposed by Herzel Laor in US patent number 6,097,860, issued to Astarte Fiber Networks, Inc., for directing a beam of light from a fiber focused to a fixed mirror and reflected to an array of moveable mirrors.
Laor discloses a complex control system for detecting angle deviation. Because the optical path includes a first and a second reflection (in a Z pattern) between launching a focused beam and coupling a switched beam to a selected output, a cumulative error will be detected at the output. To determine the angle error of each mirror is complex and difficult.
Further to make multiple passes between moveable mirrors increases the complexity of determining the angular position error of each mirror.
An accurate sensing and control system for an optical cross-connect system employing multiple path changes by moveable mirrors is needed.
Summary of the Invention The present invention has found that by providing an element having optical power in an optical path between a first and a second moveable mirror, and a pair of wavefront sensors for sensing an output in a first direction and a second direction along the optical path, that an angle deviation from the optical path can be detected for each of the first and second moveable mirrors simultaneously and independently.
Accordingly, the present invention provides an optical cross-connect switch comprising at least one input for launching a signal;

a plurality of outputs for selectively receiving the signal from the at least one input via an optical path;
a first array of independently moveable mirrors in the optical path for redirecting the signal from the at least one input to a second array of independently moveable mirrors, said second array of independently moveable mirrors for redirecting the signal to the selected output;
an element having optical power in the optical path between the first and second array of moveable mirrors;
a first wavefront sensor for detecting an angle of the signal received from the second array of moveable mirrors; and a second wavefront sensor for detecting an angle of a second signal received from the first array of moveable mirrors launched in an opposite direction on a substantially same optical path from the selected output to a same at least one input.
Advantageously, each wavefront sensor can detect an angle deviation error of one mirror.
Each mirror of each array can be checked and corrected. Using a simultaneous control signal, this can be done in real time.
Brief Description of Figures Fig. 1 is an example of a preferred optical cross-connect switch;
Fig. 2 illustrates a first embodiment of a control in accordance with the present invention for the switch of Fig. 1;
Fig. 3 illustrates a second embodiment of a control in accordance with the present invention for the switch of Fig. 1;
Fig. 4 illustrates the isolation of error for a first mirror on an optical path from left to right, and for a second mirror on an optical path from right to left, both optical paths beginning from the ideal path and experiencing deviation at the mirrors;
Detailed Description of Preferred Embodiments An example of an optical cross-connect switch for use in the present invention is shown in Fig. 1 Fig. 1 shows a large optical cross-connect arrangement 1200 in accordance with the present invention. Optical switch 1200 is scalable to 4000x4000 and is based on arrays of two-dimensional tilt mirrors 1210 and 1220 and ATO lens 1230. An input fiber bundle 1240 is shown on the left hand side of Fig. 12. An input micro-lens array 1250 is placed at an end face of the input fiber bundle L240 having one micro-lens centered on an optical axis of each fiber. An input relay lens 1260 is provided between the micro-lens array 1250 and a first MEMS chip 1210 having an array of two-dimensional tilt mirrors/micro mirrors. The distance between the input micro-lens array 1250 and the input relay lens 1260 and the input relay lens 1260 and the first MEMS chip corresponds to a focal length of the input relay lens 1260. This input relay lens 1260 sends a beam of light incident thereon through a hole 1270 in the first MEMS
chip 1210.
The first MEMS chip 1210 is followed by an ATO lens 1230, i.e. an element having optical power whose focal length corresponds to the near zone length (multi mode) or Rayleigh range (single mode) of the beam incident on the 2D tilt mirrors, and a second MEMS chip array 1220 having an array of two-dimensional tilt mirrors/micro mirrors and a hole 1280 disposed thereon. Both, the first MEMS chip 1210 and the second MEMS chip 1220 are arranged at a distance from the ATO lens 1230 which corresponds to the focal length of ATO lens 1230. The second MEMS chip 1220 is followed by an output relay lens 1290 which focuses the light to an output micro-lens array provided at an end face of an output fiber bundle 1310 having one micro-lens centered on an optical axis of each fiber. The distance between the second MEMS chip 1220 and the output relay lens 1290 and the output relay lens 1290 and the output micro-lens array corresponds to a focal length of the output relay lens 1290. All components are arranged along an optical axis OA. Such an arrangement provides for an even more compact design of an optical switch in accordance with the present invention, and lessens aberration effects of the lens. In order to demonstrate more clearly how optical switch 1200 functions, an exemplary beam of light L is traced along an optical path A
to H
through switch 1200. The beam L exits an input fiber at point A at an end face thereof having a miro-lens disposed thereon. The beam L propagates parallel to the optical axis OA until it reaches point B on the input relay lens 1260. Input relay lens 1260 sends beam L at an angle to the optical axis OA to point C on the ATO lens 1230 through the hole 1270 in the first MEMS chip 1210. The ATO lens 1230 sends beam L parallel to the optical axis OA to point D on one of the micro-mirrors on the second MEMS chip 1220.
The mirror on the second MEMS chip 1220 switches beam L to point E on one of the micro-mirrors on the first MEMS chip 1210 after passing through the ATO lens 1230.
The micro-mirror on the first MEMS chip 1210 sends the light back to point F
on the ATO lens 1230 parallel to the optical axis OA and then at an angle to the optical axis OA
to point G through hole 1280 in the second MEMS chip 1220. The output relay lens 1290 collects the beam of light L coming from hole 1280 in the second MEMS
chip 1220 and images it on the output micro-lens array 1300. An output fiber in the output fiber bundle 1310 collects beam L from the output micro-lens array 1300. It is apparent that this switch also functions in reverse, i.e. the output fiber bundle then functions as the input fiber bundle and so forth.
First embodiement:
This invention which builds on a previously disclosed OXC design, the in-line ATOM
(shown in Fig. 1), proposes an optical architecture and control scheme (shown schematically in Fig. 2) that addresses this challenge.
As seen in Fig. 2, the internal alignment of the switch core (i.e.
independently of the input and output fiber array) is performed using two optical sources (S 1 and S2), one copropagating and the other one counterpropagating with respect to the input fiber light.
