Optical line protection with digital dispersion compensation module

Embodiments of present invention provide an optical signal transportation system. The system includes a first and a second optical line protection (OLP) node; a working signal transmission medium and a protection signal transmission medium between the first and second OLP nodes providing transportation paths for an optical signal from the first OLP node to the second OLP node; and at least one digital dispersion compensation module (DDCM) connected to at least one of the working and protection signal transmission media inside the second OLP node, wherein the DDCM includes a plurality of dispersion compensation units (DCUs) with each DCU being capable of providing either a positive or a negative dispersion selected by an optical switch to the optical signal, and wherein the DDCM is capable of providing the optical signal a total dispersion determined by the optical switch of each of the plurality of DCUs.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a continuation-in-part (CIP) application of a concurrently filed U.S. patent application entitled “Digital Dispersion Compensation Module”, and claims benefit of priority to a provisional U.S. patent application Ser. No. 61/957,352 filed Jul. 1, 2013, the content of which are both incorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates generally to system and configuration thereof for optical data transportation and, in particular, relates to an optical line protection system with digital dispersion compensation module.

BACKGROUND

Optical signal transmission and transportation is a key enabling force in today's high speed digital communication infrastructure which supports vast amount of data transportation that are essential for many data centric informational applications such as, for example, internet application. With ever increasing demand for transportation bandwidth, new optical signal transmission and transportation systems are constantly being developed which trend toward higher data rate and higher channel density count.

Optical signal, in a format of binary or multi digital level, usually experiences certain amount of distortion during transportation that, together with other causes such as noise, affects overall system performance. Generally, the higher the data rate of and the longer a distance traveled by an optical signal, the bigger the amount of distortion that the optical signal usually experiences. Among many factors contributing to the optical signal distortion, chromatic dispersion of the transportation media such as fiber is a main factor. The amount of dispersion that an optical signal is able to tolerate in a transmission system varies inversely proportional to the square of the data-rate. As a general rule of thumb, for a 40 Gb/s direct detection system, the dispersion window is typically less than the equivalent of 10 km of SMF-28 fiber at 1550 nm wavelength.

FIG. 1is a simplified functional block diagram of an optical signal transportation system with line protection scheme as is known in the art. Under normal working conditions, optical signals are usually transported over working fiber paths31and32, as a bidirectional optical transportation system1, between terminal10and terminal20. If there is a fault such as fiber cut in one or both of the working fiber paths31and32, the amount of optical signal received at photo-detector PD2in terminal10and/or at photo-detector PD5in terminal20will generally decrease to a level below a pre-defined threshold. As a result, this decrease in signal level triggers optical line protection (OLP) switches, such as SW1and SW2in terminal10and SW3and SW4in terminal20, to switch and cause the system to transmit and receive optical signals via protection fiber paths41and42instead of working fiber paths31and32. The same event, such as fiber cut, may also generate a system alarm to alert the happening and existence of such a fault in the working fiber paths31and32. Optical signals transported over protection fiber paths41and42may continue to be monitored by photo-detectors PD3and PD6. In the bidirectional transportation system1illustrated inFIG. 1, photo-detectors PD1and PD4are used to monitor optical signal levels launched into the fibers31/41and/or32/42, in both directions.

However, the above optical system configuration may not work well on fiber links with a narrow dispersion window due to difference in total fiber dispersion between the working fiber paths31/32and the protection fiber paths41/42. This is especially true in a DQPSK direct detection system where data rate of the optical signal is around 40 Gb/s or even higher such as 100 Gb/s. Generally, in the above system in order to expand dispersion window that an optical signal may be able to tolerate, fiber-bragg gratings (FBG) and/or more frequently Etalon-based channelized tunable dispersion compensation modules (TDCM) (both of which are not shown inFIG. 1) are used at the receiving end of each channels of their respective terminals.

