In all-optical networks, optical switching and routing become the most important issues for interconnecting the transport network layers. This invention describes a novel tunable optical add/drop filter for the all-optical wavelength-division-multiplexing (WDM) network applications. This filter can add or drop part of the high transmission capacity signals of a WDM link. It can be used to decentralized access point in the access network or as small core network node to realizing branching points in the network topology. It works in both wavelength and space domains. It has the advantages of: 1) High throughput and low voltage operation; 2) Wide tuning range and therefore, high channel capacity; 3) High isolation and high directivity between input and output ports; 4) Compact device packaging is possible as compares to the conventional grating and mechanical switching type of add/drop filter; 5) Multiple ports add/drop tunable filters can be realized with this invention to interconnect multiple WDM networks. This novel add/drop filter can be used in various WDM topologies. It enhances the performance of the conventional tunable filter by re-routing the rejected wavelengths back to network, which not only save the precious optical energy, but also cut down the return loss of the device.
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20. A tunable add/drop filter comprising
a nonreciprocal setup for optical channels return to WDM network, a tunable filter for predetermined optical channel selection, and a reciprocal optical add/drop setup for add/drop operations, wherein said reciprocal optical add/drop setup comprises two optical birefringent elements, and wherein said two optical birefringent elements of said reciprocal optical add/drop setup have polarization eigen planes oriented at 45° and -45° relative to a polarization eigen plane of said birefringent element in said non-reciprocal setup, respectively.
1. A tunable add/drop filter which comprises comprising:
a nonreciprocal setup for optical channels return to WDM network, a tunable filter for predetermined optical channel selection, and a reciprocal optical add/drop setup for add/drop operations, where, the non-reciprocal optical setup is a combination of an optical birefringent element which has a thickness of d, a faraday rotator that rotates eigen polarization of input light beam by 45°, a polarization beam combiner and a right angle prism placed at an entrance side of the birefringent element to recombine returned optical signals; the tunable filter for selectively dropping or adding the optical channels; and the reciprocal optical add/drop setup comprises two optical birefringent elements with thickness of (1/2)d, and with their polarization eigen planes oriented at 45° and -45° relatively to a polarization eigen plane of the birefringent element in the non-reciprocal setup, respectively. wherein said non-reciprocal optical setup comprises a combination of an optical birefringent element, which has a thickness of d, and a faraday rotator that rotates eigen polarization of input light beams by 45°, and wherein said non-reciprocal optical setup further comprises a polarization beam combiner and a right angle prism placed at an entrance side of said birefringent element to recombine returned optical signals.
7. A tunable add/drop filter comprising:
a tunable filter which is selectively adjustable to pass only desired frequencies of light waves; means for splitting an optical input polarization into two eigen orthogonal polarizations; means for rotating said two orthogonal eigen polarizations by 45° and passing the rotated orthogonal eigen polarizations to a first side of said tunable filter; means for spatially combining an optical output from a second side of said tunable filter; means for spatially combining undesired frequencies of said two orthogonal eigen polarizations which are rejected by said tunable filter; and means for outputting a combination of said undesired frequencies.
28. A tunable add/drop filter comprising
a nonreciprocal setup for optical channels return to WDM network, a tunable filter for predetermined optical channel selection, and a reciprocal optical add/drop setup for add/drop operations, wherein said non-reciprocal optical setup comprises a combination of an optical birefringent element, which has a thickness of d, and a faraday rotator that rotates eigen polarization of input light beams by 45°, and wherein said reciprocal optical add/drop setup comprises two optical birefringent elements, each having a thickness of (1/2)d, and with their polarization eigen planes oriented at 45° and -45° relative to a polarization eigen plane of said birefringent element in said non-reciprocal setup, respectively.
9. A tunable add/drop filter comprising:
selectively adjustable means for passing only desired frequencies of light waves; means for splitting an optical input signal into two eigen orthogonal polarizations; means for rotating said two orthogonal eigen polarizations by 45° and passing the rotated orthogonal eigen polarizations to a first side of said selectively adjustable means; means for spatially combining an optical output from a second side of said selectively adjustable means; means for redirecting frequencies of said two orthogonal polarizations, which are rejected by said selectively adjustable means, through said means for splitting such that said rejected frequencies follow different paths than the split optical input signal; and means for spatially combining said rejected frequencies of said two orthogonal eigen polarizations.
