WO2010077581A2

WO2010077581A2 - Low latency optical interconnect using refractive index reduced fiber optics

Info

Publication number: WO2010077581A2
Application number: PCT/US2009/066897
Authority: WO
Inventors: Lawrence Gunn
Original assignee: Lawrence Gunn
Priority date: 2008-12-08
Filing date: 2009-12-04
Publication date: 2010-07-08
Also published as: WO2010077581A3

Abstract

Hollow core optical fibers are used to provide interconnect between a financial exchange and a computer system. The low latency provided by the low refractive index of the hollow core allows remote receipt of financial market data in a timeframe not possible by other means. Similarly, transaction orders may be transmitted and processed more rapidly than allowed by other means.

Description

LOW LATENCY OPTICAL INTERCONNECT USING REFRACTIVE INDEX

REDUCED FIBER OPTICS

Priority Application

[0001] This application claims priority under 35 U.S. C. § 119(e) from U.S. Provisional Patent Application Serial Number 60/xxxxxx, entitled "Low Latency Optical Interconnect," filed December 08, 2008, which is hereby incorporated herein by reference in its entirety.

Background of the Invention Field of the Invention

[0002] The present invention is directed to communication systems, in particular to low latency fiber-optic communication systems. Description of the Related Art

[0003] Algorithmic trading firms use low-latency connections to financial market information, combined with a computer running an algorithm to make rapid trading decisions that enable them to take advantage of short-lived market conditions. As there are a number of financial firms engaged in this practice, there is competition, and there is a drive to lower latency access to market data, and to faster algorithms to make decisions. The drive for lower latency has led to proximal or co-location of the computers with the exchange servers, in the order to eliminate time-of-flight (TOF) latency. However, there is still some TOF latency, due to cabling and the size of the data centers. In addition, there is considerable latency between exchanges that are geographically separated. A means to reduce the latency of these connections would be advantageous to a player in this space. [0004] The major financial institutions are currently investing in ways to eliminate single-digit microseconds from the market data latency, and some are starting to focus on nanosecond-level improvements. Examples of latency sensitive entities that would benefit from the low latency communications system are the major financial exchanges, (such as stock, commodity, option, and currency exchanges) and their customers, which can be the major financial institutions or market data resellers.

[0005] The data rates required for these low latency connections have driven the adoption of fiber-optic transmission media. However, these fiber optic cables are composed of glass, in which light travels -66% the speed of light in vacuum or air. Thus, if one were to launch two signals at the same time, one down a fiber-optic cable, and the other in free-space, then the free space signal would travel substantially faster than the fiber optic signal. However, the free-space signal has other disadvantages, such as losses in the atmosphere, beam spreading, impact of local weather conditions, and path disruptions by mechanical objects, such as birds. Thus, the ability to combine the speed of free-space communications with the advantages of fiber-optics is desirable.

[0006] One technology that has been developed in recent years is the hollow-core fiber. A hollow-core fiber can be of two main types, a fiber containing a Bragg reflector in the cladding, which serves as a mirror to keep the light confined to the core, or a fiber containing a photonic crystal reflector region surrounding the lower refractive index core. This reflector need only reflect light at the glancing angles required for fiber transmission. It can be drawn from parallel placed preform glass tubes, and by leaving out tubes in the center of the fiber, it's possible to draw a fiber that both supports single mode operation, and has adequate loss characteristics. Such a fiber is disclosed in US patent 6,985,661, which is hereby incorporated in entirety by reference. Note that there are a wide variety of examples of similar hollow-core fibers.

[0007] The high power handling capacity of hollow-core fibers has led to their use in deliver of high power laser beams for use in cutting or adhesive curing applications. Additionally, the unique dispersion properties of the fiber have led work with super-continuum generation and other scientifically useful phenomena. However, the high optical losses, comparably high dispersion, high cost and difficulty of implementation have not led to the adoption of these hollow-core fibers in communications systems. Furthermore, the use of the low-latency properties of these fibers has not been exploited in communications systems specifically designed for latency reduction, such as those required in financial trading operations.