In order to get optical transportation system1back to work or recovered once being interrupted due to, e.g., fiber cut, the tunable dispersion compensation module (TDCM) in each receiving channel is required to change or modify their dispersion setting so as to compensate any difference in the amount of total dispersion between the working (31/32) and the protection (41/42) fiber paths. However, dispersion of this channelized TDCM is normally tuned through gradual temperature change which is generally considered being slow, in the range of seconds if not in the tens of seconds. Together with the process of using forward error correction (FEC) algorithm for feedback or other feedback mechanism to find the right setting for the TDCM, the entire process of recovering optical transportation system1from fiber cut, for example, for just one channel may take several seconds and sometimes close to tens of seconds. It is generally known in the industry that for dynamic line protection application it is required that the system recovery time be less than 50 ms. Obviously, thermally-tuned TDCM is unable to meet the 50 ms recovery time requirement for the protection scheme of an optical transportation system.

SUMMARY

Embodiments of present invention provide an optical signal transportation system. In one embodiment, the system includes a first and a second optical line protection (OLP) node; a working signal transmission medium and a protection signal transmission medium between the first and second OLP nodes providing transportation paths for an optical signal from the first OLP node to the second OLP node; and at least one digital dispersion compensation module (DDCM) connected to at least one of the working and protection signal transmission media inside the second OLP node, wherein the DDCM includes a plurality of dispersion compensation units (DCUs) with each DCU being capable of providing either a positive or a negative dispersion selected by an optical switch to the optical signal, and wherein the DDCM is capable of providing the optical signal a total dispersion determined by the optical switch of each of the plurality of DCUs.

According to one embodiment, at least one of the DCUs includes a piece of fiber-bragg grating (FBG) having a first and a second terminal. The DCU is capable of providing the positive dispersion by connecting the optical switch to the first terminal and providing the negative dispersion by connecting the optical switch to the second terminal. In one embodiment, value of the positive dispersion is same as value of the negative dispersion inside the at least one of the DCUs.

In one embodiment, the at least one DDCM is connected to the working signal transmission medium or to the protection signal transmission medium through an optical switch.

In another embodiment, the above at least one DDCM is a first DDCM and is connected to the working signal transmission medium, and the system further includes a second DDCM that is connected to the protection signal transmission medium.

According to one embodiment, the first OLP node includes a test signal generator capable of generating a test optical signal being launched into the working or protection signal transmission medium, the test optical signal includes a plurality of digital optical signals at different wavelengths, the plurality of digital optical signals having a data rate ranging from about 10 Mb/s to about 155 Mb/s.

According to another embodiment, the second OLP node includes a signal processing unit capable of receiving the test optical signal from the first OLP node, dividing the test optical signal into the plurality of digital optical signals according to their respective wavelengths, detecting group delay differences among the plurality of digital optical signals, and determining a total dispersion that the test optical signal experienced from the first OLP node to the second OLP node based on the group delay differences.

In one embodiment, the signal processing unit further includes a plurality optical delay lines capable of adding delays to the plurality of digital optical signals received at the second OLP node.

According to one embodiment, the above optical signal transportation system further includes a second working signal transmission medium and a second protection signal transmission medium between the first and second OLP nodes providing transportation paths for a second optical signal from the second OLP node to the first OLP node. In one embodiment, the first and second signal transmission media are optical fibers.

Embodiments of present invention provide an optical signal transportation system which, in one embodiment, includes a first and a second digital dispersion compensation unit (DDCU); a first and a second working signal transmission medium between the first and second DDCUs providing transportation paths for a first optical signal from the first DDCU to the second DDCU and a second optical signal from the second DDCU to the first DDCU, wherein the first and second DDCUs are capable of providing the first and second optical signals, respectively, with a total amount of dispersion that compensates for proper detection of the first and second optical signals.

