Abstract:
A precise, tunable, low loss add/drop filter for use in fiber optic communication systems utilizes a cascaded system of Mach Zender-like stages, along with reflective elements to return passed channels to the output pass+add signal while transmitted signal(s) appear at the drop port. The optional add signal follows the reverse path of the drop signal and is added to the pass signal.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to tunable optical filters. In particular, the present invention relates to tunable optical add, drop, and add/drop filters.  
           [0003]    2. Description of the Prior Art  
           [0004]    Optical fiber communications systems are theoretically capable of extremely high data rates (terabits per second), meaning that many channels of gigabit rate data can theoretically be carried on a fiber, via wavelength division multiplexing. The utility of fiber optic systems has been limited, however, because of the cost and complexity of the electronics required to separate out a specific wavelength channel or channels at every node in the communications system. Currently, optical add/drop filters are used to extract desired frequencies. FIG. 1 (prior art) shows a conventional fixed-wavelength optical add/drop filter system  100 , based on a thin-film interference filter  108 .  
           [0005]    An add/drop filter  100  such as that shown in FIG. 1, which extracts (drops) and reinserts (adds) a fixed wavelength. A multiplexing filter system which drops and adds four fixed wavelengths is many thousands of dollars. In the future, when fibers may carry as many 256 wavelengths, the cost of an add/drop multiplexor might be $1 million or more.  
           [0006]    Since most nodes in add/drop multiplexor will not need to extract all of the wavelengths, much of the hardware will be idle most of the time. Yet configuring an add/drop filter to extract only particular wavelengths precludes any flexibility at the node.  
           [0007]    Recent progress in tunable lasers has produced communication quality lasers which are capable of precise tuning across the entire fiber communication band. This removes the expense of providing dedicated lasers for each channel at each node. The obvious companion to the tunable laser would be a tunable filter, which can drop and add any given wavelength on the fiber. In addition, it would be desirable if the tunable filter could also act as a monitor for the tunable laser so that it could be accurately locked to the dropped channel wavelength. Each node would require only as many tunable laser/tunable filter sets as the maximum number of channels to be read at that node.  
           [0008]    Unfortunately, the universal, tunable, drop/add filter described above does not exist. Current drop/add multiplexor filters capable of handling the required close wavelength spacing either use dedicated filters for each channel, or use a cascade of band-splitting filters which result in a separate output fiber for each wavelength. Both methods require network designers to either limit the node&#39;s usable wavelengths or use redundant hardware.  
           [0009]    Tunable filters of the Fabry-Perot type are available, but it is not currently feasible to achieve the necessary degree of finesse in these filters. If they can tune the entire WDM range, they do not have narrow enough channels, and if they have narrow enough channels, they can only tune over a portion of the required band. For systems which will require 256 channels, finesse of over 250 is required. A Fabry-Perot filter with this kind of finesse would require very uniform and high reflectivity mirrors, and would be very susceptible to environmental effects such as temperature changes and vibration.  
           [0010]    [0010]FIG. 2 a  (prior art) shows a 3-stage Lyot filter  200 . Lyot filters were invented in 1933, and are known for achieving a high degree of finesse. The finesse of a Lyot filter increases as 2 N , where N is the number of filter stages. Lyot filters having finesse of over 250 are easily achievable.  
           [0011]    Referring to the example Lyot filter of FIG. 2 a  and the frequency plot of FIG. 2 b,  the operation as follows. The first polarizer  202  polarizes input light  210 . Stage 1, comprising delay block  204  and a polarizer  202 , passes half of the channels, while discarding the other half. So, for example, the area above the line is passed, while the area below the line is absorbed by the polarizer. Stage 2, comprising delay block  206  and a polarizer  202 , does the same thing with the light it receives, passing half of those channels and discarding the other half. Stage 3, comprising delay block  208  and a final polarizer  202 , passes half of the channels it receives and discards the other half. Thus, the output light is the light passed through all three stages. The output bands are much narrower than the bands passed by the first stage, but are separated by as much as the centers of the bands passed by the first stage. Further stages make the output bands narrower.  