Summary of the Invention

[0008] The present invention involves a communications system wherein replacement of solid core optical fibers is performed, at least partially, with hollow-core fibers. This will allow the propagating signals to traverse the communications link more rapidly. The amount of latency removed from the system is proportional to the distance to be traveled. In a conventional fiber optic cable, light travels lkm in 4.9μsec, whereas in vacuum, light takes only 3.3μsec to traverse lkm. Thus, there is a 1.6μsec advantage for every kilometer traveled in free space vs. a conventional fiber. For a 300m link this is ~500nsec advantage, but the advantage for a link of -1000km, say between Chicago and New York, is 1.6ms. This is particularly advantageous when the data is time-sensitive, or when there is an advantage to receiving the data more quickly than by traditional means. [0009] The immediate problem is that free space transceivers and hollow fibers do not have the range of common single mode fiber optics due to relatively high optical losses. Thus, if one is to construct a link of substantial distance, it is necessary to employ amplification and/or regeneration periodically. The trick is to balance the frequency of regeneration and the associated latency, with the quality of the data link. For example, in the case of the hollow core fiber, it might be necessary to place repeaters or amplifiers more frequently. One way to do this is to place transceivers with contain regeneration, such as CDR circuitry, back to back. This could be done by placing two XFP transceivers in back to back format at the appropriate distances.

[0010] One issue with regular fiber optical transmission is that the launch power is restricted due to non-linear effects in the fiber material. In a hollow core fiber, these limitations are substantially altered. As a result, one exemplary architecture is to have an optical amplifier immediately after the transmitter, thus boosting the optical power prior to transmission. This is not an option with standard links, but exploits the advantage of having the majority of the optical power in air.

[0011] Another issue with deployment is that the current transmission standards allow for regeneration or amplification once every 40 or 80km. However, the loss of the hollow-core fiber is likely to be too high in order to transmit that far between the transmitter and receiver without amplification. While the addition of intermediate access points that allow regeneration or amplification is possible, it is also costly, and as a result is less than desirable. Thus, it might be worthwhile to include distributed amplifiers in the fiber cable itself. In this case, it would be necessary to provide the optical pump power remotely, from the existing access points. Thus, it is possible to add additional fibers to the cable which serve the primary purpose of distributing the optical pump power to the amplifier locations. This could be achieved with one new fiber for each amplifier, or by adding taps that distribute an appropriate amount of power from a pump distribution fiber to each amplifier. This pump could be co or counter-propagating, or both.

[0012] Another issue with hollow-core fiber links is splicing between lengths of the fiber, which is commonly available only in shorter lengths, or splicing between hollow-core fiber and solid core fiber, such as amplifiers. However, these splices commonly create unwanted reflections. As such it might be necessary to create a free- space optical connection between the two fibers, where the solid fiber can be polished with an angle to prevent reflections. This could be accomplished using a pair of collimators, packaged in a narrow form factor such that it can be included in the fiber optic cabling. As it is undesirable to have water or other absorbing media enter into the core of the waveguide, it will perhaps be necessary to provide a means of sealing, perhaps hermetically, these open splice points. One can take advantage of the free space splice in order to provide simultaneously, and in the same hardware, a sealing interface that allows for optical transparency between the two fibers.

[0013] There are two major options to consider for low-latency optical links: a free-space optical (FSO) link and a link composed of hollow- fiber. Intermediate options include operating a standard fiber at a wavelength that runs slightly faster in the fiber than a standard wavelength, or using fibers, such as reduced refractive index cladding fibers, or reducing the core refractive index. These are technically easier to achieve, however, the benefit is substantially less, probably under 1% whereas the free-space or hollow-fiber can be up to -33% less.

