Patent Publication Number: US-2017371104-A1

Title: Tunable add-drop filter with an active resonator

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 61/834,113 filed Jun. 12, 2013, which is incorporated herein in its entirety. 
    
    
     GOVERNMENT INTEREST 
     This invention was made with government support under Grant No. W911NF-12-1-0026 awarded by the U.S. Army Research Office. The government may have certain rights in the invention. 
    
    
     BACKGROUND 
     The field of the invention relates generally to optical add-drop filters (ADF), and more particularly to add-drop filters including at least one active resonator that provides gain. 
     Optical add-drop filters (ADFs) have been utilized in applications ranging from optical communication (e.g., modulators, multiplexers, and switches) to optical sensing. These devices typically include two waveguides and a filter. For communication applications, it is important that ADFs have high add-drop efficiencies, low crosstalk, filter tunability to select different wavelengths to add and/or drop, and even different bandwidths. In at least some known systems, Bragg gratings, photonic crystal structures, and whispering gallery mode (WGM) ring resonators have been used as filter components in ADFs. With their micro-scale size, ultra-high quality (Q) factor (which translates into narrow bandwidth), and large free spectral range, WGM resonators have emerged as a relatively attractive candidate for ADFs. However, in addition to fabrication challenges, two problems encountered by at least some known ADFs including WGM resonators are the non-unitary add-drop efficiencies and differences in the efficiencies and crosstalks of the add and drop channels. 
     Accordingly, current technologies may suffer from significant crosstalk and low drop efficiency, resulting in relatively high losses. Furthermore, at least some known ADFs are not optically tunable. Rather, in at least some known ADFs, bandwidth and frequency tuning may be performed by varying a refractive index of a polymer coating of the WGM resonator, or by directly heating the resonator. 
     BRIEF DESCRIPTION 
     In one embodiment, an add-drop filter for transmitting at least one signal is provided. The add-drop filter includes at least two optical waveguides capable of carrying the at least one signal, and at least one active resonator coupled between the optical waveguides, wherein the at least one active resonator provides gain that counteracts losses for the at least one signal. 
     In another embodiment, an optical communication system is provided. The optical communication system includes an add-drop filter for transmitting at least one signal, the add-drop filter including at least two optical waveguides capable of carrying the at least one signal, and at least one active resonator coupled between the optical waveguides, wherein the at least one active resonator provides gain that counteracts losses for the at least one signal. The optical communication system further includes a gain stimulation device coupled to the add-drop filter, the gain stimulation device configured to provide a gain stimulation signal that excites a gain medium of the at least one active resonator. 
     In yet another embodiment, a method of transmitting at least one signal through an add-drop filter is provided. The method includes directing the at least one signal into the add-drop filter, wherein the add-drop filter includes at least two optical waveguides, and at least one active resonator coupled between the optical waveguides, and providing, using the at least one active resonator, gain that counteracts losses for the at least one signal as the at least one signal is transmitted through the add-drop filter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments described herein may be better understood by referring to the following description in conjunction with the accompanying drawings. 
         FIG. 1  is a schematic diagram of an exemplary active add-drop filter (ADF). 
         FIGS. 2A-2F  are perspective views of exemplary resonators that may be used with the ADF shown in  FIG. 1 . 
         FIG. 3  is a schematic diagram of an exemplary optical communication system that includes the ADF shown in  FIG. 1 . 
         FIG. 4  is a graph plotting experimental data of drop efficiency versus pump power of an optical gain stimulation device. 
         FIG. 5  is a graph plotting experimental data showing the change in transmission as a function of the pump power. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The systems and methods described herein are directed to an add-drop filter (ADF) that includes an active resonator (e.g., a whispering-gallery-mode (WGM) resonator). In one suitable embodiment, the WGM resonator includes an optically active gain medium such that it is an active resonator. Accordingly, the systems and methods described herein provide an optically controllable ADF. That is, by introducing an optically active gain medium in a WGM resonator, the features and performance of an ADF including the WGM resonator may be optically controlled. 
     Modern optical communication systems and networks generally require state-of-the-art ADFs, narrowband optical filters, optical routers, and optical modulators. At least some known optical filters suffer from relatively poor linewidths of resonances, as this is limited by an absorption loss of the material used for the filter. To compensate such losses, photons may be provided from a gain medium into the same resonance line of interest. An active resonator with its incorporated optically active gain medium will have resonance lines much narrower than what is achievable with passive resonators (i.e., resonators without an optically active gain medium). At least some known ADFs suffer the same problem that optical filters suffer. Thus, an ADF with active gain medium will have an improved resolution, allowing adding and/or dropping of signals whose wavelengths are spaced relatively closed. 