13. A tunable add/drop filter comprising:
at least two input ports for inputting optical signals; selectively adjustable means for passing only desired frequencies of light waves; means for splitting an optical input signal from each said input port into two eigen orthogonal polarizations; means for rotating each pair of said two orthogonal eigen polarizations by 45° and passing the rotated orthogonal eigen polarizations to a first side of said selectively adjustable means; means for spatially combining outputs received from a second side of said selectively adjustable means to form optical outputs, wherein each optical output corresponds to a respective pair of said two orthogonal eigen polarization inputted to said first side of said selectively adjustable means; means for redirecting frequencies of each of said pairs of said two orthogonal polarizations, which are rejected by said selectively adjustable means, through said means for splitting such that said rejected frequencies follow different paths than the split optical input signals; and means for spatially combining said rejected frequencies, wherein each spatial combination of rejected frequencies corresponds to a respective pair of said two orthogonal eigen polarizations rejected from said first side of said selectively adjustable means.
2. The tunable add/drop filter of
3. The tunable add/drop filter of
4. The tunable add/drop filter of
5. The tunable add/drop filter of
6. The tunable add/drop filter of
8. The tunable add/drop filter of
means for splitting said optical signal into two eigen orthogonal polarizations and passing the split optical signal to said second side of said tunable filter; and means for spatially combining desired frequencies of said split optical signal which are outputted from said first side of said tunable filter with said undesired of frequencies of said two orthogonal eigen polarizations which are rejected by said tunable filter and outputting a combined signal through said means for outputting.
10. The tunable add/drop filter of
11. The tunable add/drop filter of
means for splitting a second optical input signal into two eigen orthogonal polarizations and passing the split second optical signal to said second side of said selectively adjustable means; means for directing frequencies of said split optical signal, which pass through said selectively adjustable means, along said different paths and with the same polarizations as said rejected frequencies of said first optical signal; and means for spatially combining said rejected frequencies of said first optical signal and the passed frequencies of said second optical signal.
12. The tunable add/drop filter of
14. The tunable add/drop filter of
15. The tunable add/drop filter of
16. The tunable add/drop filter of
at least two input ports for inputting optical add signals; means for splitting said optical add input signal into two eigen orthogonal polarizations each, and passing the split optical add signals to said second side of said selectively adjustable means; means for directing frequencies of said split optical add signals, which pass through said selectively adjustable means, along said different paths and with the same polarizations as said rejected frequencies of said optical input signals; and means for spatially combining said rejected frequencies of said optical input signal and the passed frequencies of said optical add signals, wherein each spatial combination of rejected frequencies and passed frequencies corresponds to a respective pair of said input ports for inputting optical signals and said input ports for inputting optical add signals.
17. The tunable add/drop filter of
18. The tunable add/drop filter of
19. The tunable add/drop filter of
21. The tunable add/drop filter of
22. The tunable add/drop filter of
23. The tunable add/drop filter of
24. The tunable add/drop filter of
25. The tunable add/drop filter of
26. The tunable add/drop filter of
27. The tunable add/drop filter of
29. The tunable add/drop filter of
30. The tunable add/drop filter of
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In this invention, a tunable add/drop filter for the wavelength-division-multiplexing (WDM) network applications is described. This filter can add or drop part of the high transmission capacity signals of a WDM link.
The communication environment is evolving towards increasingly heterogeneous but interconnected networks. The growth of demand for existing services and the introduction of new advanced services is expected to create a large increase of traffic flow in the near future. The current evolution of telecommunication network is led by asynchronous and synchronous transfer modes (Asynchronous Transfer Mode(ATM), Synchronous Optical Network (SONET), Synchronous Digital Hierarchy (SDH)), which require primarily electronic technologies for processing and switching. Although the necessary hardware building blocks are available to design wide area networks, complex staycomposes of a comprises a tunable filter 55, a non-reciprocal optical return setup 54, and a reciprocal optical setup 56, as shown in FIG. 1. The spectra changed in the filter can be understood from 700 and 701 for the adding and dropping operations, respectively. In 700 adding operation (FIG. 1a), the channel (wavelength) to be added into the WDM network is in 711 and enters from 52. It combines with the spectrum 712, which already exists in the network, and exit exits at 53 with a combined spectrum 713. In dropping operation 701 (FIG. 1b), the network spectrum is 722. It drops part of the spectrum 721 to 52. The rest of the returned channels then re-routes re-route through 53 and go back to the network with a spectrum 723.