It will be appreciated that for simplicity and clarity purpose, elements shown in the drawings have not necessarily been drawn to scale. Further, in various functional block diagrams, two connected devices and/or elements may not necessarily be illustrated to be connected, for example, by a continuous solid line or dashed line but rather sometimes a small gap between two lines extended from the two devices and/or elements may be inserted intentionally in order to illustrate the individual devices and/or elements even though their connection is implied. In some other instances, grouping of certain elements in a functional block diagram may be solely for the purpose of description and may not necessarily imply that they are in a single physical entity or they are embodied in a single physical entity.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 2is a demonstrative illustration of a functional block diagram of an optical digital dispersion compensation module according to one embodiment of present invention. DDCM200may include four dispersion compensation units211,212,213, and214that are connected to a multi-port optical circulator215, such as a 6-port optical circulator, in a cascaded manner. Dispersion compensation unit211,212,213, and214may each contain one two-port dispersion element221,222,223, and224respectively. The two-port dispersion element has reciprocal dispersion characteristics depending on which port an optical signal enters the dispersion element. For example, an optical signal entering into port Ai (i=1, 2, 3, and 4) and propagating towards port Bi may get reflected back before reaching port Bi and may experience positive dispersion. Alternately, an optical signal entering into port Bi and propagating towards port Ai may get reflected back before reaching port Ai and may experience negative dispersion. In other words, dispersion element221,222,223, and224may serve as either a positive dispersion element or a negative dispersion element depending upon which port is used for signal entry.

Dispersion element221,222,223, and224may be made, in one embodiment, from non-uniform long fiber-bragg grating (FBG) which has continuous operation bandwidth up to 100 nm. DDCM200may thus be able to compensate dispersion over a broad wavelength range anywhere within 1260 nm to 1680 nm to cover multiple optical spectrum bands. By using FBG as dispersion element, DDCM200may further be able to compensate dispersion profile such as slope of dispersion, linear or non-linear, or even derivative of slope of dispersion that are considered as high-order dispersion. Compensation of high-order dispersion has been considered as crucial for current and/or future super-channel transmission which generally has up to 400 Gb/s or even terabit transmission capacity.

Dispersion compensation units211,212,213, and214may be able to provide a base amount, or a certain integer multiple thereof, of dispersion compensation to an input optical signal. The base amount of dispersion compensation may be determined by the granularity of compensation required by the system where DDCM200is used, which is often affected and/or determined by the rate of digital optical signal such as whether the digital optical signal is a 10 Gb/s, 40 Gb/s, or 100 Gb/s optical signal. Furthermore, assuming DCU211is designed to have a base amount of dispersion (both positive and negative), equivalent in value to a piece of SMF-28 fiber of n km in length, wherein n may be any suitable number, and having a dispersion amount of y ps/nm/km at a nominal wavelength, DCU212,213, and214may be designed to have their dispersions equivalent to 2i-1×n km of the same SMF-28 fiber where i=2, 3, and 4.

By setting optical switches231,232,233, and234at either port Ai (for positive dispersion) or port Bi (for negative dispersion), where i=1, 2, 3, and 4, the total equivalent dispersion that DDCM200may be able to provide may range from −15 n km to +15 n km of SMF-28 fiber with an incremental step of 2 n km. Thus, when being used to compensate a fiber-optic link of a total dispersion equal to a piece of SMF-28 fiber of −16 n km to +16 n km, net dispersion of the fiber-optic link after compensation may be reduced down to within +/−n km. This reduction in net dispersion dramatically eases the required tolerance range of the transmitting and/or receiving devices communicating through the fiber-optic link.

It is to be noted that the above configuration of DDCM200may be generalized to include a dispersion compensation module having N dispersion compensation units cascaded by an optical circulator of at least N+2 ports, with N being any suitable digital number such as 4 for DDCM200illustrated inFIG. 2. Each dispersion compensation unit k (k=1, 2, . . . N) may be able to selectively provide either a positive or a negative dispersion with a value equivalent to 2k-1×n km SMF-28 fiber. The N dispersion compensation units may be cascaded randomly, in an ascending order, or in a descending order along the optical circulator. The range of dispersion compensation provided by this DDCM may be from −(2N−1)×n km to +(2N−1)×n km, capable of adjusting a fiber-optic link of with no more than 2N×n km dispersion down to +/−n km.