     Passive ADFs fabricated using a WGM resonator have add and drop efficiencies smaller than one, due to non-zero intrinsic losses of the resonator and different coupling losses between the resonator and fiber taper waveguides. However, as described herein, introducing gain into a WGM resonator and optically pumping the gain below the lasing threshold not only allows loss compensation to achieve add and drop efficiencies higher than with a passive ADF, but also reduces crosstalk and improves tunability. For example, according to at least some embodiments, an active ADF fabricated using an erbium-ytterbium co-doped microsphere may achieve 100-fold enhancement in the intrinsic quality factor, 3.5 fold increase in drop efficiency, bandwidth tunability of 35 MHz, and a crosstalk of only 2%. In other embodiments, other dopants with similar properties for providing optical gain may be used. Also, in at least some other embodiments, the WGM resonator may be any other WGM resonator, such as a micro-toroid, micro-ring, micro-bubble, or micro-bottle. By providing gain, the systems and methods described herein facilitate achieving a ratio of add and drop efficiencies very close to one, and those efficiencies are higher than those of passive ADFs. 
     In some embodiments, resonances are provided in both the optical pump band (980 nm) and the emission band (1550 nm—telecommunication band). In one embodiment, when the gain medium is pumped in the 980 nm band (e.g., using an optical pump), emission from the erbium ions into the 1550 nm band compensates a portion of the losses in the system, facilitating resonances with narrower linewidths. The narrower linewidths may be shown by comparing the linewidths of the resonances with and without an optical pump (i.e., when pump is inactive, the ADF works like conventional passive ADF). Accordingly, the active ADF has a much a narrower linewidth than the passive filter, enabling dropping and/or adding signals with wavelengths (or frequencies) very close to each other. 
     At least some known ADFs have different add and drop efficiencies. Ideally, the add and drop efficiencies are equal to each other. However, this is possible only when losses are completely compensated for. The better the compensation, the closer the add and drop efficiencies. Active ADFs, such as those described herein, address this issue as well. 
     More specifically, by increasing an optical pump power, the linewidth gets narrower as the losses are compensated more and more. Sufficiently increasing the pump power brings the effective loss relatively close to zero, such that an add-drop efficiency ratio approaches unity. Accordingly, in the embodiments described herein, the add and drop efficiencies are relatively close to each other. 
     Active ADFs in accordance with the embodiments described herein also demonstrate a reduction in the crosstalk between different ports as compared with passive ADFs. As such, the active ADFs described herein provide reduced crosstalk, similar values of add and drop efficiencies, higher add-drop efficiencies, and more precise tunable bandwidth as compared to passive ADFs. Further, in some embodiments, the drop and add wavelengths in active ADFs with gain medium can be tuned optically. In other embodiments, thermal tuning may be used to tune the add and drop wavelengths. 
     In some embodiments, an ADF fabricated using a WGM micro-resonator with a doped optically active medium provides higher add and drop efficiencies, reduces crosstalk, helps to obtain similar efficiencies for adding and dropping (i.e., the ratio of add and drop efficiencies approach unity), enables dropping and/or adding signals with smaller wavelength separations, and provides bandwidth tunability. Some of these are performance improvements over ADFs with passive WGM resonators, and others are possible only in an active ADF utilizing an active WGM resonator. Thus, active ADFs are significant tools for use in present and future optical communication networks. 
     As described herein, in some embodiments, an ADF is provided in which an erbium-ytterbium (Er3+-Yb3+) co-doped microsphere resonator is side-coupled to a pair of tapered fibers. In such ADFs, the optical gain provided by Er3+ ions helps to compensate losses in the resonator, therefore enabling a tunable add-drop bandwidth, efficiency and crosstalk. Further, in this co-doped active resonator, Yb3+ ions are doped to improve the efficiency of the optical pumping of Er3+ ions, and tunability occurs from the ability to tune the optical gain by increasing or decreasing a pump power. Different rare-earth ions can be doped singly or co-doped multiply in any concentration to provide gain at the spectral band of choice or to cover gain in many different bands. Further, as described herein, techniques other than ion doping may be utilized to provide active gain in an ADF. 