           [0012]    Unfortunately, the Lyot filter has several significant downsides. First, it requires polarized light, so half of the light is lost up front. Second, there is no complementary output—the light removed at each stage is discarded at the polarizers  202 . A 9 stage filter results in a loss of more than 4 dB. Thus, Lyot filters have primarily been used in solar studies, where plenty of light is available and high finesse is essential.  
           [0013]    The high finesse of a Lyot filter, without its corresponding loss of light and lack of a complimentary output, would be ideal for use in fiber optic system add/drop filters and multiplexors.  
           [0014]    A need remains in the art for a precise, tunable, low loss add/drop filter for use in fiber optic communication systems.  
         SUMMARY OF THE INVENTION  
         [0015]    It is an object of the present invention to provide a precise, tunable, low loss add/drop filter for use in fiber optic communication systems. This object is accomplished with a cascaded system of tunable filters.  
           [0016]    A tunable drop filter according to the present invention has an input port, a drop port, and an output port and includes means for providing an input signal consisting of channels to the input port, a plurality of filter stages connected to the input port, each filter stage operating to selectively transmit either even or odd channels and reflect either odd or even channels respectively, means for providing reflected channels as a pass signal at the output port, and means for providing a transmitted channel at the drop port.  
           [0017]    Each filter stage could comprise a fiber Mach Zender interferometer having a selective delay for transmitting the selected channels and a mirror for reflecting channels not transmitted by the fiber Mach Zender interferometer. The means for providing reflected channels as a pass signal at the output port and the means for providing an add signal at the add port such that the add signal follows the reverse path of the drop signal could comprise circulators.  
           [0018]    As a feature, each filter stage could include a delay applied to any reflected channels, each delay selected to synchronize the pass signal channels.  
           [0019]    As an alternative, each stage could comprise a bulk optics Mach Zender interferometer having a selective delay for transmitting the selected channels and a mirror for reflecting channels not transmitted by the bulk optics Mach Zender interferometer.  
           [0020]    Or, each stage could comprise a selective delay block, a polarizing beam splitter adjacent to the delay block for transmitting the selected channels, and a mirror for reflecting channels not transmitted by the polarizing beam splitter.  
           [0021]    A tunable add/drop filter according to the present invention has an input port, an output drop port, an input add port and an output pass+add port and includes means for providing an input signal consisting of channels to the input port, a plurality of filter stages connected to the input port, each filter stage operating to selectively transmit either even or odd channels and reflect either odd or even channels respectively, means for providing reflected channels as a pass signal at the output port, means for providing a transmitted channel at the drop port, means for providing an add signal at the add port such that the add signal follows the reverse path of the drop signal; and means for combining the add signal and the pass signal at the pass+add output port. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIG. 1 (prior art) shows a conventional optical add/drop filter system for use in fiber optic communications systems.  
         [0023]    [0023]FIG. 2 (prior art) shows a Lyot filter.  
         [0024]    [0024]FIG. 3 shows a block diagram of an improved tunable add/drop filter according to the present invention.  
         [0025]    [0025]FIG. 4 shows a first embodiment comprising a fiber implementation of a tunable add/drop filter according to the present invention.  
         [0026]    [0026]FIG. 5 shows a second embodiment comprising a bulk optics tunable add/drop filter according to the present invention.  
         [0027]    [0027]FIG. 6 shows a third embodiment comprising an add/drop filter without circulators according to the present invention.  
         [0028]    [0028]FIG. 7 shows a fourth embodiment of a tunable add/drop filter according to the present invention, configured to compensate for delays.  
         [0029]    [0029]FIG. 8 shows a fifth embodiment of of a tunable add/drop filter according to the present invention, comprising a modified Lyot filter. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]    [0030]FIG. 3 shows a high level block diagram of an improved tunable add/drop filter  300  according to the present invention. Tunable add/drop filter  300  is a 4 port device. Input signals, comprising a plurality of optical signals at distinct wavelengths λ 1 , λ 2 , λ 3 , . . . , enter at network input  302 . The signal at one wavelength, for example λ 2 , which is to be selected for by filter  300 , appears at drop output port  308 . A new signal λ 2 ′, at wavelength λ 2 , may be inserted at add input port  304 . Output signals which appear at pass+add port  306  comprise signals λ 1 , λ 2 ′, λ 3 , . . . (If λ 2 ′ is inserted at port  304 ). Thus, the pass signals are all of the signals not selected for by filter  300 , i.e. λ 1 , λ 3 , . . . The add signal (if used) is λ 2 ′.  