DDCM200inFIG. 2uses a set of optical switches231,232,233, and234to control and configure a set of dispersion compensation units to provide a combination thereof, thus delivering a right amount of dispersion to a system where it is used. When being compared with conventional dispersion compensation scheme which, for example, uses slow thermally-tuned tunable dispersion compensation module, digital dispersion compensation module controlled by optical switches electronically provides a much fast response time, which is typically determined by the speed of optical switches, of 10 milliseconds (ms) or less.

DDCM200demonstratively illustrated inFIG. 2and other DDCMs such as those that are described in the concurrently filed U.S. patent application “Digital Dispersion Compensation Module”, which is incorporated herein by reference in its entirety, may be used in an optical line protection scheme to protect an optical signal transportation system from general transmission path failure such as a sudden fiber cut. The fast response time of 10 milliseconds (ms) or less offered by the DDCM provides the needed speed by the industry standard of 50 ms. Various optical line protection schemes using digital dispersion compensation module are described below in more details with reference toFIGS. 3-5. It is to be noted that in the below description, DDCM used in the system configurations may be the one illustrated inFIG. 2, may be those described in the concurrently filed U.S. patent application “Digital Dispersion Compensation Module”, or any other types of dispersion compensation modules that offer a fast response time.

FIG. 3is a demonstrative functional block diagram illustration of an optical signal transportation system employing optical line protection using digital dispersion compensation module according to one embodiment of present invention. More specifically, as a non-limiting example, inFIG. 3optical signal transportation system300is demonstratively illustrated to have a first optical line protection (OLP) node310, a second OLP node320, and at least a first working signal transmission medium331and a first protection signal transmission medium341between first and second OLP nodes310and320. Signal transmission media331and341provide transportation paths for optical signals, for example optical signal S1, to be transported or to transmit or propagate from first OLP node310to second OLP node320.

In one embodiment, system300may be a bi-directional optical signal transportation system and, in addition to first working signal transmission medium331and first protection signal transmission medium341, have a second working signal transmission medium332and a second protection signal transmission medium342providing transportation paths for optical signals, for example optical signal S2, to be transported or to transmit or propagate from second OLP320to first OLP node310. InFIG. 3, system300is demonstratively illustrated as a point-to-point signal transportation system. However, embodiments of present invention are not limited in this respect and may be applied to other types of systems such as, for example, a ring system or a mesh system wherein working and protection signal transmission media may take different geographic routes.

In system300, signal transmission media331,341,332, and342may be optical fibers including conventional single mode fiber such as SMF-28, dispersion shifted fiber (DSF), or any other currently existing or future developed fibers. However, other types of signal transmission medium such as free space, bulk optics, or any combination of the above transmission media may be used as well, all of which are fully contemplated herein by applicants. Nevertheless, in the following description, for simplicity without losing generality, signal transmission media331,341,332, and342may be referred to as optical fibers or, simply as fibers.

Generally when disruption such as fiber cut happens to a working fiber331, or a pair of working fibers331and332in a bi-directional system, optical signals are routed to a protection fiber341or a pair of protection fibers341/342. However, the protection fiber or fibers in general have different amount of total dispersion from that of the working fiber or fibers and the difference in the amount of dispersion needs to be properly compensated before data communication may be restored properly or re-established.

According to one embodiment of present invention, OLP nodes310and320may be able to protect continuity of optical signal transportation in the event of fiber cut or other disruption to transmission media331and/or332, and the protection may be achieved within 50 milliseconds (ms) or less, possibly within 10 ms. The above fast data restoration is achieved by embodiment of present invention through the application of a digital dispersion compensation module (DDCM) that has a set of dispersion compensation modules with pre-determined dispersion values. Inside the DDCM, the set of dispersion compensation modules may be quickly and electronically configured through a set of optical switches to provide a combination thereof with desired total dispersion value to the protection fiber341and/or342. One of such DDCMs is described above in detail with reference toFIG. 2, and more examples may be referenced to the concurrently filed U.S. patent application “Digital Dispersion Compensation Module”.