       FIG. 1  is a schematic diagram of an active ADF  100  according to one embodiment. ADF  100  includes a resonator  102  optically coupled to a first optical waveguide  104  and a second optical waveguide  106 . In this embodiment, resonator  102  is a microsphere, and first and second optical waveguides  104  and  106  are tapered optical fibers. Alternatively, resonator  102  and first and second optical waveguides  104  and  106  may be any optical components that enable ADF  100  to function as described herein. For example, in some embodiments, first and second optical waveguides  104  and  106  are planar waveguides that perform equivalently to an actual optical fiber. Further, resonator  102  is not limited to WGM resonators. That is, gain-induced performance enhancement, as described herein, can be realized in ADFs built using any suitable filter or resonator. For example, photonic crystal cavities suffer from losses that may be compensated by introducing active gain. 
     ADF  100  includes an input port  110 , a through port  112 , an add port  114 , and a drop port  116 . In operation, one or more optical signals enter ADF  100  through input port  110  and exit ADF  100  at through port  112 . Further, one or more optical signals may be added through add port  114  or dropped through drop port  116 . 
     As noted, first and second optical waveguides  104  and  106  are tapered optical fibers in this embodiment. Each of first and second optical waveguides  104  and  106  includes a first normal portion  120 , a first tapered portion  122 , a narrow portion  124 , a second tapered portion  126 , and a second normal portion  128 . In first normal portion  120  and second normal portion  128 , the tapered optical fiber has a first diameter. In narrow portion  124 , the tapered optical fiber has a second diameter smaller than the first diameter. In the exemplary embodiment, each tapered optical fiber has a length of approximately 10 millimeters (mm), and the diameter of narrow portion  124  is approximately 0.8 micrometers (μm). Alternatively, the tapered optical fibers may have any dimensions and/or characteristics that enable ADF  100  to function as described herein. 
     In the exemplary embodiment, each tapered optical fiber is prepared from a standard communication single-mode fiber having a core radius of approximately 4 μm and a cladding radius of approximately 62.5 μm. The standard communication single-mode fiber is heated and pulled above a hydrogen flame to generate the tapered optical fibers. Alternatively, the tapered optical fibers may be prepared using any methods and/or components that enable ADF  100  to function as described herein. 
     As will be understood by those of skill in the art, first and second optical waveguides  104  and  106  are optically coupled to resonator  102  at narrow portions  124 . More specifically, as optical signals pass through first tapered portion  122 , the light spreads out into the surrounding area. Accordingly, narrow portion  124  allows optical signals to couple into and out of resonator  102 . 
     As shown in  FIG. 1 , κ1 denotes coupling losses between first optical waveguide  104  and resonator  102 , κ2 denotes coupling losses between second optical waveguide  106  and resonator  102 , and κ0 denotes an intrinsic energy decay rate of resonator  102 . The intrinsic energy decay rate will be reduced by any optical gain ξ provided by ADF  100 . Specifically, at resonance, the transmission and drop efficiency of ADF  100  are given by Equations 1 and 2, respectively: 
     
       
         
           
             
               
                 
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     To avoid crosstalk, the transmission should be zero at resonance in both adding and dropping conditions. This can be satisfied for add and drop channels simultaneously only when the intrinsic loss of the cavity κ0 is zero, which is relatively difficult to achieve, due to the fact that the material or resonator  102  has a non-zero absorption loss. However, Equation 2 suggests that in order to increase the drop efficiency, the intrinsic losses should be decreased. To achieve this, optical gain can be utilized to compensate for the losses (ideally with ξ substantially equal to κ0). Then, by tuning the loss and gain in ADF  100 , one can decrease resonance linewidths, increase drop efficiency, and substantially eliminate the crosstalk. 
     As noted, in this embodiment, resonator  102  is a microsphere. More specifically, resonator  102  may be, for example, a 80 μm diameter silica microsphere fabricated by reflowing the end of a fiber tip with a high power CO2 laser. Alternatively, resonator  102  may be fabricated using any process that enables ADF  100  to function as described herein. For example, in some embodiments, resonator  102  may be fabricated using a semiconductor material. 
     In this embodiment, gain is provided in ADF  100  by introducing a gain medium to resonator  102  and using a pump laser (not shown in  FIG. 1 ) to excite the gain medium. In order to introduce the gain medium, in this embodiment, the silica microsphere is dip-coated with a layer of Er3+-Yb3+ co-doped sol-gel silica. In other embodiments, resonator  102  may be any suitable WGM resonator, and resonator  102  may be doped using other ion doping methods. For example, the resonator  102  may be doped using at least one of an ion implantation method in which ions are implanted into the material, a dip-coating method in which resonator  102  is dipped one or more times into and coated with a sol-gel material having the gain dopants, and/or a direct fabrication method wherein resonator  102  is fabricated from a sol-gel material already doped with the gain medium. 
