Abstract:
Optical systems are provided. One such system includes an optical transmission path that is defined, at least partially, by a variable optical delay system. The variable optical delay system incorporates a variable refractive index component that is arranged to receive an optical signal. The variable optical delay system provides a control input to adjust a refractive index of the variable refractive index component so that latency of the optical signal can be altered. Methods and other systems also are provided.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to optics. In particular, the invention relates to systems and methods for providing optical time delays to optical signals. 
   2. Description of Related Art 
   Much of modern progress is associated with the increasing prevalence of computers and related devices. As computers have become interconnected, e.g., via the Internet, they place increasing demand on communications systems for additional bandwidth. In response to this demand, there has been a trend to replace electrical communication channels with optical channels. Concomitantly, there has been a drive to develop optical counterparts to electrical-based communication devices. For example, optical fiber was developed for propagating optical signals, whereas copper wire has been used for propagating electrical signals. 
   As is known, packet-switching networks divide data into packets that can be communicated separately. The electrical switches that implement routing for each packet typically store each packet while a header of the packet is read. In particular, the header of a packet includes information corresponding to the intended destination of the packet. Therefore, by reading the header, routing of the packet can be determined. 
   Conventionally, electrical memory devices are used to store a packet while the header of the packet is read and to delay the packet until an appropriate channel becomes available for routing the packet. However, converting an optical signal to and from electrical form so that the packet can be stored by an electrical memory device adds delay to the switching operation. Thus, a packet may be delayed for a longer period of time than that required for routing. Additionally, converting an optical signal to and from electrical form adds to expense, e.g., component expense associated with demultiplexing the optical signal, discretely delaying each of the constituent signals, and re-multiplexing the signals. 
   Therefore, it should be appreciated that there is a need for improved systems and methods that address the aforementioned and/or other perceived shortcomings of the prior art. For instance, what is needed are systems and methods for achieving more precise control of packet delays so that communications throughput can be enhanced. 
   SUMMARY OF THE INVENTION 
   The present invention involves optically delaying optical signals. In particular, systems and methods in accordance with the invention use components that exhibit indexes of refraction that can be changed. Since the speed of light is dependent upon the index of refraction of the material through which the light propagates, propagation delay of an optical data signal can be controlled by controlling the index of refraction of the material. 
   An optical system in accordance with the invention includes an optical transmission path that is defined, at least partially, by a variable optical delay system. The variable optical delay system incorporates a variable refractive index component that is arranged to receive an optical signal. The variable optical delay system provides a control input to adjust a refractive index of the variable refractive index component so that latency of the optical signal can be altered. 
   A method for delaying an optical signal propagating along an optical path in accordance with the invention includes: receiving the optical signal, and altering a refractive index of at least a portion of the optical path to alter a propagation delay of the optical signal. 
   Clearly embodiments of the invention may exhibit features and/or advantages in addition to, or in lieu of, those mentioned above. Additionally, other systems, methods, features and/or advantages of the present invention will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems and/or methods be included within this description, be within the scope of the present invention, and be protected by the accompanying claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Many aspects of the invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views 
       FIG. 1  is a schematic diagram of an optical system in accordance with the present invention. 
       FIG. 2  is a flowchart depicting functionality of the variable optical delay system of FIG.  1 . 
       FIG. 3  is a schematic diagram of an embodiment of a variable optical delay system that can be used in the optical system of FIG.  1 . 
       FIG. 4  is a schematic diagram of another embodiment of a variable optical delay system that can be used in the optical system of FIG.  1 . 
       FIG. 5  is a schematic diagram of another embodiment of an optical system in accordance with the present invention. 
       FIG. 6  is a flowchart depicting functionality of the variable optical delay system of FIG.  5 . 
       FIG. 7  is a schematic diagram of an embodiment of a variable optical delay system that can be used in the optical system of FIG.  5 . 
       FIG. 8  is a schematic diagram of another embodiment of a variable optical delay system that can be used in the optical system of FIG.  5 . 
   

   DETAILED DESCRIPTION 
   As will be described in detail here, the present invention involves delaying optical signals. In this regard, reference is made to  FIG. 1 , which is a schematic diagram depicting an embodiment of an optical system  10  in accordance with the present invention. 
   As shown in  FIG. 1 , optical system  10  includes a variable optical delay system  100 . Variable optical delay system  100  receives optical signals, such as input optical signal  110 , and routes the optical signals to provide output signals, such as output optical signal  120 . As will be described in greater detail later, variable optical delay system  100  is able to delay a received optical signal optically. Functionality of the variable optical delay system  100  of  FIG. 1  will now be described with reference to the flowchart of FIG.  2 . 