Embodiments of present invention provide optical line protection through the use of one or more OLP nodes such as OLP node310and OLP node320. Each OLP node provides protective function for both transmitting and receiving of optical signals. More specifically, for example, at the transmitting side, OLP node310may include a two-by-two (2×2) optical switch311(SW1) that, during normal operation, receives an optical signal S1and transmits the optical signal S1to working fiber331. Optical switch311may also receive a testing signal D1, either during normal operation or in the event of transmission media disruption such as fiber cut, and may transmit the test signal D1to protection fiber341for total dispersion set up or optical signal restoration during disruption. Embodiments of present invention may also include sending test signal D1to working fiber331for the initial set up of working fiber total dispersion. Test signal D1may be generated by a transmitter Tx1through a signal generating unit SGU1, the function of which is described below in more details with reference toFIG. 6.

On the receiving side, OLP node310may include a two-by-two (2×2) optical switch312(SW2) that, during normal operation, receives an optical signal S2from working fiber332and passes it through for normal transmission. OLP node310may also receive a test signal D2, either during normal operation or in the event of transmission media disruption such as fiber cut, to be processed by a receiver Rx1and a signal processing unit SPU1for set up of total dispersion of working fiber332or protection fiber342or for optical signal restoration through protection fiber342during disruption. OLP node310may include a first digital dispersion compensation module DDCM1in working fiber332and a second digital dispersion compensation module DDCM2in protection fiber342. Test signal D2may enter DDCM1or DDCM2, and pass through optical switch312(SW2) to be received and processed by Rx1and SPU1, function of which are described below in more details with reference toFIG. 6.

More specifically, during normal working operation, DDCM1of OLP node310may provide a properly determined dispersion to working fiber332, through proper setting of optical switches therein automatically or manually, for example during an initial set up process of working fiber332. Optical switch312may be in a pass-through position such that optical signal S2from OLP node320passes through optical switch312and exit OLP node310. In the meantime, OLP node320may send test signal D2to propagate through protection fiber342, through DDCM2, to be received by Rx1and processed by SPU1. The amount of dispersion of protection fiber342may thus be determined by SPU1which subsequently sets, automatically or manually, a proper total dispersion of DDCM2by electronically controlling a set of optical switches inside thereof such that protection fiber342is pre-conditioned to be ready for optical signal transportation from OLP node320to OLP node310in the event of a fiber cut. For example, the total dispersion of DDCM2may be decided based on optical signal S2being properly detected. The set up of protection fiber342may be performed during normal operation when working fiber332is working properly, or be performed on-demand when a fiber cut, for example, is detected and restoration of optical signal transportation is requested. Embodiments of present invention are able to achieve signal restoration within 50 ms or less even on-demand because of the use of digital dispersion compensation modules that are controlled by a set of optical switches.

OLP node310may additionally include photo-detectors PD1, PD2, and PD3monitoring optical signal power level at various points in association with the operation of optical switches311and312, and/or monitoring and triggering of various system operation alarms.

InFIG. 3, system300is a bi-directional optical signal transportation system, and OLP node320operates similarly as OLP node310. For example, OLP node320may include an optical switch322(SW3) on a transmitting side to send optical signal S2and/or test signal D2generated by transmitter Tx2and signal generating unit SGU2. On a receiving side, OLP node320may include two digital dispersion compensation modules DDCM3and DDCM4cascaded to working fiber331and protection fiber341, respectively. OLP node320may include an optical switch321(SW4) to receive optical signal S1and/or test signal D1and test signal D1may be received by a receiver Rx2and processed by a signal processing unit SPU2for the set up of DDCM3and/or DDCM4, automatically or manually. OLP node320may include photo-detectors PD4, PD5, and PD6.

