Low differential delay chromatic dispersion compensator

A chromatic dispersion compensator with low differential delay is provided that includes a chirped fiber Bragg grating in a compensating optical fiber core. The chirped fiber Bragg grating includes wavelength gratings spaced at distances varying with respect to the length of the compensating optical fiber core to compensate for differential delay in a synchronous time protocol for a bidirectional computer data communication link. The chromatic dispersion compensator also includes an optical junction to optically couple the chirped fiber Bragg grating to an optical fiber of the bidirectional computer data communication link.

BACKGROUND

The present invention relates to communication networks, and more specifically, to providing a low differential delay chromatic dispersion compensator for latency-sensitive networks.

In optical communication systems, chromatic dispersion of light signals propagating over long distances causes light pulses to spread out as they travel along an optical fiber. Chromatic dispersion occurs because different spectral components at different wavelengths in a pulse travel at slightly different speeds. For example, in normal dispersion situations, short wavelengths (blue) travel faster than long wavelengths (red). First-order chromatic dispersion, D, is given in ps/nm-km by the expression:

In this expression, λ0is the fiber's zero dispersion wavelength, S0is the fiber's zero dispersion slope, and λcis the operating center wavelength. The resulting pulse spread can cause pulses in succession to overlap and interfere with each other, producing bit errors. The optical power penalty associated with first order dispersion, Pd, is given (in dB) by:
Pd=5 log(1+2π(BDαλ)2L2)
In this expression, B is the bit rate, L is the link length, and Δλ is the root mean square (RMS) spectral width of the source. Dispersion can become a limiting factor in optical communication systems, since it grows worse for longer links and higher bit rates.

SUMMARY

According to one embodiment of the present invention, a chromatic dispersion compensator with low differential delay is provided that includes a chirped fiber Bragg grating in a compensating optical fiber core. The chirped fiber Bragg grating includes wavelength gratings spaced at distances varying with respect to the length of the compensating optical fiber core to compensate for differential delay in a synchronous time protocol for a bidirectional computer data communication link. The chromatic dispersion compensator also includes an optical junction to optically couple the chirped fiber Bragg grating to an optical fiber of the bidirectional computer data communication link.

An additional embodiment is a system that includes an optical fiber in a bidirectional computer data communication link to support optical communication between servers. The system also includes a chromatic dispersion compensator coupled to the optical fiber. The chromatic dispersion compensator has a low differential delay and includes a chirped fiber Bragg grating in a compensating optical fiber core. The chirped fiber Bragg grating includes wavelength gratings spaced at distances varying with respect to the length of the compensating optical fiber core to compensate for differential delay in a synchronous time protocol for a bidirectional computer data communication link. The chromatic dispersion compensator also includes an optical junction optically coupling the chirped fiber Bragg grating to the optical fiber in the bidirectional computer data communication link.

A further embodiment is a method of providing chromatic dispersion compensation with a low differential delay. The method includes linking at least two servers via a bidirectional computer data communication link comprised of optical fibers. The method additionally includes coupling a chromatic dispersion compensator with low differential delay to one of the optical fibers of the bidirectional computer data communication link. The chromatic dispersion compensator includes a compensating optical fiber core with a chirped fiber Bragg grating spaced at distances varying with respect to the length of the compensating optical fiber core to compensate for differential delay in a synchronous time protocol for the bidirectional computer data communication link. The chromatic dispersion compensator also includes an optical junction to optically couple the chirped fiber Bragg grating to the optical fiber of the bidirectional computer data communication link. The method further includes reducing chromatic dispersion of an optical pulse received at the chromatic dispersion compensator from the bidirectional computer data communication link via reflecting longer wavelengths of the optical pulse through an optically shorter path of the chirped fiber Bragg grating and reflecting shorter wavelengths of the optical pulse through an optically longer path of the chirped fiber Bragg grating.

DETAILED DESCRIPTION

The invention as described herein provides a low differential delay chromatic dispersion compensator for latency-sensitive networks. Differential time sensitive protocols (e.g., a synchronous time protocol) may be used in high-reliability computer systems, where servers operate synchronously for synchronous disaster recovery and business continuity applications. One example of such as protocol is Server Time Protocol (STP). STP is a synchronous time protocol that embeds time stamps within data streams, allowing multiple servers to synchronize themselves to a common time-of-day clock. Latency compensation for bidirectional links may be used to maintain a common time base with a high degree of accuracy between the servers. A bidirectional link between two servers can include separate fibers for transmission in each direction to and from the servers. If the length of the fibers in each direction of the bidirectional link varies substantially, e.g., over 900 meters, the resulting latency variation in each direction can cause synchronization errors, since it takes a greater amount of time to transmit in one direction versus the opposite direction.