[0014] For purposes of this application, hollow core fibers will be defined as those where a portion of the optical mode travels in a gaseous or vacuum region, and reduced index fibers will be defined as those where all of the light travels in a physical medium, but where the index of refraction is reduced compared to standard optical fibers today. An embodiment of a fiber containing both attributes would be a fiber with a core comprising sol-gel material, which has voids, yet also a physical presence. There are numerous ways to construct low latency fibers using a combination of these techniques which can be substituted for hollow core fibers in this discussion, though the simple single mode hollow core fiber is a preferred embodiment. Note that a common property of the lower-latency hollow-core fibers is that the refractive index of the core is less than that of the cladding, requiring replacement of refractive index based guiding. Refractive index guiding is based on satisfying the phase conditions of reflections of light in the core off of the lower index cladding, which is guaranteed by Snell's law under a critical angle. The other techniques, which have been developed more recently, rely in thin film mirrors or diffractive optics such as photonic crystals, for confinement. Much of this information is obvious to one skilled in the art of optical fiber communications and is covered in detail in many texts, such as in Jeff Hecht's "Understanding Fiber Optics," which is hereby incorporated herein by reference in its entirety.

[0015] Note that in the case of free-space optical links, there are a number of issues with uptime and link availability due to weather and atmospheric conditions. As a result it is typical to employ coding schemes, such as Forward Error Correction (FEC) to lower the bit error rate. However, this incurs a very large latency penalty, and as such is not useful for a low latency link. In a FSO link, it would be necessary to implement a simple repeater system, without considerable data storage at each node that is required for FEC. Note that coding such as 8b 10b can be implemented at the source and terminal ends, and simply repeated through the nodes. Similarly, FEC could be implemented only at the source and terminal, while each of the intermediate nodes simply serves as a repeater.

[0016] For the low latency fiber options, it is possible to employ a number of well known telecommunications techniques such as Wavelength Division Multiplexing (WDM), which can increase the bandwidth or subscribers that a single fiber can accommodate. Also, it is possible to use WDM for bi-directional transmission, thus reducing the costs of fiber deployment.

[0017] During deployment of a new fiber, there are major costs associated with the physical deployment of the fiber itself. Thus, the most efficient deployment scheme might use existing fiber in metro, urban and other areas where deployment is expensive, but install the hollow-core fiber in low-cost areas, such as rural deployments and areas where conduit is already installed.

[0018] It is sometimes desired to both transmit and receive data in a low latency form, such that both financial trade data and trade instructions can be transmitted with low latency. As such, not only will trade information be broadcast, but it is also potentially advantageous to have the ability for a customer to transmit, on demand, data or instructions back to the exchange. [0019] In order to evaluate trading opportunities based on information between two geographically distinct trading systems, it might be preferable to place an algorithmic trading center physically between the two exchange computers, so that the combined latency of the two exchanges is only a fraction of the distance between the two exchanges instead of the entirety of the distance between two exchanges. Thus placing an algorithmic trading computer between two exchanges would allow a latency optimized algorithm which requires input from both exchanges. The optimal positioning would be such that exactly half of the latency of the total path is incurred on either side of the algorithmic trading center. It is likely that this half-time point is similar to the halfway point physically between the two exchanges, but there are likely variances on this due to details of the fiber plant, routes taken, etc.

[0020] For long distance links, it will be necessary to employ hollow core fiber which is single mode, however, for short lengths, it is possible to use multimode hollow- core fiber.

[0021] Some links can be submarine links, passing under large bodies of water. In such a case, not all the link need be hollow core, but if a portion of the link is hollow core, there will still be a substantial latency advantage. In this case, the receivers/transmitters on either end, perhaps in one country, could be transmitting data to another country via a link that consists at least partially of a hollow core fiber. This could also be a terrestrial link, although there are many interesting submarine links as well.

Brief Description of the Drawings

[0022] Preferred embodiments of the present invention are described below in connection with the accompanying drawings. [0023] FIGURE 1 is an exemplary embodiment of the communication system involved in transmission of low latency data to a remote data analysis system.

[0024] FIGURE 2 illustrates the communication system involved in transmitting low-latency commands from a remote analysis system to a system capable of processing those commands.

[0025] FIGURE 3 provides an exemplary embodiment where only a portion of the communications link is implemented with hollow-core fiber.