FIG. 4is a demonstrative functional block diagram illustration of an optical signal transportation system with optical line protection employing digital dispersion compensation module according to another embodiment of present invention. InFIG. 4, optical signal transportation system400includes a first OLP node410and a second OLP node420. Comparing with OLP node310in system300being illustrated inFIG. 3, OLP node410includes only DDCM1that is placed after a 2×2 optical switch412(SW2) and shared by working fiber432and protection fiber442. Therefore, during normal operation, DDCM1may be properly set up for dispersion compensation of working fiber432only, and thus may not be set up for dispersion compensation of protection fiber442. Protection fiber442may only be set up for dispersion compensation by DDCM1during signal restoration in the event of fiber cut in the working fiber432.

On the transmitting side, OLP node410works similarly as OLP node310. For example, OLP node410may includes a 2×2 optical switch411(SW1) and may transmit an optical signal S1and/or a test signal D1to working fiber431and/or protection fiber441, and may include transmitter Tx1and signal generating unit SGU1for generating test signal D1. OLP node410may also include photo-detectors PD1, PD2, and PD3for various optical signal detection and alarm processing.

As a bi-directional optical signal transportation system, OLP node420may operate similarly to OLP node410. For example, OLP node420may include an optical switch422(SW3) on a transmitting side to send optical signal S2and/or test signal D2generated by transmitter Tx2and signal generating unit SGU2. On a receiving side, OLP node420may include only DDCM3that is placed after a 2×2 optical switch421(SW4) and shared by working fiber431and protection fiber441. OLP node420may receive optical signal S1and/or test signal D1and test signal D1may be received by receiver Rx2and processed by signal processing unit SPU2for set up of DDCM3such as setting of the optical switches therein automatically or manually. OLP node420may include photo-detectors PD4, PD5, and PD6.

FIG. 5is a demonstrative functional block diagram illustration of an optical signal transportation system with optical line protection employing digital dispersion compensation module according to a further embodiment of present invention. InFIG. 5, optical signal transportation system500may include a first OLP node510and a second OLP node520that are respectively connected to a first digital dispersion compensation unit (DDCU)550and a second DDCU560. Different from embodiments illustrated above inFIG. 3andFIG. 4, OLP node510includes, in addition to photo-detectors PD1, PD2and PD3, only an input one-by-two (1×2) optical switch (SW1) that selects an optical signal from DDCU550to transport either through a working fiber531or a protection fiber541, and an output 1×2 optical switch (SW2) that selects an optical signal from either working fiber532or protection fiber542to output to DDCU550for processing. Similarly, OLP node520includes, in addition to photo-detectors PD4, PD5, and PD6, only an input one-by-two (1×2) optical switch (SW3) that selects to transport an optical signal from DDCU560either through a working fiber532or a protection fiber542, and an output 1×2 optical switch (SW4) that selects an optical signal from either working fiber531or protection fiber541to output to DDCU560for processing.

System500ofFIG. 5also includes DDCU550and DDCU560connected to OLP510and OLP520respectively. DDCU550, on a transmitting side, includes a multiplexer that combines an optical signal S1with a test signal D1, generated by transmitter Tx and signal generating unit SGU, and transmits the combined signal to OLP510. On a receiving side, DDCU550receives a combined signal of an optical signal S2and a test signal D2from OLP510and a demultiplexing divider subsequently directs the optical signal S2through a digital dispersion compensation module DDCM1and the test signal D2to a receiver Rx and signal processing unit SPU, which detects the amount of dispersion of working fiber532or protection fiber542and adjusts automatically a total dispersion amount of DDCM1for proper compensation. DDCU560functions similarly as DDCU550to include a transmitting side with a multiplexer, a Tx, and a SGU and a receiving side with a demultiplexing divider, a DDCM3, a Rx, and a SPU. It is to be noted that the digital dispersion compensation module, such as DDCM3, may be optionally placed before the demultiplexing divider. The amount of additional dispersion DDCM3experienced by test signal D1may be taken into account during dispersion set up by SPU.