The light wave propagates propagation within the add/drop filter can be further explained as follow follows. In the dropping operation, the incoming network signals carry carrying multiple wavelengths enter from port 51. The non-reciprocal optical setup 54 passes spectrum 722 to the tunable filter 55. The selected channel 721 passes through the filter and the reciprocal setup 56, and exit exits at port 52. The rejected channels by the tunable filter, on the other hand, reflects reflect back to the non-reciprocal setup 54. Because of the non-reciprocal property of 54, light propagates backward in a different path as in from the forward propagating direction path. Therefore, it exits at port 53 and completes the dropping operation.
For the added operation, optical signal 711 to be added into the network enters from port 52. Because of the reciprocal setup of 56, light traveling in the reverse direction follows exactly the same path as it did in the forward direction. Therefore, spectrum 711 passes through the filter 55 that has been tuned to the channel and enters the non-reciprocal setup 54. Because of the non-reciprocal optical path arrangement, this added channel joins the rest of the rejected channels rejected by the filter in the backward propagating direction and exits at port 53. This completes the adding operations.
A preferred structure of this invention is shown in FIG. 2. The non-reciprocal setup is built by a combination of a Faraday rotator 25, a birefringent element 21, a polarization beam combiner 15, and a right angle prism 14. The optical reciprocal setup is comprised of a pair of birefringent elements 22/23 with their polarization eigen planes 212/213 perpendicular to each other, and are ±45° relative to the polarization eigen plane 211 of the birefringent element in the non-reciprocal setup. The polarization eigen plane is defined by the plane that contains the optical axis of the birefringent element and also is the plane that contains the two orthogonal polarization states, when an unpolarized light is incident onto the element. The add/drop channel is selected by the tunable filter 26. The add/drop port is designated by 12. The input and output ports to the WDM network are 11 and 13.
The detailed operations of the add/drop tunable filter are shown in FIG. 3, which is the top view of the device. The polarization progression within the filter is also indicated. In FIG. 3a, the forward dropping operation is realized by splitting the input polarization into two eigen orthogonal polarizations using the birefringent element 21. These two light beams with polarization at (0°, 90°) are then rotated another 45° by the Faraday Rotator 25 which sits inside a magnet 29 and incident onto the filter 26. The dropping channel passes through tunable filter 26 where it has been tuned to the desired resonant condition. The two spatially separated signals are recombined by the second and third birefringent element elements 22 and 23 oriented at ±45° and collected by the output lens 12. Since the thickness of each of 22 and 23 is chosen to be only 1/2 of the first birefringent element 21, the two polarizations can be combined into a single beam by orientating 23 at 90° with respect to 22. This arrangement of beam displacement allows any incoming state of polarization to be efficiently transmitted through the add/drop filter in the forward direction.
For the channels (wavelengths) that are rejected by the tunable filter 26, they backward propagate to 25 and are rotated another 45°. Because this is a non-reciprocal effect, the returned polarizations are in (90°, 0°) states and are orthogonal to their original input states. Hence, they travel at different paths when passing through 21, as shown in FIG. 3b. These two light beams are recombined by the right angle prism 14 and the polarization beam combiner 15 and send back to the WDM network.
Similarly, the added operation can be traced as shown in FIG. 3b. The light signal to be added into the WDM network first splits its polarization by 22 and 23 combination with polarization angles of (+45°, -45°). This is based on the fact that the input and output of the combined elements (22 and 23) are reciprocal. This means that light traveling in the reverse direction (i.e. the adding operation) must follow exactly the same path as it does in the forward direction. Therefore, at the exit of this the combined birefringent element (22/23), the spatial walk-off and the polarizations are identical for both forward and backward traveling light waves. With filter 26 tuned to the added wavelength, light signal passes the filter and enters 25. By adding another 45° polarization to its original state, the output polarizations become (90°, 0°), which are the same as the rejected wavelengths. They are then collected by the prism 14 and polarization combiner 15 and go into the WDM network as was explained above. This completes the add/drop operations.
The elements used in this invention are listed below for illustration. These shall not limit to the application, but are for illustration purposes only. The Faraday rotator can be those based on magneto-optic materials, for examples, yttrium iron garnet (YIG), bismuth-substituted rare earth iron garnet (RBilG), and holmium and terbium doped garnet crystals (HoTbBi)lG. The filter in this invention can be, piezo-tuned Fabry-Perot optical filters, liquid-crystal based Fabry-Perot tunable filters (U.S. Pat. No. 5,111,321, by Patel), tunable polarization interference filters (A. Title, Tunable birefringent filters, Optical Engineering, Vol. 20, pp. 815, 1981.), and acoustooptical tunable filters (X. Wang, Acousto-optic tunable filters spectrally modulate light, Laser Focus World, May 1993 1992.). When fixed filters, for example the interference filters, are used in the invention, they result in fixed add/drop filters. The polarizing materials used for the reciprocal operation can be materials with optical anisotropy, for examples calcite, rutile, lithium niobate (LiNbO3), and yttrium orthovanadate YVO4. All these Faraday rotators, filters, and polarizing crystals are commercially available.