The beam area of these monitoring optical sources has been expended to cover a size similar to that of the input/ouput fiber bundle to eliminate the need of two additional collimated fiber arrays (another possible option but more complex and expensive). A hole plate could be inserted in the path to create a beamlet array if flat illumination is not appropriate. The wavelengths of the two optical sources are selected in a region outside the intended operating range of the switch so that monitoring is concurrent with the live traffic. Each collimated beam is combined colinearly with the input/output using a WDM
beam combiner. Light from the co- and counterpropagating beams is detected at each end using a wavefront sensor. Due to the imaging property of the switch, there is a one to one correspondence between every MEMS mirror inside the switch and each wavefront sensor pixel. Consequently, light originating from S 1 which is reflected from the MEMS
arrays, M1 then M2, gets imaged on the output wavefront sensor WS 1 and light originating from S2 which is reflected from M2 then M1 gets imaged at the input wavefront sensor WS2. This arrangement provides a unique wavefront sensor pixel for every individual MEMS mirror.
Typical wavefront sensor would consist in an array of microlenses coupled to an array of quadrant detectors. Quadrant detection will generate four electrical signals, which will correspond to a particular mirror orientation. These signals can be processed to generate an error signal and through a feedback loop move the MEMS mirror (left, right, up and down for example) to a predetermined position.
Although it is intended for the two monitoring optical sources to be very closely aligned to the input and output fiber array, small imperfections in the array fabrication will lead to slight misalignment between the light emerging or incident on the fiber array (signal light) and the sources S 1 and S2. This misalignment can be taken care in an initial calibration of the feed-back mechanism of the switch where every connection would be established, the mirror position optimized for maximum throughput and the data (four voltage values for a quadrant detector) stored in a look up table. The misalignment between the signal and S1/S2 will lead to the alignment beam being off center on the quadrant detectors. This situation will be taken care in the feedback electronics through an offset voltage setting.
Over time there could be an independent movement between the input/output fiber array and the monitoring optical sources, S1 and S2. This relative motion would introduce an increase in the OXC insertion loss if not corrected. We propose to use several probe beams, for example one emerging from each corner in the input fiber array, which will make an end to end point connection to a similar set of fibers on the output array. The probe beams would be kept in a closed feedback loop for optimum transmission.
Any differential movement between the input/output fiber array, the collimating lenses, the MEMS mirrors and ATO lens will create a shift between the initial quadrant detector alignment readings and the new ones (created from the tracking loop of the probe beams).
Using signal processing, an appropriate correction could be applied to the initial calibration connectivity table, which would be periodically updated using data from the probe beams. This method will ensure low transmission loss assuming the individual input fibers are not moving independently of each other.
Second embodiement:
In an other embodiement, shown in Figure 3, it is proposed to have out-of-band pilot tone signals added to each input and output fibers. Four wavefront sensors are disposed before and after the switch core and receive light reflected from wavelength sensitive beam sputters (that partially reflects pilot tone light, and let the real traffic wavelength pass through unaffected). This is either a unique component or the association of a beam sputter and a filter.
Pilot tones travelling from left to right split on the first beam sputter.
Part of the light is hitting the wavefront sensor WS3, and the other part of the pilot light travels through the switch. Part of this light then hits the wavefront sensor WS4. Pilot tones travelling from right to left split on the second beam sputter. Part of the light hits WS l, and the rest travels through the switch. It then splits on the first beam sputter and part of this light hits WS2.
The detected wavefront from WS3 and WS 1 are then the target for WS2 and WS4 respectively. Any deviation from this target would cause angular misalignment of the beam in front of the micro-collimators, therefore adding insertion losses.
Therefore, the feed-back signal is the difference between the wavefront reading of WS3-WS2 and WS 1-WS4. With wavefront sensors consisting of a microlens array and an array of quadrant detectors, 2 signed errors signals are obtained per wavefront sensor. The 2 error signals from WS3-WS2 are fed back to control micro-mirrors M1, while the 2 error signals from WSl-WS4 are fed-back to control micro-mirrors M2. There is a unique and fixed reliationship between the pixel on the wavefront sensors and the corresponding micromirrors. This enables to have constant feed-back loops established.
The advantage of this embodiement is that there is no calibration of the feed-back mechanism required. Indeed, ideal fiber coupling corresponds to a wavefront measured on WS2 being identical to the one obtained on WS3 and respectively for WS4 and WS1.
There is no teaching of the switch required.
When the switch is assembled, its look-up table is loaded with initial value defined assuming ideal ATO imaging (ie linear angle per port assignement). When the 4 wavefront sensors are turned on, they immediately provide the real target for switch alignement, regardless of the siwtch state. Feed-back signals are used to correct the switch look-up table in a converging manner. An initial scan could be performed to guarantee that all states have been updated, but it may not be necessary since this control mechanism provides with both the error signal and a permanently recalibrated target.
This control mechanism is actually providing with initial set-up, real time calibration and feed-back mechanism at the same time.
Wavefront sensor design (both schemes):
Since the beams impinging on the MEMS micro-mirrors are relatively big, they are significantly more sensitive to angular misalignment than to lateral misalignment. This is also true for the beams coming from the first microlens/fiber array assembly.
As an example, the beam generated by the fiber bundle+microlens array that we plan to use has a radius of 66.6 microns. The tolerance for 1dB of extra loss is: +/-32 microns, which is very loose, compared to +/- 0.2° for the angle.
To be able to sense this angular misalignement, it is proposed to use a wavefront sensor consisting in a microlens array with focal length of 8 mm on a pitch of 250 microns coupled to a quadrant detector scheme also on a 250 microns pitch (ie each detector of the quad could be approx. 100x100 microns2).
The displacement of the beam on the quad is +/- 28 microns, with a beam diameter (3w) of 178 microns. It is therefore pretty easy to detect, while keeping the beam inside its own cell (3w + 2 displ. = 234 microns < pitch).