It is to be noted here that there may be various variations of configuration of the optical transportation system500illustrated inFIG. 5that are fully within the spirit of present invention. In one embodiment, some optical signal transportation system may operate without optical line protection (OLP). For example, a system may be similar to system500inFIG. 5; may not include OPL510and OPL520and protection fibers541and542; and may include only working fibers531and532that are directly connected to DDCUs550and560. During system setup, DRT modules inside one DDCU unit may work with DRR modules in the other DDCU unit to determine the amount of dispersion of the working fibers531and532, and automatically decide the amount of dispersion compensation to be provided by DDCM1and DDCM3for the respective fibers531and532. Such a system may include, for example, an un-protected 100 Gb/s non-return-to-zero (NRZ) optical signal transportation system.

FIG. 6is a demonstrative illustration of a system diagram for dispersion measurement of an optical link according to one embodiment of present invention. According to one embodiment of present invention, multiple optical sources may be combined and used to detect the amount of dispersion of an optical transmission link or medium, and thus any dispersion differences among different transmission media. It is known in the art that optical signal experiences a group delay during transportation that differs for different wavelengths, and the difference may be expressed as D×Δλ with D being dispersion of the transmission medium and Δλ being difference in wavelength of the optical signals. Thus, by measuring the group delay difference among a set of optical signals with known difference in their wavelengths, total dispersion D of a transmission medium may be determined. Dispersions in different transmission media and their difference may be determined as well.

More specifically, the transmitter and signal generating unit (Tx1and SGU1, Tx2and SGU2, or Tx and SGU inFIGS. 3, 4, and 5) may be implemented as is illustrated in OLP610ofFIG. 6, according to one embodiment of present invention. OLP610may include multiple light sources, such as laser sources or laser signals, of LS1. . . LSn that are modulated or controlled by a signal generating unit SGU. Alternatively, the multiple light sources may be implemented by applying multiple synchronized frequency sources to modulate a single laser source. Lights from the multiple light sources may be combined by a wavelength division multiplexer (WDM) combiner to become a test signal D1which is then launched into transmission media or fibers631and/or641, via optical switch SW1, for the measurement of dispersion.

After propagating through transmission media of working fiber631or protection fiber641, test signal D1is received by OLP node620. Test signal D1is then divided into individual light signals LS1. . . LSn, according to their respective wavelengths, by a WDM divider and subsequently be detected by their respective photo-detectors Det1. . . Detn. Delays among different wavelength signals may be detected and actual dispersion of the transmission medium may be determined.

FIG. 7is a demonstrative block diagram illustration of an optical line protection node for dispersion measurement according to one embodiment of present invention. InFIG. 7, OLP node700includes various components such as power supply unit (PSU), digital dispersion compensation module DDCM1and DDCM2for the working and protection fibers, optical switch SW1at the transmitting side (Tx), optical switch SW2at the receiving side (Rx), and a plurality of photo detectors PD1, PD2, and PD3. In addition, according to one embodiment, OLP node700may include a delay signal generator (DG), which generates dispersion test signals at for example two different wavelengths λ1and λ2which are then combined by WDM1and transmit to either the working fiber or the protection fiber selected by optical switch SW1. OLP node700may also include a delay signal detector (DD), which measures a delay between two individual signals of different wavelengths λ1and λ2that are received via optical switch SW2and separated by WDM2, and determine the dispersion value of transmission medium. InFIG. 7, CCU denotes a central control unit that controls the two digital dispersion compensation modules and the two optical switches. CI is a communication interface while PSU provides electric power for circuit operation.

FIG. 8is a demonstrative block diagram illustration of implementation of delay signal generator and delay signal detector according to one embodiment of present invention. Delay signal generator DG may generate digital test signals with pre-determined relative delay in a data rate between about 10 Mb/s and about 155 Mb/s which may be used to modulate multiple, such as two, laser signals of different wavelengths. Delay signal detector DD may include two delay lines such as fixed optical delay lines or adjustable electrical delay lines. The delay lines may be used to enhance dispersion measurement capability. Delay signal detector DD may also include a phase detector that detects a phase difference between the two test signals received via the two delay lines. ADC is an analog-to-digital convertor that converts the detected phase difference from analog format to digital format, which is subsequently sent to a microprocessor uC to determine the dispersion value.