         [0031]    Filter  300  comprises a series of stages similar to the stages of the Lyot filter  200  shown in FIG. 2 (Prior Art). Filter  300  has the high finesse of a Lyot filter  200 , without its corresponding loss of light and lack of a complimentary output. The filter is precisely tuned using a cascade of low precision phase shifters, each of which need only be capable of, at most, a one wavelength shift with a precision of no better than 10%. The free spectral range of the filter is determined freely by one stage of the filter, and the finesse increases as 2 N , where N is the number of stages. Thus, a moderate number of stages ( 8  or  9 ) are sufficient to cover the entire WDM band with high selectivity.  
         [0032]    Examples of the stages of filter  300  are shown in FIGS.  4 - 8 .  
         [0033]    [0033]FIG. 4 shows a first embodiment comprising a fiber implementation of a tunable add/drop filter  300   a  according to the present invention. Add/drop filter  300   a  has the same combined input signal  302 , pass+add signal  306 , drop signal  308 , and add input  304  as described with respect to FIG. 3.  
         [0034]    To understand the operation of filter  400 , consider that each stage of filter  400  is a fiber Mach Zender interferometer, if fiber mirrors  410 ,  418 , and  428  are removed. Thus, like the Lyot filter of FIG. 2, filter  400  drops half of the channels at each stage and passes the other half. The dropped channels appear at mirrors  410 ,  418 , and  428 . In a Mach Zender interferometer, the signals at  410 ,  418 , and  428  would be outputs. But in the present invention, these signals are reflected back to form part of the pass signal  306 .  
         [0035]    Thus, input signal  302  comprises a plurality of optical signals at distinct wavelengths λ 1 , λ 2 , λ 3 , . . . Circulator  402  provides this combined input signal to the network, and provides reflected signals as pass+add output  306 . Couplers  404 ,  408 ,  412 ,  416 ,  420 , and  426  are a 50/50 coupler, meaning that each provides 50% of the signal to the upper branch and 50% to the lower branch. These couplers are what divide the signal so that bands may be selected for.  
         [0036]    For example, in the first stage, input signal  302  encounters coupler  404 , which divides the light between the upper branch and the lower branch of the first stage. The signal in the upper branch passes through block  406 , which applies a path difference, phase delay, of 4Γ (+/−π) to the light, while the signal in the lower branch does not have a phase delay applied. 50/50 coupler  408  sends half of the light on to 50/50 coupler  412  and half down into mirror  410 . The delay applied by block  406  is selected such that coupler  408  passes either the even bands it encounters or the odd bands. 4Γ+π selects one set while 4Γ−π select the other set.  
         [0037]    The second stage operates similarly to the first. Coupler  412  divides the light into it between the upper branch and the lower branch of the second stage. The signal in the upper branch passes through block  414 , which applies a path difference of 2Γ (+/−π) to the light, while the signal in the lower branch does not have a phase delay applied. Coupler  416  sends half of the light on to coupler  420  and half down into mirror  418 . The delay applied by block  414  is selected such that coupler  416  passes either the even bands it encounters or the odd bands.  
         [0038]    Finally, in the third stage, coupler  420  divides the light into it between the upper branch and the lower branch of the third stage. The signal in the upper branch passes through block  422 , which applies a path difference of Γ (+/−π) to the light, while the signal in the lower branch does not have a phase delay applied. Coupler  426  sends half of the light on to circulator  430  and half down into mirror  428 . The delay applied by block  422  is selected such that coupler  426  passes either the even bands it encounters or the odd bands.  
         [0039]    As an example, suppose that the input signal  302  contained signals λ 1 , λ 2 , λ 3 , λ 4 , λ 5 , λ 6 , λ 7 , and λ 8 . If drop output λ 6  were desired, the stages would pass the bands as follows:  
         [0040]    Stage 1: λ 2 , λ 4 , λ 6 , and λ 8    
         [0041]    Stage 2: λ 2  and λ 6    
         [0042]    Stage 3: λ 6    
         [0043]    The other bands are reflected back by the fiber mirrors  410 ,  418 , and  428 .  