The microlenses to be used are very easy to fabricate since their F# is 32.
They can be made using refractive or diffractive lenses.
Miscellaneous:
If the microcollimator arrays are perfect, all light travelling through the switch depicted in Fig. 1 have to pass through the optical via-holes in the MEMS arrays.
Therefore, this information could be used as a multiplexed error signal. This would provide only a warning signal as opposed to a real feed-back signal, indicating that the switch has drifted and that calibration is required. For example, one could have taps comparing optical power entering the switch to the light passing through a pin-hole similar to the via-hole in the MEMS arrays. Similarly, an optical fiber could be used as a pin-hole. Real traffic or pilot tones could be used, either added on each fibers or added in the free space region of the beams.

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Claims

Claims What is claimed is:

1. An optical cross-connect switch comprising at least one input for launching a signal;
a plurality of outputs for selectively receiving the signal from the at least one input via an optical path;
a first array of independently moveable deflectors in the optical path for redirecting the signal from the at least one input to a second array of independently moveable mirrors, said second array of independently moveable deflectors for redirecting the signal to the selected output;
an element having optical power in the optical path between the first and second array of moveable deflectors;
a first wavefront sensor for detecting an angle of the signal received from the second array of moveable deflectors; and a second wavefront sensor for detecting an angle of a second signal received from the first array of moveable deflectors launched in an opposite direction on a substantially same optical path from the selected output to a same at least one input.