STP may be transported on optical fibers using a multiplexing technique that merges multiple wavelengths and/or frequencies to increase throughput. An example of such a multiplexing technique is wavelength division multiplexing (WDM). WDM switches multiple optical carrier signals on an optical fiber by using different wavelengths of laser light to carry different signals. In a WDM system, pulses at different wavelengths typically suffer different amounts of dispersion. Another example of a multiplexing technique that can be used to increase the throughput of STP is time division multiplexing (TDM). TDM can multiplex several lower data rate signals onto a higher aggregate data rate carrier (e.g., 10 Gbit/s, 40 Gbit/s, or 100 Gbit/s), where higher data rates are more susceptible chromatic dispersion errors.

As the data rate and distance for STP links increases, for example, over 100 km, chromatic dispersion compensation may be performed. One approach to dispersion compensation is insertion of dispersion compensating fiber (DCF) in the communication path, where the DCF has an opposite and much stronger dispersion characteristic than normal fiber. This approach for dispersion compensation requires adjusting the length of the DCF to balance out the normal link dispersion. However, latency-sensitive protocols, such as STP, may demand that transmit and receive fibers in a link experience roughly the same differential time delay; otherwise, the time-of-day clocks can lose synchronization. Adding an arbitrary amount of DCF to one or more fibers in the link can result in a sufficiently large differential latency that prevents latency-sensitive protocols from functioning properly. This can limit the maximum distance of links between the servers, for instance, less than 100 km. Since dispersion depends on the fiber's physical properties as well as transmission laser characteristics, both fibers in a bidirectional link (assuming one fiber in each transmission direction) may experience different amounts of dispersion even if they follow the same physical path. After some DCF is added to one fiber in the bidirectional link, the fibers in the bidirectional link can be of significantly different physical lengths, e.g., one kilometer or more difference.

In an exemplary embodiment, a low differential delay chromatic dispersion compensator (also referred to simply as a “compensator” herein) with a chirped fiber Bragg grating is inserted in the communication path (bidirectional link) between servers. The additional path length of the compensator is minimal, (e.g., 3 meters or less) to minimize latency differences between compensated and uncompensated links. The chirped fiber Bragg grating recompresses optical signals that have spread due to chromatic dispersion, which enables higher throughput and/or longer distances between servers. Adding the compensator to fiber in only one direction of a bidirectional link has a near-zero differential latency impact, as the additional path length attributable to the compensator is negligible. Thus, the compensator provides low differential delay chromatic dispersion compensation relative to each direction of a bidirectional link. Moreover, while DCF typically uses a reduced diameter core as part of its compensation (which results in a transmission loss), the chirped fiber Bragg grating of the compensator can be embedded in an optical fiber core with a core diameter that is approximately the same as the link it compensates, resulting in a minimal transmission loss. The low differential delay chromatic dispersion compensator with chirped fiber Bragg grating is described in greater detail herein.

Turning now to the drawings, it will be seen that inFIG. 1there is a block diagram of a system100upon which low differential delay chromatic dispersion compensation for latency-sensitive networks is implemented in exemplary embodiments. The system100ofFIG. 1includes a first server102in communication with a second server104over a network106. The system100may be a systems complex (sysplex), where the servers102and104execute synchronously to each other under normal operating conditions. In an exemplary embodiment, the server102includes at least one processing circuit108(e.g., a CPU) capable of reading and executing instructions, as well as accessing memory110. The processing circuit108may be a general purpose or application specific microprocessor, and/or can include multiple processing cores, e.g., a multi-core module (MCM). The memory110can be a combination of volatile and non-volatile memory, including cache and/or secondary storage, for program and/or data storage. Similarly, the server104includes at least one processing circuit112and memory114, that may be the same technologies as described in reference to processing circuit108and memory110.

The server102may include STP transmit and receive logic (TX/RX)116to transmit and receive data streams on the network106using a latency sensitive protocol. Time interface118accesses a time base, such as a time-of-day clock, that may be internal or external to the server102, providing time information to sync logic120. The sync logic120maintains synchronization between the servers102and104, and can synchronize the server102to other servers (not depicted) of the system100. Similarly, the server104includes STP TX/RX122, time interface124, and sync logic126to provide substantially the same functionality as the STP TX/RX116, time interface118, and sync logic120of server102.