An example of the tunable add/drop filter can be realized by using a liquid-crystal Fabry-Perot tunable filter as shown in FIG. 4. A pair of halfwave plates are inserted in front of and behind of the liquid crystal filter. A halfwave plate satisfies the equation Δnd=λ/2, where Δn and d are the birefringence and thickness of the wave plate, and λ is the light wavelength. The first wave plate 27 is added into the light path 800 to change the polarization of the decomposed input light to match the 45° optic axis of the filter 99. The second halfwave plate 28, which is placed on the opposite side of the filter, rotates the extra-ordinary light wave into ordinary in light paths 801. The two then recombines recombine by the birefringent elements 22 and 23. The rest of the operations are explained in the previous embodiment.
Due to the spatial-light-modulation capability (2-Dimensional) of a liquid-crystal Fabry-Perot filter, a multiple-port add/drop tunable filter can be realized based on the current structure. As shown in FIG. 5, this multi-port add/drop tunable filter can be easily fabricated by patterning a liquid-crystal Fabry-Perot filter into sections, and spatially aligning a series of inputs and outputs ports together. Remember, this multi-port tunable add/drop filter has exactly the same number of birefringent elements, Faraday rotator, and filter as in the single-port design. The patterning of the liquid-crystal Fabry-Perot can also be achieved photolithographically on the controlling transparent indium-tin-oxide (ITO) electrodes. Therefore, costs saving on materials and a compact packaging are possible for this multi-port filter. Potential applications include, but not limit to, are not limited to multiple WDM networks interconnections where simultaneously simultaneous performances of add/drop channels at this filter node can be achieved.
It can also combine with a N×N optical switch at the add/drop ports. In this case, multiple WDM networks are interconnected to each other and exchange information on this optical node. It operates in wavelength-space domain and is transparent to users and operators. This versatile filter will release the complex design of the high-capacity WDM network and decentralized access point decentralize access points in the access networkor as . It may also be used as a small core network node to in realizing branching points in the network topology.
When a fixed filter, for example the interference filter, is used in this invention a high throughput passive add/drop filter is realized. Here, the add/drop channel is pre-defined by the interference filter. However, only such a wavelength can go in and out of the ports.
When an 1×2 optical switch is added onto the add/drop port, as shown in FIG. 6a, the three-port add/drop filter becomes a four-port add/drop filter with it's input- and output-port separated.(See FIG. 6b) Two of this these add/drop filters 811 can be further interconnected to form a wavelength-space switching node for multi-layered WDM systems. In FIG. 6c, one of the add/drop port ports 823 of the each of the add/drop filter 811 is linked to the each other filters 811 are linked together. The channels between the two WDM systems 801 and 802 can then be shared through this interconnected optical node. Furthermore, because of the reciprocal nature of this add/drop filter at the add/drop port ports 821 and 822, optical channels can still be loaded up and down from the its WDM network 801 and 802, respectively. This greatly increase increases the flexibility design from the system's perspective.
This tunable add/drop filter can be regarded as a combination of a tunable filter and an optical circulator. It has the merits of:
1. High throughput because all of the optical energies are preserved by the re-routing characteristics of the add/drop operations.
2. Wide tuning range when liquid-crystal Fabry-Perot, piezoelectric Fabry-Perot, or acoustooptic tunable filter are used. Therefore, high channel capacity is obtainable.
3. High isolation and high directivity between input and output ports because of the use of Faraday rotator and birefringent materials.
4. Compact device packaging is possible, as compares compared to the conventional grating and mechanical switching type of add/drop filter.
5. When the tunable filter is a liquid-crystal Fabry-Perot type, multiple-port add/drop tunable filters can be realized by patterning the liquid-crystal Fabry-Perot filter into sections and spatially aligning an array of input and output fibers together. With the output ports connected to an N×N switch, a space-separated, wavelength-division demultiplexer can be realized. This multiple-port add/drop tunable filter can potentially be used to link multi-WDM networks without complicated electrooptic conversion at each networking node.
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