   As shown in  FIG. 2 , the functionality (or method)  100  may be construed as beginning at block  210 , where an optical signal is received. In block  220 , a refractive index of at least a portion of an optical path is altered. More specifically, at least a portion of the optical path through which the optical signal is to propagate is altered to exhibit a change in refractive index. This enables selected optical delays to be imparted to the optical signal. 
     FIG. 3  is a schematic diagram of another embodiment of an optical system in accordance with the invention. In  FIG. 3 , optical system  300  includes an optical source  302 . Note, optical source  302  can be selected from a variety of configurations including lasers, for example. 
   Optical system  300  defines an optical path  308  and includes a variable refractive index component  310  arranged along the optical path. Variable refractive index component  310  incorporates a material, the refractive index of which is capable of being changed. By way of example, the refractive index of the material may change in response to the application of an electric field. For ease of description, variable refractive index component  310  will be described as a liquid crystal cell, the refractive index of which is changed by applying or removing an electric field from the cell. 
   An electric field generator  312  applies an electric field across cell  310 . More specifically, the liquid crystal cell  310  is arranged between plates  314  and  316  of the electric field generator. An electric field is generated by opposing electrical charges, provided from electrical source  320 , accumulating on the plates. The flow of electrical charges from the electrical source to the plates is initiated by controller  322  providing a control input for actuating switch  324 . 
   Controller  322  receives electrical signals from an optical-to-electrical converter  326 . Optical-to-electrical converter  326  receives portions of the optical signals propagated along optical path  308  and converts the received portions of the optical signals to electrical signals. The controller  322  analyzes the electrical signals and provides a control input to the switch  324 . Note, controller  322  can be implemented in software, firmware, hardware, or a combination thereof. 
   Subjecting the liquid crystal material of cell  310  to an electric field causes a change in the refractive index of the cell, which changes the speed of the optical signals propagating through the cell. Consequently, since the cell is arranged along the optical path of optical signals  302 , application of the electric field causes the propagation time of the optical signals through the optical time delay system  301  to change, e.g., the optical signals are optically delayed differently than that which would have occurred without changing the refractive index. 
     FIG. 4  illustrates another embodiment in accordance with the invention. In  FIG. 4 , optical system  400  includes two variable refractive index components,  402  and  404 , that are arranged in series along an optical path  406 . An electric field generator  408  applies an electric field selectively to one or more of the components  402 ,  404 . More specifically, the electric field generator includes two pairs of plates  410 ,  412  and  416 ,  418  that are electrically coupled in series with a DC voltage source  420 . 
   Switches  422  and  424 , which control the flow of charges from DC voltage source  420 , are selectively actuated by controller  430 . In particular, controller  430  enables electric fields to be applied to none, either one, or both of the components  402 ,  404 , based upon the respective switch positions. As shown in the embodiment of  FIG. 3 , an optical-to-electrical converter ( 432 ) converts portions of the optical signal into electrical signals for use by the controller. 
   Another embodiment of an optical system in accordance with the invention is depicted schematically in FIG.  5 . As shown in  FIG. 5 , optical system  500  includes a variable optical delay system  502  that includes a variable refractive index system  510  and a path routing delay system  520 . An input transmission medium  530  propagates optical signals to the variable optical delay system, and an output transmission medium  540  receives optical signals from the variable optical delay system. 
   Variable refractive index system  510  is used to impart selected optical delays to optical signals in manners similar to those described before with respect to  FIGS. 3 and 4 . In particular, variable refractive index system  510  alters the refractive index of a material through which the optical signals propagate in order to affect the propagation delay of the optical signals. 
   In contrast, path routing delay system  520  is used to impart selected optical delays to optical signals by directing the optical signals to various optical path segments (not shown in FIG.  5 ), each of which imparts a fixed optical delay to an optical signal. For instance, each of the optical path segments could be formed of the same material and exhibit a different length than that of another segment. Thus, each of the optical path segments would impart a different optical delay to an optical signal. In some embodiments, the routing system provides course delays to optical signals while the variable refractive index system is used to add fine delays. Functionality of optical system  500  is depicted in the flowchart of FIG.  6 . 
   As shown in  FIG. 6 , the functionality (or method)  500  may be construed as beginning at block  610 , where an optical signal is received. In block  620 , a determination is made as to whether the optical signal is to be delayed. By way of example, the optical signal may be delayed in order to read a header associated with a data packet carried by the optical signal. If it is determined that the optical signal does not require delaying, the process may proceed to block  630 , where the optical signal is routed, i.e., the signal is provided to an output transmission medium. If, however, it is determined that delaying is required, the process may proceed to block  640 . 