         [0044]    The path difference applied by each stage changes by a factor of two, though this may be fine tuned to achieve certain objectives, such as flatter bandpass. Numerically optimizing the filter is one way to systematically achieve such objectives. The phase shifters  406 ,  414 ,  422  are normally capable of shifting the phase of the light passing through them by ± π, where π is the approximate wavelength of the center of the filter FSR.  
         [0045]    Add input  304  is routed by circulator  430  back to the network. The simplest example is to assume that input  304  is also at λ 6  call it λ 6 ′. Then the add input λ 6 ′ passes back through the network the same way λ 6  passed forward.  
         [0046]    [0046]FIG. 5 shows a second embodiment comprising a bulk optics tunable add/drop filter  500  according to the present invention. Again, each stage is a Mach Zender interferometer, if  512  and  526  are ignored. Mirrors  512  and  526  act to reflect the passed channels back to join the pass+add output  306 . Note that circulators as shown in FIG. 3 are required at each end of filter  500  to enable the adding and dropping of channels.  
         [0047]    Stage 1 comprises beam splitters  502  and  510 , prism mirrors  506  and  508 , mirror  512 , and delay block  504 , which adds a delay of 4Γ. Thus, light at beam splitter  510  either cancels out, and therefore reflects back to pass+add output  306 , or combines to continue to stage 2.  
         [0048]    Stage 2 comprises beam splitters  514  and  524 , prism mirrors  520  and  522 , mirror  526 , and delay block  518 , which adds a delay of 2Γ. Thus, light at beam splitter  524  either cancels out, and therefore reflects back to pass+add output  306 , or combines to continue to drop output  308 . Add input  304  travels back through filter  500  the same way the drop output travelled forward.  
         [0049]    [0049]FIG. 6 shows a third embodiment comprising an add/drop filter  600  without circulators according to the present invention. Filter  600  operates in a similar manner to filter  400  of FIG. 4, and thus similar reference numbers are used to indicate similar elements. However each stage, instead of having a mirror to reflect passed channels back has a return stage to send the pass channel along a separate path. Return stage 1 comprises couplers  632  and  636  and adjustable delay  634 . Return stage 2 comprises couplers  630  and  626  and adjustable delay  628 . Return stage 3 comprises couplers  620  and  624  and adjustable delay  622 . Preferably delays  604  and  634  are controlled together, as are each delay in the other stages. Add input  304  enters return stage 3, and pass+add output  306  comes out of return stage 1.  
         [0050]    [0050]FIG. 7 shows a fourth embodiment of a tunable add/drop filter  700  according to the present invention, configured to compensate for delays. Again, its operation is similar to filter  400 , so similar reference numbers are used for similar elements. The difference appears in the fiber mirrors  702  and  706 . Each includes a variable delay element, which is used to ensure that returned pass+add signals arrive at the same time. In other words, the delays synchronize the bands. Delay  704  is the longest.  
         [0051]    [0051]FIG. 8 shows a fifth embodiment of a tunable add/drop filter  800  according to the present invention, comprising a modified Lyot filter. The stages comprise Lyot filters, as shown in FIG. 2 a,  replacing the polarizers with polarizing beam splitters  802 , and adding mirrors  803  facing the perpendicular output of the polarizing beam splitters in order to reflect the light back into the filter. Only input signal  810  and drop signal  812  are shown here. Note that circulators such as those shown in FIG. 4, or other elements such as the return stages of FIG. 6, are required to enable the input, add, drop, and pass+add ports.  
         [0052]    Those skilled in the art will appreciate that various modifications to the exemplary embodiments are within the scope of the patent. For example, the technique of eliminating the need for circulators by using return stages paired with the stages, shown in FIG. 6, can be used with any of the embodiments. The fiber mirrors of FIG. 4 could be replaced with standard mirrors. The stages could be built with planar optical wave circuits, rather than fiber or bulk optics.