The server102may communicate through multiple links128and130to a multiplexing (mux) module132. The mux module132includes STP to/from mux conversion transmit and receive logic (STP/MUX TX/RX)134to convert STP data into a multiplexed format for long distance communications, e.g., over 40 km. The mux module132optically encodes and decodes data on one or more bidirectional fiber optic links, such as links136and138, also referred to as bidirectional computer data communication links136and138. In one embodiment, links128and130are also bidirectional fiber optic links. In an alternate embodiment, links128and130are wired or wireless electronic communication paths. The links128and130may be comprised of unidirectional links140,142,144, and146, providing communication paths to and from the server102and mux module132. In an exemplary embodiment, the bidirectional computer data communication links136and138include optical fiber links148,150,152, and154(or simply “optical fibers”), providing a communication path between mux module132and mux module156. Similar to the mux module132, the mux module156includes STP/MUX TX/RX158to convert multiplexed data from the bidirectional computer data communication links136and138to STP for the server104, as well as conversion in the opposite direction. The mux module156and server104can communicate via bidirectional links160and162, which may be further comprised of unidirectional links164,166,168, and170. The bidirectional links160and162may be similar to the bidirectional links128and130.

To maintain integrity of communications over the bidirectional computer data communication links136and138, chromatic dispersion compensators can be added. In the example depicted inFIG. 1, chromatic dispersion compensator172is on link148, chromatic dispersion compensator174is on link150, and chromatic dispersion compensator176is on link154; however, chromatic dispersion compensation is not used on link152. The chromatic dispersion compensators172-176provide near-zero latency chromatic dispersion compensation for the bidirectional computer data communication links136and138. In an exemplary embodiment the chromatic dispersion compensators172-176use passive optics to perform chromatic dispersion compensation.

AlthoughFIG. 1depicts two servers102and104in communication via a combination of links128,130,136,138,160, and162through mux modules132and156, the scope of the invention is not so limited. It will be understood that any number of links can be used to maintain low-latency differential communication between two or more servers with optical fibers. Moreover, the mux modules132and156can be incorporated in the servers102and104. The mux modules132and156can use a variety or combination of multiplexing techniques to transport time synchronous protocol data with multiple wavelengths and/or frequencies via bidirectional computer data communication links136and138. Examples of multiplexing techniques include wave division multiplexing (WDM) and time division multiplexing (TDM).

FIG. 2depicts an example of chromatic dispersion compensator172ofFIG. 1in accordance with exemplary embodiments. The chromatic dispersion compensator172provides low differential delay compensation on link148relative to link150ofFIG. 1. An example of an input pulse202in link148is depicted prior to reaching the chromatic dispersion compensator172. The input pulse202is comprised of multiple wavelengths (λ1, λ2, . . . , λM) that become spread or broadened as it propagates in optical fiber core204of the link148. Shorter wavelengths (λM) travel faster than longer wavelengths, such as λ2and λ1, resulting in further spreading over greater distances. Rather than using DCF, which could add one or more kilometers to the path length of link148, the chromatic dispersion compensator172performs first order chromatic dispersion compensation using a chirped fiber Bragg grating206in a compensating optical fiber205. The chirped fiber Bragg grating206receives the input pulse202traveling through the link148using optical junction208. The optical junction208may be an optical splitter, tap, or circulator.

The chirped fiber Bragg grating206can be manufactured by selectively doping compensating optical fiber core210of the compensating optical fiber205to make it sensitive to ultraviolet light, then exposing the compensating optical fiber core210using a phase mask in a manner similar to conventional lithography. A pattern of alternating high and low refractive index is written into the compensating optical fiber core210, which acts as a diffraction grating. Light propagating through the chirped fiber Bragg grating206is scattered by Fresnel reflection from each successive refractive index perturbation (refractive index perturbation212, refractive index perturbation214, up to refractive index perturbation216), in the compensating optical fiber core210. A periodic index perturbation with period P causes high reflectivity in the vicinity of the Bragg wavelength, defined as λB=2 nP, where n is the effective modal index of the compensating optical fiber core210. The light efficiency can approach 100% within a narrow wavelength window (1 nm or less) around the Bragg wavelength. The grating period can be made to vary linearly with distance in the compensating optical fiber core210to produce a chirped distribution of refractive index perturbations212,214. . .216(also referred to as wavelength gratings212,214. . .216). As light with different wavelengths (λ1, λ2, . . . , λM) passes through the chirped fiber Bragg grating206, each refractive index perturbation212,214. . .216, reflects a limited range of wavelengths while allowing the rest to pass through.