[0026] FIGURES 4 and 5 provide more detailed examples of how hollow-core fiber would be implemented in a system as a portion of the communications link as shown in FIGURE 3.

[0027] FIGURE 6 illustrates an exemplary embodiment of a hollow-core fiber link configured with optical amplification.

[0028] FIGURE 7 illustrates an exemplary embodiment of a hollow-core fiber configured to carry bi-directional traffic through the construction of wavelength-division- multiplexing (WDM) elements on both ends of the link.

[0029] FIGURES 8 and 9 illustrate exemplary embodiments where the data source, low-latency links and command interface have various geographical arrangements.

Detailed Description of the Preferred Embodiment

[0030] A preferred embodiment of the invention is illustrated in FIGURE 1, where a dynamic system 101, such as a financial exchange, is connected at least partially via a hollow core fiber 107 to a remote analysis system 109. In many systems, such as is the case with financial exchanges, there is an interface 102 provided at one or more locations, which can be used to retrieve financial information through query or by monitoring streaming data. The data is provided via a link 103 operating using one of a number of standard communications protocols, such as Ethernet. The data received from this link enters a communications system 104 which is used to convey the data via another standard link 108 to a remote analysis system 109. Inside of the communications system 104 a variety of protocols can be implemented and utilized during transmission, which are well known to one skilled in the art of communications and which will not be detailed here in the interest of brevity. However, at some point in the system 104, transmission over some distance is done at least partially with a hollow-core fiber 107 which is configured between a transmitter 105 and a receiver 106 which are physical layer devices capable of converting the incoming data stream, which could be either optical or electronic, into the appropriate optical signal for transmission down the link containing the hollow core fiber 107.

[0031] Upon receipt of the data at the remote analysis location, the data is processed and commands are issued, such as buy or sell orders. The issue of these commands is covered in FIGURE 2. In this drawing, the remote analysis system could be the same as that shown in FIGURE 1 , or it could be located in proximity to an information source, such as 101. The key element in this embodiment is that the location of the command origin 209 and the command destination 201 are remote from each other. In this case, the communications system is similar in description to that shown in FIGURE 1 , though instead of carrying a data stream, it will be carrying commands from the remote analysis system 209 to the financial exchange 201. [0032] Note that the definition of the term "remote" spans systems which are distant enough to require utilization of optical communications. In some instances this is as short as 10s of meters or even less, while more commonly this is longer distance spanning many kilometers. The preferred embodiment envisions utilization of the latency benefits that accrue from transmission over long distances, but it is also envisioned that shorter distances will become increasing relevant as data rates and computational power increase.

[0033] In an alternative embodiment of the invention shown in FIGURE 3 includes a link only partially traversed with hollow-core fiber 107, and which may contain one for more lengths of conventional fiber 303, 305. While this length of hollow core fiber 107 is shown mid-span, this need not be the case, nor would the number of hollow core spans be limited to one. The key element is that the incoming command or data 301 is transmitted by a transmitting apparatus 302 down a span containing at least a partial span of hollow-core fiber 107 before arriving at the receiver 306 after which it is transmitted via a link 307 to the remainder of the communications system.

[0034] An alternative embodiment of the span between the initial transmitter 302 and receiver 306 is shown in FIGURE 4. In this example, a conventional fiber optic link 303 (optional) delivers data to a transponder 401 which then transmits the data down a hollow-core fiber 107 to another transponder 402. Note that the number of transponders employed depends on the distance between transponders and the total distance between the ultimate transmitter 302 and receiver 306. In most cases it will be advantageous to utilize hollow core fiber for 100% of the distance, but in some cases, where pre-existing infrastructure or challenging transmission conditions such as undersea cables or in a dense metropolitan environment, it might be advantageous to use a conventional link 303. The transponders consist of a receiver which converts the optical signal to an electrical signal, where it is amplified and conditioned before being retransmitted as an optical signal. Since this is an active device, a source of local electrical power 403, 404 is required to power the transponders. This electrical power could be provided via cable that runs alongside the fiber optics to a distant power source, or it could be sourced locally. Each source of power may provide electricity to more than one transponder via cabling.

