The field of the invention is wavelength selective devices using tunable optical filters.
There exists a well known category of optical devices that perform optical filtering and can be tuned to select one or more narrow band of wavelengths from a wider wavelength spectrum. These devices are used in a variety of optical systems. Of specific interest are wavelength division multiplexed systems that operate typically over wavelength bands of tens of nanometers. These systems require optical performance monitoring (OPM) to ensure that signal power, signal wavelength, and/or signal to noise ratios (OSNR) are within specified limits. Other applications for tunable optical filters, inter alia, are for optical noise filtering, noise suppression, wavelength division demultiplexing, and optical routing.
Complex state of the art DWDM systems have many channels operating across a wide optical spectrum. To monitor these channels requires many measurements. Monitoring equipment that reduces the time and complexity of performing these measurements can significantly increase overall system performance and reduce system costs.
Other applications using wavelength selective devices in WDM systems are for selectively routing channels in large optical systems.
We have developed tunable multiport optical devices that perform various optical functions involving multiple optical channels in a convenient and cost-effective manner. The tunable multiport optical devices of the invention are simple in design and have few optical components. The basic elements are a wavelength dispersion element and a rotating mirror operating with arrays of input and output optical fibers. Preferred applications include optical channel monitoring (OCM), optical switches for routing optical channels between multiple optical light paths and Reconfigurable Optical Add/Drop Multiplexers (ROADM).
The invention may be better understood when considered in conjunction with the drawing in which:
The embodiments shown are described in the context of OPM applications. However, it should be understood that the basic devices described here are also useful as wavelength selective devices for routing selected WDM channels.
The specific optics as represented by ray optics, for the embodiment of
The optical fibers are shown only schematically in the figures. Typically they will be standard single mode fibers with a cladding diameter of 125 microns and a core diameter of 10 microns or less. In the portion of the array shown, i.e., the portion addressed by the wavelength selection elements, the optical fibers are stripped of the usual polymer coating. This allows greater precision in the array, producing, in many cases, a predictable spacing between cores of the fibers. Recognizing that a variety of options in the format of the array may be desirable, as will be discussed in greater detail below, optical fibers with sizes other than the conventional 125 microns may be useful. For example, cladding diameters of 50, 62.5, 250, may be used to advantage to vary the overall aperture (size) of the array. It is expected that small aperture arrays may be most cost effective.
Only one of the beam components (wavelength channels), in this case components represented by arrow 17b, will be normal to the mirror 16. That beam component is reflected back along a path represented by the dashed line. Other beam components, like the two shown in the y-axis cross section of
When mirror 16 is rotated in the y-z plane, another beam component (wavelength channel) will be normal to the mirror 16 and will be selectively reflected back through output fiber 13 and its properties measured. In this manner, the wavelength spectrum of the input beam to optical fiber 12 may be scanned and the properties of all of its beam components can be measured.
Thus the device achieves wavelength selection and provides an optical filter. The wavelength of the filter is tuned by the rotational orientation of mirror 16.
It should be noted that a similar result can be obtained if the axis of the dispersive element is rotated by 90 degrees and the mirror is tilted in the same axis that the beam is dispersed. In this configuration the light beam from the grating is dispersed into the wavelength components of the signal beam along the same axis of the fiber array, and there is some likelihood that the spectra from a fiber port will overlap with an adjacent or non-adjacent fiber port. The wavelength components which are diffracted from the dispersion element can be distinguished by increasing the separation of the fiber ports, although this will require a large optical aperture. To obtain satisfactory performance fiber port separation would be increased to three or more times larger than the separation required when the axis of the dispersive element is orthogonal to the fiber array.
It will be understood that a function of the rotating mirror 16 is to select a wavelength component of the incident beam and return it to a fixed position, in the case of the arrangement of
It will be recognized that the optical paths in the x-axis cross section of
Another WDM channel may be input as an input beam to optical fiber 18. The output of the beam components from this channel are directed through output optical fiber 19 and measured by the associated photodetector as shown in the top portion of
It will be appreciated by those skilled in the art that, while the array of input optical fibers, e.g., 12 and 18, and the array of output fibers 13 and 19 are shown closely packed and precisely aligned, the device input optical fibers and the device output optical fibers may have any length and be routed in any suitable fashion to other components/couplers in the system. For example, the photodetectors 21 are shown as an array of elements receiving light beams directly from the closely packed array of output optical fibers. However, the optical fiber 13 may route an optical signal to a photodiode non-aligned with respect to the output array of optical fibers.
The detection device may take any of a variety of forms, and measure a variety of optical beam properties. The arrangement shown is simple and useful for illustration. If the input beams are suitably time division multiplexed, a single detection device may be used. Alternatively a single spectrum analyzer may be used as the detection device.