     During operation of ADF  100 , first and second optical waveguides  104  and  106  and resonator  102  are substantially fixed relative to each other. For example, in some embodiments, first and second optical waveguides  104  and  106  are mounted to a supporting material (not shown), such as, for example, a glass base. Further, although first and second optical waveguides  104  and  106  are shown as substantially straight in  FIG. 1 , first and second optical waveguides  104  and  106  may alternatively be curved, u-shaped, and/or fixed in any shape that enables ADF  100  to function as described herein. 
     In one embodiment, ADF  100  is assembled as follows. Using a nanopositioning system, resonator  102  is placed between first and second optical waveguides  104  and  106  such that one of first and second optical waveguides  104  and  106  is very close to the resonator  102 . Then, the other of first and second optical waveguides  104  and  106  is pushed close to resonator  102  using a fiber tip placed on a positioning stage. In some embodiments, once resonator  102  and first and second optical waveguides  104  and  106  are properly positioned, the components of ADF  100  are secured (i.e., by encasing the components in a thermoplastic material) such that ADF  100  forms a packaged device. That is, ADF  100  may be packaged as a “black-box” component that leaves ports  110 ,  112 ,  114 , and  116  exposed, but encloses and protects first and second optical waveguides  104  and  106  and resonator  102 . Further, one or more components of ADF  100  may be fabricated using optical lithography and/or semiconductor processes. 
       FIGS. 2A-2F  are perspective views of exemplary WGM resonators that may be used with ADF  100 . For example, a ring resonator  202  (shown in  FIG. 2A ), a capillary resonator  204  (shown in  FIG. 2B ), a bubble resonator  206  (shown in  FIG. 2C ), a microsphere resonator  208  (shown in  FIG. 2D ), a disk resonator  210  (shown in  FIG. 2E ), and/or a microtoroid resonator  212  (shown in  FIG. 2F ) may be used in ADF  100 . Alternatively, other types of WGM resonators maybe utilized. 
       FIG. 3  is a schematic diagram of an optical communication system  300  that includes ADF  100 . System  300  includes a pump laser  302  that generates light (also referred to as a gain stimulation signal) to excite the gain medium, and a probe laser  304  that emits at least one optical signal (i.e., light) to be transmitted through ADF  100 . The light emitted by pump laser  302  and the light emitted by probe laser  304  pass through respective polarization controllers  306  and  308  and are combined before passing through a first fiber spool  310  and entering ADF  100  at input port  110 . 
     Through port  112  is coupled to a first filter  312 , which is in turn coupled to a first photodetector  314 . First photodetector  314  measures a power of the optical signal transmitted through first filter  312  and the detected power may be output to a computing device (not shown) for further processing. In this embodiment, drop port  116  is coupled to a second filter  320  through a second fiber spool  322 . Further, second filter  320  is coupled to a second photodetector  324 . Second photodetector  324  measures a power of the optical signal transmitted through first filter  320  and the detected power may be output to a computing device (not shown) for further processing. In this embodiment, first and second filters  312  and  320  are 980 nm to 1550 nm wavelength-division multiplexing filters. Alternatively, first and second filters  312  and  320  may be any filters that enable system  300  to function as described herein. 
     In this embodiment, a controller  330  is communicatively coupled to pump laser  302  and probe laser  304 . Controller  330  enables a user to control pump laser  302  and probe laser  304  (e.g., to optically tune pump laser  302  and control optical signals emitted by probe laser  304 ). Controller  330  may be a computing device or any other hardware component that enables the user to control pump laser  302  and probe laser  304 . 
     In this embodiment, pump laser  302  emits light in the 980 nm band, and probe laser  304  emits light in the 1550 nm band. Alternatively, pump laser  302  and probe laser may emit light in any bandwidths that enables system  300  to function as described herein. When pumping at around 980 nm, the Erbium ions in resonator  102  emit light at 1550 nm. This light compensates for a portion of the losses of ADF  100 . The Ytterbium, if present, acts as a sensitizer. 
     By changing a power of the signal from pump laser  302 , the level of compensation, and thus the performance of ADF  100 , can be varied. For example, as shown in graph  400   FIG. 4 , as an input power of pump laser  302  increases, the drop efficiency of ADF  100  (see Equation 2) increases. Accordingly, adjusting the pump power (e.g., using controller  330 ) facilitates selectively tuning the performance characteristics of ADF  100 . 