The chirped fiber Bragg grating206enables longer wavelengths to reflect earlier, while shorter wavelengths are reflected later, as the input pulse202travels in and reflects out of the chirped fiber Bragg grating206. Thus, faster wavelengths experience a longer path length, while the slower wavelengths experience a shorter path length. For example, λ1travels an optically shorter path218in the chirped fiber Bragg grating206before reflecting as compared to optically longer paths220and222reflecting λ2and λMin the chirped fiber Bragg grating206of the compensating optical fiber core210. The net effect recompresses the spectrum of the input pulse202to the output pulse224, which travels through the optical junction208back to the link148. Even if two separate chromatic dispersion compensators (e.g., chromatic dispersion compensators172and174) are used on the same bidirectional link (e.g., bidirectional computer data communication link136), the differential delay is nearly zero (approximately 15 nanoseconds or less) and is much lower than DCF solutions.

In an exemplary embodiment, the chromatic dispersion compensator172is optically passive, resulting in a low cost, simple compensator. The length226of the compensating optical fiber core210may be approximately 3 meters of less, adding very little distance to the total optical path length between servers102and104ofFIG. 1, which can be over 100 km. Conventional DCF achieves its properties, in part, by reducing the diameter of the fiber core, which in turn increases the fiber's transmission loss. For example, commercial DCFs for 100 km standard single mode fiber have about 10 dB excess loss compared with conventional fibers. To minimize optical power losses attributable to the chromatic dispersion compensator172, the diameter of the compensating optical fiber core210is equivalent to the optical fiber core204of the link148within a manufacturing tolerance. Reducing optical power losses due to compensation can extend the maximum distance at which the servers102and104can sustain high-speed time synchronous communications.

Turning now toFIG. 3, a process300for low differential delay chromatic dispersion compensation will now be described in accordance with exemplary embodiments, and in reference toFIGS. 1 and 2. At block302, at least two servers102and104are linked via bidirectional computer data communication link136that includes optical fiber links148and150supporting communication in opposite directions. The servers102and104can also be in communication via additional redundant communication paths, such as bidirectional link138that includes optical fiber links152and154. Communication between the serves102and104may be a multiplexed synchronous time protocol, such as STP over WDM.

At block304, a chromatic dispersion compensator is coupled to one of the optical fiber links148and150of the bidirectional computer data communication link136, such as chromatic dispersion compensator172. Chromatic dispersion compensation can be applied to either or both of the optical fiber links148and150. As depicted inFIG. 2, the chromatic dispersion compensator172may include compensating optical fiber205having compensating optical fiber core210with chirped fiber Bragg grating206including wavelength gratings212-216(refractive index perturbations) spaced at distances varying with respect to the length226of the compensating optical fiber core210. The chromatic dispersion compensator172also includes optical junction208to optically couple the chirped fiber Bragg grating206to the optical fiber link148.

At block306, the chromatic dispersion compensator172reduces chromatic dispersion of optical pulse (e.g., input pulse202) received from the bidirectional computer data communication link136via reflecting longer wavelengths of the optical pulse through an optically shorter path (e.g., λ1through optically shorter path218) of the chirped fiber Bragg grating206and reflecting shorter wavelengths of the optical pulse through an optically longer path (e.g., λ2through optically longer path220) of the chirped fiber Bragg grating206.

A bidirectional link may contain multiple chromatic dispersion compensators, which can vary in design characteristics. For example, chromatic dispersion compensators172,174, and176ofFIG. 1can use different types of optical junctions208, have different lengths226of the compensating optical fiber core210, and/or different degrees of chromatic dispersion compensation. However, the net effect of inserting the chromatic dispersion compensators172,174, and176into communication paths between the servers102and104is increased throughput at longer distances than could otherwise be achieved, while minimizing the differential latency impact between each direction of the bidirectional computer data communication links136and138.

In an exemplary embodiment, the chromatic dispersion compensators172,174, and176are insensitive to ambient temperature changes. If the compensating optical fiber205is uniformly heated or stretched, the period of the chirped fiber Bragg grating206is changed, and accordingly the Bragg reflection wavelength is also changed, but the dispersion remains unchanged.

Technical effects include providing a low differential delay chromatic dispersion compensator for latency-sensitive networks. The use of a chirped fiber Bragg grating compensates for chromatic dispersion that can occur when transmitting multiple wavelengths and/or frequencies over long distances. The small length of compensating optical fiber added to the path length, in conjunction with no feedback loop delays, enables time-sensitive communications to stay synchronized between two or more servers.