In this description the optical elements are shown as separate elements. These represent functional elements. The physical elements providing these functions may, in some cases, be combined as a single module. For example, a grating may have a reflective surface or an attached or integral lens.
In the embodiments of
The fiber array may have other formats.
In the systems described it is not required that the ports be tuned simultaneously. If the mirror orientation can be controlled in 2 axes, i.e., both x- and y-, then the ports can be read in series, i.e., there will be one input fiber and a number of output fibers. In
It should be evident that the number of optical fibers in a multiport optical filter according to the invention may vary widely. In embodiments like
The light waveguides described above are optical fibers. However, other waveguide arrays may be substituted. For example, the arrays of optical waveguides may comprise optical integrated circuits (OICs) where parallel waveguides are formed in a common substrate such as lithium niobate, doped glass, or indium phoshide. The term “waveguide” used herein is intended to include any suitable light guiding element.
The placement of the optical fibers, both for the input side and the output side may vary significantly. In the embodiments of
As mentioned earlier, the devices described above may be used as wavelength selective devices in any application requiring that function. The embodiment shown in
The device of
A wide variety of applications exist for wavelength selective devices. For example, channel selectors in DWDM transmission and display require a single channel to be selected among a large number of channel options. Recognizing that the reflector element can itself provide added functionality, optical systems may be employed in which the reflector is tilted to transmit a predetermined sequence of wavelengths. These may be used in coding devices.
In the embodiments described above, the reflector is tilted with respect to the dispersive element to achieve wavelength selectivity. However, devices may be designed in which the dispersive element is moved and the reflector is fixed. Likewise other optical elements, for example, lenses, may be used to achieve the same effect. All arrangements in which some controlled predetermined movement of a dispersive element with respect to a reflector or refractive element to achieve the purpose of the invention should be considered equivalent.
As mentioned earlier, the beam steering element is preferably a light reflecting element or a light refracting element. In both cases the element is typically operated as a moving element, e.g., a MEMS mirror or the like. Optionally, the beam steering function may be provided by a non-moving element, for example, an electro-optic device. In one embodiment using an electro-optic device the beam steering element relies on changes in refractive index of an electro-optic medium. The changes in refractive index may be used to change the direction of diffraction of a light beam being analyzed or switched.
With reference to
Alternatively, the device shown in
In another alternative embodiment the parallel readout implementation of the embodiment of
These embodiments illustrate the very large versatility of devices operating according to the invention that operate with a wavelength selective mirror (for example) that may be tilted around more than one axis. In the arrangement shown, the tilt around the x-axis selects the beam position, i.e., the output port, while the tilt around the y-axis selects the wavelength. The biaxial tilt allows a selected wavelength component of an input beam to be directed as an output beam to any point in the x-y plane. This gives to rise to another level of versatility in the design of devices operating according to the principles of the invention. This will be described in conjunction with the illustrations in
The view in
While five wavelengths are illustrated in
The output array in
In
It is important to note that the precision of the fiber array spacing in either x or y axes does not affect the performance provided the rotation of the mirror in the x or y axes can be optimized to minimize loss. This can be facilitated through a calibration process which stores a look up table with the location of the fibers stored, or using an optimization algorithm such as a hill-climbing algorithm that seeks to minimize the loss in any optical light path.
To identify or monitor the positions of the output optical fibers in an organized or random array it may be useful to employ a monitor that periodically locates the position of the members of the array. One suitable device for doing this is a CCD or CMOS imaging device which, when the output optical fibers are illuminated, can record the spatial position of each member in the array. A CCD imaging device is also useful as a light output detector in the normal operation of any of the devices or systems described earlier. If the CCD image plane is made as large as the aperture of the device it can serve as an output detector without regard to the precise locations of the individual members in the array.
As will be understood by those skilled in the art, arrays similar to those shown in
Reference to minimal dispersion in the preferred embodiments means essentially no dispersion. However, in some geometries the light may be dispersed somewhat in the x-z plane. Dispersion that is not used functionally in the device may be considered to be minimal dispersion.
In concluding the detailed description it is evident that various additional modifications of this invention may occur to those skilled in the art. All deviations from the specific teachings of this specification that basically rely on the principles and their equivalents through which the art has been advanced are properly considered within the scope of the invention as described and claimed.
This application is a continuation-in-part and claims the benefit of U.S. application Ser. No. 12/804,627, filed Jul. 26, 2010 now U.S. Pat. No. 8,577,192. That application is incorporated by reference herein in its entirety.
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Number | Date | Country | |
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Number | Date | Country | |
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Parent | 12804627 | Jul 2010 | US |
Child | 12927066 | US |