     Further, as shown in graph  500  of  FIG. 5 , increasing the pumping power also increases the Q factor of ADF  100  and decreases the bandwidth, allowing for finer frequency selectivity when adding and dropping optical signals. For example, in some embodiments, by increasing the pump power, the intrinsic Q factor may increase from its initial value of 5.4×106 to 1.1×108, resulting a bandwidth increase from 38.7 MHz to 72 MHz. Further, as shown by the bottom curve on graph  500 , at a sufficiently high pumping power, ADF  100  will be critically coupled such that all optical signals input into ADF  100  are coupled into resonator  102 , substantially eliminating any crosstalk. 
     Although in some embodiments, gain is provided by optically pumping a gain medium, alternatively, alternatively ADF  100  may provide other types of gain. For example, ADF  100  may utilize Raman gain, parametric gain, gain generated using quantum dots, gain provided by material properties of resonator  102 , etc. Further, in some embodiments, ADF  100  may be fabricated from semiconductor materials and electrical pumping may be utilized to provide gain. 
     Further, although the illustrated embodiments show a single resonator  102 , in some embodiments, ADF  100  and/or system  300  includes a plurality of resonators  102 . For example, ADF  100  may include an array of resonators  102  and optical waveguides  104  and  106  in some embodiments. 
     In at least some of the embodiments described herein, controlling the optical gain in a WGM resonator-based ADF can be utilized to increase add-drop efficiency and bandwidth, and reduce crosstalk. The ability to tune the optical gain by the pump power also provides tunability. This concept can be used in other types of ADFs based on WGM structures such as microrings, microtoroids, or any other optical resonator with a circular cross-section, as well as photonic crystal structures doped with appropriate gain media. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims. 
     In view of the above, it will be seen that the several advantages of the disclosure are achieved and other advantageous results attained. As various changes could be made in the above processes and composites without departing from the scope of the disclosure, it is intended that all matter contained in the above description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. 
     When introducing elements of the present disclosure or the various versions, embodiment(s) or aspects thereof, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of the elements. It is also noted that the terms “comprising”, “including”, “having” or “containing” are intended to be open and permits the inclusion of additional elements or steps. 
     All of the compositions and/or methods disclosed and claimed herein may be made and/or executed without undue experimentation in light of the present disclosure. While the compositions and methods of this disclosure have been described in terms of the embodiments included herein, it will be apparent to those of ordinary skill in the art that variations may be applied to the compositions and/or methods and in the steps or in the sequence of steps of the method described herein without departing from the concept, spirit, and scope of the disclosure. All such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the spirit, scope, and concept of the disclosure as defined by the appended claims. 
     It will be understood by those of skill in the art that information and signals may be represented using any of a variety of different technologies and techniques (e.g., data, instructions, commands, information, signals, bits, symbols, and chips may be represented by voltages, currents, electromagnetic waves, magnetic fields or particles, optical fields or particles, or any combination thereof). Likewise, the various illustrative logical blocks, modules, circuits, and algorithm steps described herein may be implemented as electronic hardware, computer software, or combinations of both, depending on the application and functionality. Moreover, the various logical blocks, modules, and circuits described herein may be implemented or performed with a general purpose processor (e.g., microprocessor, conventional processor, controller, microcontroller, state machine or combination of computing devices), a digital signal processor (“DSP”), an application specific integrated circuit (“ASIC”), a field programmable gate array (“FPGA”) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. Similarly, steps of a method or process described herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. Although preferred embodiments of the present disclosure have been described in detail, it will be understood by those skilled in the art that various modifications can be made therein without departing from the spirit and scope of the disclosure as set forth in the appended claims. 
     A controller, computing device, or computer, such as described herein, includes at least one or more processors or processing units and a system memory. The controller typically also includes at least some form of computer readable media. By way of example and not limitation, computer readable media may include computer storage media and communication media. Computer storage media may include volatile and nonvolatile, removable and non-removable media implemented in any method or technology that enables storage of information, such as computer readable instructions, data structures, program modules, or other data. Communication media typically embody computer readable instructions, data structures, program modules, or other data in a modulated data signal such as a carrier wave or other transport mechanism and include any information delivery media. Those skilled in the art should be familiar with the modulated data signal, which has one or more of its characteristics set or changed in such a manner as to encode information in the signal. Combinations of any of the above are also included within the scope of computer readable media. 
     This written description uses examples to disclose the disclosure, including the best mode, and also to enable any person skilled in the art to practice the disclosure, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the disclosure is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.