   In block  640 , the optical signal is directed to an optical path segment which imparts a fixed optical delay to the optical signal. Typically, multiple optical path segments are provided, each of which is capable of imparting a different optical delay to an optical signal. Thus, optical delays of various durations can be imparted to the optical signal. In block  650 , a subsequent determination can be made as to whether additional delaying of the optical signal is required. If it is determined that no further delaying is required, the process may return to block  630  (described before). If, however, it is determined that additional delaying is required, the process proceeds to block  660 . 
   In block  660 , the refractive index of a portion of the optical path along which the optical signal propagates can be altered. In particular, the refractive index of a portion of the optical path is altered to delay the optical signal. Thereafter, the process may proceed to block  630 , where the optical signal is routed. 
     FIG. 7  is a schematic diagram depicting an embodiment of an optical system  700  in accordance with the invention that includes a variable optical delay system  702 . The variable optical delay system incorporates a path routing delay system  704  and a variable refractive index system  706 . 
   Path routing delay system  704  includes a switch  705  that selectively, optically communicates with multiple optical path segments, e.g., segments  707 ,  708 ,  710  and  712 . Each of the segments imparts a different optical delay to an optical signal. In the embodiment of  FIG. 7 , each of the optical segments is shaped as a loop that is formed of optical fiber. Clearly, various other shapes and media can be used. 
   Switch  705  receives optical signals  720  via input transmission medium  722  and selectively routes the optical signal to one of the optical path segments. Depending upon the length of the segment selected, the optical signal is delayed by one of the four delays provided by the segments. The delayed optical signal may then be further delayed by the variable refractive index system  706 . 
   Variable refractive index system  706  provides a selected delay to the optical signal by altering the refractive index of a portion of the optical path through which the optical signals propagate. In particular, the variable refractive index of portion  730  of optical path  732  is altered by application of an electric field. Plates  734  and  736  of an electric field generator are used to apply the electric field. This is accomplished by controller  738  actuating switch  740  so that charges from electrical source  742  can build on the plates. Note, in this embodiment, the signals provided to the controller are maintained in the optical domain. 
   Preferably, delay provided by the variable refractive index system  706  is more refined than that provided by the path routing delay system  704 . Thus, the routing system can be used to provide coarse delays to optical signals, while the variable refractive index system  706  provides fine-tuning of the delay. 
   In some embodiments, optical signals can be routed through multiple paths segments of a path routing delay system. This can provide a range of delays for the optical signals. For instance, an embodiment can include path segments that impart 0.5 psec, 1 psec, 2 psec, 4 psec and 8 psec delays, respectively. Thus, by directing the optical signals through various combinations of the segments, various incremental optical delays are provided. 
     FIG. 8  is a schematic diagram depicting another embodiment of an optical system in accordance with the invention. In  FIG. 8 , optical system  800  includes a variable optical delay system  802  that incorporates a path routing delay system  804  and a variable refractive index system  806 . The path routing delay system  804  includes switches  808  and  810  that optically communicate with each other in series. Each of the switches is able to direct an optical signal along alternative optical path segments. In particular, switch  808  can direct optical signals to either segment  812  or  814 , and switch  810  can direct optical signals to either segment  816  or  818 . Clearly, various other numbers of switches and/or segments per switch can be used. 
   Each segment associated with a switch typically is able to impart a different optical delay to an optical signal. In the embodiment of  FIG. 8 , one of the segments of each switch is relatively straight, while the other segment is curved. Since the segments are formed of similar optical fibers, the curved segments delay optical signals longer than the straight segments. Clearly, various other shapes and media can be used. 
   Variable refractive index system  806  receives optical signals from the routing system. The variable refractive index system  806  provides a selected delay to the optical signals by altering the refractive index of portion  820  of optical path  822 . In particular, plates  824  and  826  are used to apply an electric field to portion  820 , such as described before with respect to portion  730  of FIG.  7 . Note, the controller(s) used for providing control inputs to the routing system and variable refractive index system have been omitted for ease of description. 
   The foregoing description has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Modifications and/or variations are possible in light of the above teachings. The embodiments discussed, however, were chosen and described to provide illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. 
   By way of example, embodiments of the invention have been described with respect to the use of liquid crystal cells, the refractive indexes of which are changed by alternately applying and removing electric fields. Various other materials, however, can be used. For instance, electro-optic materials such as electro-optic solids can be used. Examples of such solids include polymers and crystals. Materials, the indexes of refraction of which change when subjected to mechanical, thermal, and/or chemical influences, also can be used. 
   Note, propagation of optical signals may involve polarization dependency. Clearly, compensation techniques pertaining to polarization can be used by various embodiments of the invention. Additionally, the embodiments presented each include single inputs and single outputs for routing optical signals. Embodiments using various numbers of inputs and outputs can be used. Furthermore, electric field have been described with respect to using plates, however, various other structures such as electrodes could be used. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly and legally entitled.