[0035] Yet another alternative embodiment involves a span at least partially comprising one or more lengths of hollow-core fiber 107 and partially comprising one or more lengths of conventional fiber 303, 305. In this embodiment, power is not required at the transition between fiber types as passive optical mode converters 501 502 are utilized. These mode converters comprise a collimation system capable of focusing light from hollow core fiber 107 onto conventional fibers 303 or 305. Note that in this case, data or commands may be transmitted in either direction without impacting the design of the optical mode converters.

[0036] Yet another alternative embodiment involves a span comprising hollow core fiber 107 and an optical amplifier 601 which is used to compensate for the high transmission losses. For an optical amplifier such as an Erbium doped fiber amplifier (EDFA), an optical pump 602 need be provided. This can be provided either via a pump laser co-located with the amplifier, or remotely via a conventional fiber system used to distribute the optical pump power. Since an EDFA is a solid core system, some interface between the EDFA and the hollow core fiber, such as the optical mode converter 501 is required.

[0037] In a final embodiment of the link between transmitters where bidirectional traffic is shared on a single hollow-core fiber 107 through the use of WDM elements 701 702 at the ends of the fiber. In this case, data presented at the input port 703 of the WDM element 701 would be transmitted to the output port 704 of WDM element 702. Similarly, data from port 705 would be transmitted to port 706 along the same length of fiber.

[0038] Note that a number of these embodiments may be used in conjunction with each other. For example, bidirectional traffic many be carried through an EDFA. Such rearrangements of the described embodiments are well known to one skilled in the art of optical communications, and thus some deviation from these described embodiments could still fall within the bounds of the invention.

Claims

WHAT IS CLAIMED IS:

1. A Communications system configured to connect a financial market to a remote computer system wherein a hollow core optical fiber is configured to carry signals over at least a portion of the distance between the financial market server and the remote computer system.

2. A communications system as in claim 1 wherein the data comprises the live stream of market status.

3. A communications system as in claim 1 wherein the data comprises trade commands issued by the remote computer to the financial market interface.

4. A communications system as in claim 1 wherein the remote computer system utilizes an algorithm to make transaction decisions.

5. A communications system and remote computer system as in claim 4 where the algorithm is designed specifically to take advantage of the existence of the low latency link.

6. A communications system and remote computer system as in claim 4 where the algorithm is designed specifically to compute trading commands in a period of time less than the latency advantage provided by the hollow core fiber.

7. A computer system connected to a plurality of financial exchanges through communications networks where at least one of the connections comprises a hollow core optical fiber.

8. A computer system as in claim 7 wherein the computer system operates an algorithm designed to evaluate incoming market data inbound over at least one of the communications networks and to issue trading commands over at least one of the communications networks.

9. A method of performing computer assisted algorithmic trading on a financial exchange which involves: receiving exchange data from a plurality of financial exchanges over a corresponding plurality of communications networks, processing of the received data utilizing a computerized algorithm for evaluation of profitable trading opportunities, transmitting commands to a second plurality of financial exchanges over a second plurality of communication networks, where the second plurality of financial exchanges might encompass a number of the financial exchanges also contained in the first plurality, and where the second plurality of communications networks might encompass a number of the communications networks also contained in the first plurality, where at least one of the communications networks in either the first of second plurality of communications networks comprises a hollow core fiber.

10. A communications system, including a photonic crystal fiber comprising: a region of substantially uniform, lower refractive index; said lower refractive index region substantially surrounded by cladding which includes non-coaxial regions of higher refractive index and which is substantially periodic, wherein the regions of higher refractive index are made of canes, and wherein the region of lower refractive index has a longest transverse dimension which is longer than a single, shortest period of the cladding, whereby wherein the region of lower refractive index has a longest transverse dimension which is sufficiently large to provide that light is substantially confined in the lower refractive index region by virtue of a photonic band gap of the cladding material and is guided along the fiber, where the data transmitted through the telecommunications system directly connects to a financial exchange computer and a computer performing algorithmic trading.